EURYPTERIDA
BY
HENRY WOODS, M.A.
St. John’s College, Cambridge, University Lecturer in Palaeozoology.
CHAPTER XI
ARACHNIDA (CONTINUED)—DELOBRANCHIATA = MEROSTOMATA (CONTINUED)—EURYPTERIDA
Order II. Eurypterida.
The Eurypterida or Gigantostraca are found only in the Palaeozoic formations. Some species of Pterygotus, Slimonia, and Stylonurus have a length of from five to six feet, and are not only the largest Invertebrates which have been found fossil but do not seem to be surpassed in size at the present day except by some of the Dibranchiate Cephalopods. All the Eurypterids were aquatic, and, with the possible exception of forms found in the Coal Measures, all were marine. The earliest examples occur in the Cambrian deposits, and the latest in the Permian; but although the Eurypterids have thus a considerable geological range, yet it is mainly in the Silurian and the Old Red Sandstone that they are found, the principal genera represented in those deposits being Eurypterus, Stylonurus, Slimonia, Pterygotus, Hughmilleria, Dolichopterus, and Eusarcus. From the Cambrian rocks the only form recorded is Strabops;[[227]] in the Ordovician the imperfectly known Echinognathus[[228]] and some indeterminable fragments have alone been found. In the Carboniferous deposits Eurypterus and Glyptoscorpius occur, and the former survived into the Permian.[[229]]
Fig. [161].—Eurypterus fischeri, Eichw. Upper Silurian, Rootziküll, Oesel. Dorsal surface. a, Ocellus; b, lateral eye; 2–6, appendages of prosoma; 7–12, segments of mesosoma; 13–18, segments of metasoma; 19, tail-spine. (After Holm.)
The Eurypterid which is best known is Eurypterus fischeri (Figs. [161], 162), which is found in the Upper Silurian rocks at Rootziküll in the Island of Oesel (Gulf of Riga). In the Eurypterids from other deposits the chitinous exoskeleton has been altered into a carbonaceous substance, but in the specimens from Oesel the chitin is perfectly preserved in its original condition; and since these specimens are found in a dolomitic rock which is soluble in acid, it has been possible to separate the fossil completely from the rock in which it is embedded, with the result that the structure can be studied more easily and more thoroughly than in the case of specimens from other localities. Consequently Eurypterus fischeri[[230]] may, with advantage, be taken as a type of the Eurypterida.
The general form of the body (Fig. [161]) is somewhat like that of a Scorpion, but is relatively broader and shorter. On the surface of many parts of the exoskeleton numerous scale-like markings are found (Figs. [162], 163).[[231]] The prosoma or cephalothorax consists of six fused segments covered by a quadrate carapace with its front angles rounded. This bears on its dorsal surface two pairs of eyes—large kidney-shaped lateral eyes and median ocelli (Fig. [161], b, a). The margin of the dorsal part of the carapace is bent underneath to form a rim which joins the ventral part of the carapace.
On the ventral surface of the prosoma (Fig. [162]) six pairs of appendages are seen, of which only the first pair (the chelicerae) are in front of the mouth. The chelicerae are small, and each consists of a basal joint and a chela, the latter being found parallel to the axis of the body; they closely resemble the chelicerae of Limulus. The remaining five pairs of appendages are found at the sides of the elongate mouth, and in all these the gnathobases of the coxae are provided with teeth at their inner margins and were able to function in mastication, whilst the distal part of each appendage served as an organ of locomotion. The posterior part of each coxa is plate-like and is covered (except in the case of the sixth appendage) by the coxa of the next appendage behind. A small process or “epicoxite” is found at the posterior end of the toothed part of the coxae of the second, third, fourth, and fifth pairs of appendages. The second appendage consists of seven joints, whilst the remaining four consist of eight joints; none of these appendages end in chelae. The second, third,[[232]] and fourth pairs of appendages are similar to one another in structure, but become successively larger from before backwards. These three pairs are directed radially outwards; each consists of short joints tapering to the end of the limb, and bearing spines at the sides and on the under surface, and also a spine at the end of the last joint.
Fig. [162].—Eurypterus fischeri, Eichw. Upper Silurian, Rootziküll, Oesel. Restoration of ventral surface; 1–6, appendages of prosoma; m, metastoma. Immediately posterior to the metastoma is the “median process” of the genital operculum. (After Holm.)
The fifth appendage is longer than the fourth and is directed backwards; its second and third joints are short and ring-like; the others (fourth to eighth) are long and similar to one another, each being of uniform width throughout; the last joint is produced into a spine on each side, and between these two is the movable end-spine; the other joints do not bear long spines as is the case in the three preceding pairs of appendages.
The sixth appendage is much larger and stronger than the others, and like the fifth, is without long spines. The coxa is large and quadrate; the second and third joints are short, like those of the fifth appendage; the fourth, fifth, and sixth joints are longer and more or less bell-shaped; the seventh and eighth joints are much larger than the others and are flattened.
The metastoma (Fig. [162], m) is an oval plate immediately behind the mouth; it covers the inner parts of the coxae of the sixth pair of appendages, and represents the chilaria of Limulus. But, unlike the latter, it is not a paired structure; nevertheless the presence of a longitudinal groove on its anterior part renders probable the view that it is derived from a paired organ.[[233]] The front margin of the metastoma is indented and toothed. On its inner side in front is a transverse plate, the endostoma, which is not seen from the exterior, since the front margin of the metastoma extends a little beyond it.
Behind the prosoma are twelve free segments, of which the first six form the mesosoma (Fig. [161], 7–12). The tergum on the dorsal surface of each segment is broad and short, the middle part being slightly convex and the lateral parts slightly concave; the external margin is bent under, thus forming a narrow rim on the ventral surface. The tergum of each segment overlaps the one next behind. The segments increase in breadth slightly up to the fourth segment, posterior to which they gradually become narrower.
On the ventral surface the segments of the mesosoma bear pairs of plate-like appendages, each of which overlaps the one behind like the tiles on a roof. On the posterior (or inner) surfaces of these appendages are found the lamellar branchiae, which are oval in outline (Fig. [165], d). Between the two appendages of the first pair is a median process which is genital in function; this pair are larger than the other appendages, and cover both first and second segments, the latter being without any appendages, and they represent the genital operculum of Limulus (Fig. [153], 10). The form of the operculum, more particularly of the median process, differs in the male and female. In that which is believed to be the female (Fig. [162]) the median process is long, and extends beyond the posterior margin of the operculum; it is formed of two small five-sided parts at the base which are united at the sides to the two plates of the operculum; behind this is a long, unpaired part, which is pointed in front; this, together with the remaining parts, is not joined to the side-plates of the operculum, so that the latter are here separated from one another. The third part of the median process is shorter than the second, and bears at its end a pair of small pointed and diverging plates, the tips of which reach to the middle of the third plate-like appendages. On the inner side of the operculum there are, in the female, a pair of curved, tubular organs, attached to the anterior end of the median process, where they open, the free ends being closed; the function of these organs is not known, but was probably sexual.
In the male (Fig. [163], A, a) the median process is formed of two parts only, and is very short, so that the two plates of the operculum unite behind the process.
In the female a median process (Fig. [163], B) is also present between the second pair of appendages (belonging to the third segment of the mesosoma); it consists of a basal unpaired part, and of a pair of long pointed pieces which project on to the next segment. Just as in the case of the genital operculum the basal part is united in front to the appendages, the remainder being free, and separating the greater part of the two plate-like appendages. In the complete animal the median process of this segment is covered by the median process of the genital operculum. The remaining appendages of the female, and all the appendages behind the operculum in the male, are without any median process, and the plates of each pair unite by a suture in the middle line.
Fig. [163].—Eurypterus fischeri, Eichw. Upper Silurian. (After Holm.) A, Genital operculum of male; a, median process. B, Middle part of second appendage of the mesosoma in the female, showing the median process.
The metasoma (Fig. [161], 13–18) consists of six segments which become longer and narrower from before backwards. Each segment is covered by a ring-like sheath or sclerite, and bears no appendages. The posterior end of the last segment is produced into a lobe on each side, and between these lobes the long, narrow tail-spine arises (Fig. [161], 19).
The other genera of the Eurypterida do not differ in any important morphological respects from the form just described, All the genera, of which about thirteen have been recognised, are placed in one family.
Fig. [164].—Pterygotus osiliensis, Schmidt, Upper Silurian, Rootziküll, Oesel. Ventral surface. Reduced. (After Schmidt.) 1–6, Appendages of the prosoma; 7–12, mesosoma; 7, 8, genital operculum; 13–18, metasoma; 19, tail-plate; a, epistome; b, metastoma; c, coxae of sixth pair of appendages.
Fam. Eurypteridae.—The carapace varies somewhat in outline; in Slimonia it is more distinctly quadrate than in Eurypterus, whilst in Pterygotus (Fig. [164]) and Hughmilleria[[234]] it is semi-ovoid. The lateral eyes are at the margin of the carapace in Pterygotus, Slimonia (Fig. [165], a), and Hughmilleria, but in the other genera, including the earliest form, Strabops,[[235]] they are on the dorsal surface at a greater or less distance from the margin.
The pre-oral appendages of Pterygotus (Fig. [164], 1) differ from those of other genera in their much greater length and in the large size of the chelae; they probably consist of a proximal joint and chelae only, although, commonly, they are represented as having a larger number of joints. Unlike Eurypterus and Pterygotus, the second pair of appendages in Slimonia (Fig. [165], 2) differ from the third, fourth, and fifth pairs in being distinctly smaller and more slender, and it is probable that they were tactile. Whilst in Eurypterus the fifth pair of appendages are larger than the three preceding pairs, and also differ from them in structure, in the genus Pterygotus (Fig. [164], 5) they agree closely with the second, third, and fourth pairs, and in Slimonia (Fig. [165], 5) they are nearly the same as the third and fourth pairs. The sixth pair of appendages are much larger and more powerful than the fifth pair in nearly all genera; in Stylonurus (Fig. [166]), however, the sixth pair are similar to the fifth, both being greatly elongated and slender; also in Eusarcus (Drepanopterus) the sixth pair differ less from the preceding pair of appendages than is usually the case.
Fig. [165].—Slimonia acuminata, Salter. Upper Silurian. Restoration of ventral surface, × ⅑. 1–6, Appendages of prosoma; 7, 8, genital operculum; 7–12, mesosoma; 13–18, segments of metasoma; 19, tail-spine; a, lateral eye; b, metastoma, covering the inner parts of the coxae of the last pair of appendages; c, median process of genital operculum; d, branchial lamellae seen through the plate-like appendages. (After Laurie.)
In Pterygotus there is a well-developed epistome (Fig. [164], a) between the mouth and the front margin of the carapace, thus occupying the same position as the hypostome of Trilobites (p. 233). The metastoma is always well developed and forms one of the distinguishing features of the Eurypterids; in form it varies from oval in Eurypterus, to cordate in Slimonia, and lyrate in Dolichopterus.
The principal modifications seen in the genital operculum are in the form of its median process; in Slimonia this either ends in three sharp points posteriorly (Fig. [165], c), or has the form of a truncated cone; its form in Eurypterus has already been described. Glyptoscorpius differs from other Eurypterids in the possession of comb-like organs closely resembling the pectines of Scorpions. Slimonia apparently differs from other genera in that the plate-like appendages on the posterior three segments of the mesosoma do not meet in the middle line (Fig. [165], 10–12). In some forms, such as Pterygotus (Fig. [164]), there is a nearly gradual decrease in the width of the segments in passing from the mesosoma to the metasoma; but in some others, which in this respect are less primitive, such as Slimonia (Fig. [165]), the posterior five segments of the body (like those of Scorpions) are distinctly narrower and longer than the preceding segments. The long tail-spine of Eurypterus is represented in Slimonia by an oval plate produced into a spine at the end (Fig. [165], 19); whilst in some species of Pterygotus the plate is bi-lobed at the posterior end (Fig. [164], 19). In Hughmilleria the tail-spine is lanceolate.
The Eurypterids resemble the Xiphosura in many respects. In both groups the prosoma consists of at least six fused segments, and bears two pairs of eyes—one pair simple, the other grouped eyes—on the dorsal surface of the carapace. The number and position of the appendages of the prosoma in Eurypterids agree with those of Limulus. The chelicerae are closely similar in both cases. The coxae of all five pairs of legs in Eurypterids are toothed and function in mastication; similarly in Limulus all are spiny except the coxae of the last pair of legs. In both a similar epicoxite is present on the coxae. The number of joints in the legs is somewhat greater in the Eurypterids than in Limulus, and in the former none of the legs end in chelae, whereas in the latter all the walking legs, except the last, and also the first in the male, may be chelate. The metastoma of Eurypterids differs in being a large unpaired plate, but is represented in Limulus by the pair of relatively small chilaria. On the mesosoma the genital operculum and plate-like appendages with branchial lamellae are similar in both groups, but in the Eurypterids the genital operculum shows a greater development and covers the second segment, which is without plate-like appendages. A striking difference between the two groups is seen in the segments of the mesosoma and metasoma; in Eurypterids these are all free, whilst in Limulus they are fused together, but this difference is bridged over by some of the Palaeozoic Xiphosura (Fig. [159], A) in which those segments are free.
Fig. [166].—Stylonurus lacoanus, Claypole. Upper Devonian, Pennsylvania. Restoration of dorsal surface. Length nearly five feet. (After Beecher.)
The Eurypterids present a striking resemblance to Scorpions. In both groups the segments in the three regions of the body are the same in number, and the appendages of the prosoma also agree in number and position. The pre-oral appendages are chelate in both, but the second pair of appendages are chelate in the Scorpions only. In Eurypterids the coxae of the five pairs of legs are toothed and meet in the middle line, but in the Scorpions the coxae of the last two pairs do not meet; this difference, however, appears to be bridged over in the earliest known Scorpion—Palaeophonus,[[236]] from the Silurian rocks. The Eurypterids are distinguished from the Scorpions by the much greater development of the last pair of legs. The large metastoma of the former is homologous with the sternum of the Scorpion. The genital operculum is much smaller in Scorpions than in Eurypterids, and in this respect the latter agree with Thelyphonus (one of the Pedipalpi) more than with the Scorpions. The pectines are absent in the Eurypterids except in Glyptoscorpius. Instead of the lung-books of the Scorpions the Eurypterids possess branchial lamellae on the plate-like appendages; but this difference between the two groups appears to be bridged over by Palaeophonus, which was marine, and may have possessed branchial lamellae since stigmata seem to be absent.
Glyptoscorpius,[[237]] which is found in the Lower Carboniferous of the south of Scotland, is a form of considerable interest. It is about a foot in length, and agrees in many respects with Eurypterida, but it may be necessary to separate it from that group since it possesses pectines, and the legs end in a double claw; it cannot, however, be regarded as a link between Eurypterids and Scorpions, but must rather be considered as an offshoot from the former, since the latter group was already in existence at a much earlier period.
ARACHNIDA EMBOLOBRANCHIATA
(SCORPIONS, SPIDERS, MITES, ETC.)
BY
CECIL WARBURTON, M.A.
Christ’s College, Cambridge; Zoologist to the Royal Agricultural Society
CHAPTER XII
ARACHNIDA (CONTINUED)—EMBOLOBRANCHIATA—SCORPIONIDEA—PEDIPALPI
SUB-CLASS II.—EMBOLOBRANCHIATA.[[238]]
Order I. Scorpionidea.
Segmented Arachnids with chelate chelicerae and pedipalpi. The abdomen, which is broadly attached to the cephalothorax or prosoma, is divided into two regions, a six-jointed mesosoma and a six-jointed tail-like metasoma, ending in a poison-sting. There are four pairs of lung-books, and the second mesosomatic segment bears a pair of comb-like organs, the pectines.
The Scorpions include the largest tracheate Arachnid forms, and show in some respects a high grade of organisation. It is impossible, however, to arrange the Arachnida satisfactorily in an ascending series, for certain primitive characteristics are often most marked in those Orders which on other grounds would seem entitled to rank at the head of the group. Such a primitive characteristic is the very complete segmentation exhibited by the Scorpions. They are nocturnal animals of rapacious habit. In size they range from scarcely more than half an inch to eight inches in length. In the northern hemisphere they are not found above the fortieth parallel of latitude in the Old World, though in the New World they extend as high as the forty-fifth. A corresponding southward limit would practically include all the land in the southern hemisphere, and here the Order is universally represented except in New Zealand, South Patagonia, and the Antarctic islands.
Fossil scorpions are rarely found. The earliest examples known occur in the Silurian rocks, and belong to the genus Palaeophonus. In the Carboniferous Eoscorpius is found, and in the Oligocene Tityus.
Much remains to be discovered with regard to the habits of scorpions, and most of the isolated observations which have been recorded lose much of their value through the uncertainty as to the species concerned. The brief accounts given by Lankester and by Pocock,[[239]] and the more recent and elaborate studies of Fabre,[[240]] are free from this defect and contain almost the only trustworthy information we possess.
All are viviparous, and the females carry the newly-hatched young on their backs. They are predaceous, feeding for the most part on insects and spiders. These are seized by the chelate pedipalps, and, if small, are simply picked to pieces by the chelicerae and devoured, but if large the tail-sting is brought into play and the victim quickly paralysed. The process of eating is a slow one, and a Cape scorpion in captivity took two hours to devour a cockroach.
In walking, scorpions carry their pedipalps horizontally in front, using them partly as feelers and partly as raptorial organs. As regards the body the attitude varies considerably. In some cases (Parabuthus, Prionurus, etc.) it is raised high upon the legs, and the “tail” or metasoma is curved forward over the back, but in others (Euscorpius) the body is held low, and the “tail” is dragged along behind, the end only being slightly curled. In the daytime they hide away under wood or stone, or in pits which they dig in the sand. Parabuthus capensis was observed to dig a shallow pit by means of its second and third ambulatory legs, resting on its first and fourth legs aided by the chelae and the metasoma. Those that hide under wood are usually found clinging to their shelter ventral side uppermost. In captivity the creatures, though supplied with water, were never observed to drink; indeed, they are characteristic inhabitants of arid steppes and parched wastes. Like most Arachnids they can endure prolonged abstinence from food.
The only sense well developed seems to be that of touch. Notwithstanding the possession of several eyes their sight is poor. A moving object within the range of a few inches is certainly perceived, but it has to be touched before its nature is recognised. Some writers have attributed to scorpions a keen sense of hearing, and so-called “auditory hairs” are described on the tibia of the pedipalp, but Pocock came to the conclusion that Parabuthus capensis and Euscorpius carpathicus were entirely deaf, and Lankester could obtain no indication of auditory powers in the case of Prionurus. The sense of touch is extremely delicate, and seems to reside in the hairs with which the body and appendages are more or less thickly clothed. The pectines are special tactile organs. That they are in some way related to sex seems probable from the fact that they are larger in the male and sometimes curiously modified in the female, but they appear to be of use also in determining the nature of the ground traversed by the animal, being long in such species as raise the body high on the legs, and short in those that adopt a more grovelling posture. Pocock noticed that a scorpion which had walked over a portion of a cockroach far enough for the pectines to come in contact with it immediately backed and ate it.
Fig. [167].—Buthus occitanus in the mating period. (After Fabre.)
As is the case with most poisonous animals, their ferocity has been much exaggerated; they never sting unless molested, and their chief anxiety is to slink off unobserved. The fables that they kill their young, and that when hard pressed they commit suicide by stinging themselves to death, perhaps hardly deserve serious consideration. The latter accusation is disproved by the fact that a scorpion’s poison has no effect upon itself, or even upon a closely allied species. Some writers think that in the frantic waving of the “tail,” which is generally induced by strong excitement, a scorpion may sometimes inadvertently wound itself with the sharp point of its telson.
Fabre gives a fascinating account of the habits of Buthus occitanus, which occurs in the south of France. He found these scorpions plentifully in arid, stony spots exposed to the sun. They were always solitary, and if two were found under the same stone, one was engaged in eating the other. Their sight is so poor that they do not recognise each other without absolute contact.
Fabre established colonies in his garden and study, providing them with suitable soil and sheltering stones. They dug holes by reducing the earth to powder by means of the three anterior pairs of legs—never using their pedipalpi in the operation—and sweeping away the débris with the tail. From October to March they ate nothing, rejecting all food offered to them, though always awake and ready to resent disturbance. In April appetite seemed to awaken, though a very trifling amount of food seemed to suffice. At that time, too, they began to wander, and apparently without any intention of returning, and they continued daily to escape from the garden enclosure until the most stringent measures were taken to keep them in. Not till they were surrounded by glass and the framework of their cages covered with varnished paper were their attempts to climb out of their prison frustrated. Fabre came to the conclusion that they took at least five years to attain their full size.
Fig. [168].—The “promenade à deux” of Buthus occitanus. (After Fabre.)
His most interesting observations were concerned with their mating habits, in connection with which he noted some extraordinary phenomena. After some very curious antics, in which the animals stood face to face (Fig. [167]) with raised tails, which they intertwined—evidently with no hostile intention—they always indulged in what Fabre calls a “promenade à deux,” hand in hand, so to speak, the male seizing the chelae of the female with its own, and walking backwards, while the female followed, usually without any reluctance. This promenade occupied an hour or more, during which the animals turned several times. At length, if in the neighbourhood of a suitable stone, the male would dig a hole, without for a moment entirely quitting its hold of the female, and presently both would disappear into the newly-formed retreat.
After mating, the male was often devoured by the female. Moreover, after any combat with an enemy, such as a Lycosa or a Scolopendra, it appeared to be de rigueur to eat the vanquished, and on such occasions only was any considerable amount of food consumed.
The scorpions were not, however, anxious to fight, greatly preferring to retire if possible; but when incited to combat, their sting was quickly fatal to any mature insect, to spiders and to centipedes. Curiously enough, however, insect larvae, though badly wounded, did not succumb to the poison. Newly-hatched scorpions mounted on the mother’s back, where they remained motionless for a week, entirely unfed. They then underwent a moult, after which they were able to forage for themselves.
External Structure.
The chitinous plates of the prosoma are fused to form a carapace. Six segments are clearly indicated by the six pairs of appendages, but, though the development of Scorpio affords little direct evidence of the fact, there is reason to believe that there once existed a pre-cheliceral segment,[[241]] as has been clearly proved in the case of the spiders. An embryonic pregenital segment has also been recognised. The six prosomatic appendages are those proper to the Arachnida, being the chelicerae, pedipalpi, and four pairs of ambulatory legs. The mesosoma, which is broadly attached to the prosoma, comprises six segments, indicated ventrally by the genital operculum, the pectines, and the four pairs of pulmonary stigmata. The last of the broad abdominal segments, which tapers abruptly, belongs to the metasoma, which also comprises six segments, and is succeeded by the post-anal spine or sting.
Prosoma.—Near the middle of the carapace are two median eyes, and on its antero-lateral borders are usually to be found groups of smaller eyes, numbering from two to five. All the eyes are simple. There is a difference, however, in their development, the median eyes being diplostichous, or involving two layers of hypoderm, while the lateral eyes are monostichous, and pass through a stage strikingly like the permanent condition of the eyes of Limulus. The arrangement of various slight longitudinal ridges on the dorsal surface of the carapace is of systematic importance. On the ventral surface, just in front of the genital operculum, is a sternum, never large, and sometimes barely visible. Its shape and size constitute one of the principal family characteristics.
Fig. [169].—Buthus occitanus. A, Dorsal view; B, ventral view. (After Kraepelin.)
Mesosoma.—The dorsal plates or terga are distinct, and are connected by soft chitin with their corresponding sterna.
Beneath the second abdominal segments are borne the “pectines” or comb-like organs. In their structure four portions are distinguishable, an anterior lamella or shaft attaching them to the body, a middle lamella, the teeth, and the fulcra, a series of small chitinous pieces intercalated between the bases of the movable teeth.
Beneath the third, fourth, fifth, and sixth segments are the paired openings of the lung-sacs.
Metasoma.—The first segment is usually and the remainder are invariably enclosed in complete chitinous rings and show considerable variations in their comparative size and shape, and in the arrangement of the ridges and keels with which they are usually furnished. The post-anal segment is more or less globular at its base, constituting a “vesicle,” and terminates in a fine curved point, the “aculeus,” perforated for the passage of the delicate poison-duct. With the abdomen fully extended the point is directed downward, but in the attitude of attack or defence, when the “tail” is carried horizontally over the back, the sting points forward in the neighbourhood of the animal’s head.
Fig. [170].—A, Diagram of a Scorpion’s leg; 1, coxa; 2, trochanter; 3, femur; 4, patella; 5, tibia; 6, protarsus; 7, tarsus; p.s, pedal spur; t.s, tibial spur. B, Fourth tarsus of Palamnaeus swammerdami; l, lateral lobe. (After Pocock.)
Appendages.—The three-jointed chelicerae are powerful and chelate. The first joint is small, but the second is strongly developed and bears at its anterior end on the inner side a projection which forms the immovable finger of the chela. The third joint, or movable finger, is articulated on the outer side, and both fingers are armed with teeth whose arrangement is useful in distinguishing the species. The pedipalpi consist of six joints. The coxa is small and has an inwardly directed lamella which assists in feeding. The trochanter is also a small joint, bearing, normally at right angles to the longitudinal axis, the powerful humerus or femur. Then follows the brachium or tibia, again directed forward, and the last two joints form the chela or “hand,” the terminal joint or movable finger being on the outer side as in the chelicerae. In systematic determination special attention is given to the “hand.” In some forms the upper surface is uniformly rounded, while in others a “finger-keel” divides it into two flattish surfaces almost at right angles. The biting edges of the fingers are usually furnished with rows of minute teeth arranged characteristically in the different genera. The ambulatory legs are seven-jointed, though, unfortunately, authors are not agreed upon the nomenclature of the joints. Kraepelin[[242]] names them coxa, trochanter, femur, tibia, and three-jointed tarsus, and Simon[[243]] agrees with him. Pocock’s names[[244]] are coxa, trochanter, femur, patella, tibia, protarsus, and tarsus, and it is certainly convenient that each joint should have a separate name, but it must be borne in mind that the tibia of different authors is not always the same joint. Special attention must be directed to the three terminal joints, which furnish highly important characteristics. The tibia (in Pocock’s sense) is sometimes provided with a “tibial spur” at its lower distal extremity. From the soft arthrodial membrane between the protarsus and tarsus may proceed one or more dark-tipped claw-like spurs, the “pedal spurs.” The terminal joint (tarsus of Pocock) is variously furnished with hairs and teeth, and always ends in a pair of well-developed movable claws beneath which a much reduced and sometimes almost obsolete third claw is distinguishable. The tarsus generally projects in a “claw-lobe” over the base of the superior claws, and sometimes lateral lobes are present. The first and second coxae have triangular maxillary lobes directed towards the mouth. The third and fourth coxae are fused together on each side, and those on one side are separated from those on the other by the sternum. In other respects the four pairs of legs are usually similar.
Internal Anatomy.
The alimentary canal is a fairly uniform tube, nowhere greatly dilated. The very small mouth leads into a small suctorial chamber, and this is connected by a narrow oesophagus, which pierces the cerebral nerve-mass, with a slightly dilated portion which receives the ducts of the first pair of gastric glands, often called salivary glands. The succeeding portion in the prosoma receives four or five more pairs of ducts from the well-developed gastric glands. In the rapidly narrowing first metasomatic segment the intestine receives one or two pairs of Malpighian tubes, and thence proceeds to the anus, situated ventrally in the last segment.
The vascular system is of the usual Arachnid type, the heart being a seven-chambered dorsal longitudinal vessel lying in a pericardium, with which it communicates by seven pairs of valvular ostia. Lankester[[245]] has demonstrated several pairs of superficial lateral veins connecting two deep-seated ventral venous trunks with the pericardium. The lung-books are, so to speak, pushed in to dilatations of these trunks, so that some of the lateral veins carry blood newly aerated by the lung-books directly to the pericardium.
The nervous system is not greatly concentrated except in the prosoma, where there is a single ganglionic mass which innervates not only the whole prosoma but the mesosoma as far as the first and sometimes the second pair of lung-books. There are two mesosomatic ganglia, variously situated in different genera, and each metasomatic segment has its ganglion.
The generative organs are more or less embedded in the gastric glands. There are two testes, each composed of a pair of intercommunicating tubules, and connected by a common vas deferens with the generative aperture, which is furnished with a double protrusible intromittent organ. A pair of vesiculae seminales and a pair of accessory glands are also present. The female possesses a single ovary, consisting of a median and two lateral tubules, all connected by cross branches.
In addition to the external sclerites a free internal skeletal plate, situated in the prosoma between the alimentary canal and the nerve-cord, furnishes convenient fulcra for muscular attachment. It is known as the “endosternite.”
Brauer[[246]] has made the most complete study of the development of Scorpio, and two of the most interesting of his conclusions may be mentioned here. He has shown the lung-books to be derived from gills borne on mesosomatic appendages. Moreover he found in the embryo five pairs of segmental ducts—in segments 3–6 and 8—and demonstrated that those of segment 5 persisted, though without external aperture, as coxal glands, and those of segment 8 as the genital ducts.
Classification.
More than 350 species of scorpions have been described, but many of these are “doubtful,” and probably the number of known forms may be put at about 300. These are divided by Kraepelin[[247]] into six families and fifty-six genera. The best indications of the family of a scorpion are to be found in the shape of the sternum, the armature of the tarsi, and the number of the lateral eyes, while assistance is also to be derived from the shape of the stigmata and of the pectines, and from the absence or presence of a spine beneath the aculeus.
The six families are: Buthidae, Scorpionidae, Chaerilidae, Chactidae, Vejovidae, and Bothriuridae.
Fam. 1. Buthidae.—Sternum small and generally triangular. Tibial spurs in the third and fourth legs. Generally a spur beneath the aculeus. Lateral eyes three to five in number.
There are two sub-families: Buthinae and Centrurinae.
The Buthinae, which possess a tibial spur, comprise fourteen genera, most of them Old World forms. The principal genera are Buthus, which contains about 25 species, and Archisometrus with 20 species. One genus only, Ananteris, is South American, and it includes only a single species. The genus Uroplectes, with 16 species, is almost entirely African.
The Centrurinae, without tibial spur, are New World scorpions, though Isometrus europaeus (maculatus) is cosmopolitan. The principal genera are Tityus with 30 species, Centrurus with 13, and Isometrus with 6.
Fam. 2. Scorpionidae.—Sternum broad and pentagonal, with sides approximately parallel. No tibial spur, but a single pedal spur. Generally three lateral eyes.
Nearly a hundred species of Scorpionidae have been described, distributed among fifteen genera. The following sub-families are recognised: Diplocentrinae, Urodacinae, Scorpioninae, Hemiscorpioninae, and Ischnurinae.
The Diplocentrinae have a spur under the aculeus. They form a small group of only eight species. The principal genus, Diplocentrus, is entirely Neotropical, but Nebo has a single Old World representative in Syria.
The Urodacinae, with the single genus Urodacus, are Australian scorpions. As in the next sub-family, there are rounded lobes on the tarsi, but there is only a single keel on the “tail,” and the lateral eyes are two in number. Six good and three doubtful species are recognised.
The Scorpioninae are Asiatic and African forms, and are recognised by the tarsi having a large lobe on each side, by the convex upper surface of the “hand,” by the presence of two median keels on the “tail,” and by the possession of three lateral eyes. Palamnaeus (Heterometrus) has sixteen species in the Indian region. There are about thirty species of Opisthophthalmus, all natives of South Africa. Pandinus includes about ten species, but there are only two species of the type genus Scorpio, S. maurus and S. boehmei.
The sub-family Hemiscorpioninae was formed for the reception of the single Arabian species Hemiscorpion lepturus. Its most striking characteristic is the cylindrical vesicle of the tail in the male.
The Ischnurinae differ from the Scorpioninae chiefly in the absence of the tarsal lobes, the presence of a well-marked finger-keel, and the generally more depressed form of the body and hand. In the opinion of some authors they should be separated from the Scorpionidae as a distinct family, the Ischnuridae. There are more than twenty species, divided among six genera. The type genus Ischnurus has only the single species I. ochropus. There are eight species of Opisthacanthus, which has representatives in Africa and America.
Fam. 3. Chaerilidae.—Sternum pentagonal with median depression or “sulcus” rounded posteriorly. Two pedal spurs. Stigmata circular. Two lateral eyes with a yellow spot behind the second. Pectines very short.
This small family has the single genus Chaerilus with but seven species, natives of the Oriental region.
Fam. 4. Chactidae.—Two pedal spurs. Two lateral eyes (or, rarely, no eyes) but without yellow spot. Characteristic dentition on movable finger of “hand.”
There are three sub-families, Megacorminae, Euscorpiinae, and Chactinae.
The Megacorminae include but a single Mexican form, Megacormus granosus. There is a single toothed keel under the “tail,” and all the under surface is spiny. There is a row of long bristles under the tarsus.
In the Euscorpiinae the upper surface of the hand is divided into two surfaces almost at right angles by a strong finger-keel. This is a small group of about six species found in the Mediterranean region. The two genera are Euscorpius and Belisarius.
The Chactinae are without any marked keel on the hand. The scorpions of this sub-family are found in equatorial South America and the West Indies, where there are more than twenty species divided about equally between the four genera Chactas, Broteas, Broteochactas, and Teuthraustes.
Fam. 5. Vejovidae.—No tibial, but two pedal spurs. A single row of hairs or papillae under the tarsus. Sternum generally broader than long. Elongate stigmata, and three lateral eyes.
Seven of the eight genera of this family include only American forms, the principal genus being Vejovis, with about ten species. The genus Scorpiops, however, belongs to the Indian region and numbers more than ten species.[[248]]
Fam. 6. Bothriuridae.—Sternum much reduced and sometimes hardly visible, consisting of two slight, nearly transverse bars.
Of the seven genera of this family one, Cercophonius, is Australian. The other six genera include some dozen South American forms, Bothriurus having four species.
Order II. Pedipalpi.
Arachnids with non-chelate, two-jointed chelicerae, powerful pedipalpi, and four pairs of legs, of which only the last three are ambulatory, the first being used as tactile organs. The cephalothorax is usually covered by an undivided carapace, but the pedunculated abdomen is segmented. Respiration is by lung-books.
The Pedipalpi are a little-known group of animals of nocturnal habits. Though rarely seen they are widely distributed, being found in India, Arabia, the greater part of Africa, and Central and South America. They are of ancient origin, a fossil genus, Graeophonus, of the Tarantulidae (Phrynidae, see p. [312]), occurring in the Carboniferous strata in North America. They live under stones and bark, and in caves, where, when disturbed, they seek safety in crannies in the rock.
Little is known of their habits, but they are believed to feed chiefly upon insects. The female Tarantula carries the developing eggs, somewhat after the manner of the Chernetidea (see p. [434]), in a bag beneath the abdomen, the under surface of which becomes concave and dome-like during the period of gestation.[[249]]
Fig. [171].—Thelyphonus, diagrammatic ventral view; about natural size. c, Coxal joint of pedipalp; g, generative opening; p, pedipalp; sp, spiracles; st, sternal plates; 1, 2, 3, 4, ambulatory legs. (After Pickard-Cambridge.)
External Structure.—The external features which the members of this Order have in common are the segmented pediculate abdomen (9 to 12 segments), the two-jointed non-chelate chelicerae, the antenniform first pair of legs, and the presence of two pairs of lung-book stigmata beneath the abdomen. The constituent families differ so much in outward form that they must be dealt with separately.
The Thelyphonidae or “Whip Scorpions” (see p. [312]) have a long-oval carapace bearing well-developed eyes, two in front, and a group of three or five on either side some distance behind. The pedipalpi are chelate, and have their basal joints fused beneath the mouth, being thus incapable of any masticating motion.
The first legs are six-jointed, and have multi-articulate tarsi; the others are seven-jointed, and their tarsi, in some species at least, are tri-articulate. The abdomen consists of two portions, a wide nine-jointed pre-abdomen and a short narrow three-jointed post-abdomen, to which a filiform tail is articulated. Beneath the cephalothorax, between the coxae of the legs, is a distinct sternal plate in two portions (Fig. [171]). The first abdominal ventral plate is largely developed, and covers two segments. Behind it are the median genital opening and two pulmonary stigmata, while the other stigmata are behind the second ventral plate, which corresponds to the third abdominal segment. On the last abdominal segment there are often two or four light-coloured spots called “ommatoids,” and considered by some authors to be organs of sight. Laurie, however (vide infra), thinks it more probable that they are olfactory in function.
The Schizonotidae (see p. [312]) have a two-jointed carapace, and do not possess more than two eyes. There is a short unjointed tail-piece.
In the Tarantulidae (Phrynidae) the whole body is much flattened and extended laterally, the undivided carapace being reniform, and broader than long. The long non-chelate pedipalps have their basal joints free and movable, and there are several sternal plates. There are nine abdominal tergal plates, the last three diminishing rapidly in size, and the last plate covering a button-like terminal portion of the abdomen. The first abdominal ventral plate is largely developed, as in the Thelyphonidae, and the genital orifice and pulmonary stigmata are in the same situation as in that group. The Tarantulidae have glutinous glands in the first abdominal segment which are capable of spinning a few irregular threads.
In the whole group paired circular depressions are conspicuous dorsally on all the abdominal segments. These indicate the points of attachment of the dorso-ventral muscles.
Internal Structure.—The anatomy of the Pedipalpi has been very inadequately studied. Disconnected notes on various points of structure have been published by various morphologists, but no complete investigation has yet been made of the internal organs. This is largely due to the difficulty of obtaining material, and the bad state of preservation of the internal parts of such specimens as have been available for dissection.
The following points have been made out in the anatomy of Thelyphonus.[[250]]
The alimentary canal commences after the mouth with a pharynx which, though not dilated, is furnished with sucking muscles. It then narrows into an oesophagus which passes through the nerve-mass, and afterwards dilates to form the mid-gut, which immediately gives off two large lateral diverticula which extend backwards, each having five lobes. There are also two median diverticula which proceed from the ventral surface and pass through the endosternite. The abdominal portion of the canal is entirely concealed by the great “liver” mass which communicates with it by four paired ducts in the anterior part of the abdomen. Behind the fourth abdominal segment the gut is narrow till it expands in the seventh segment into an hour-glass-shaped stercoral pocket which, according to Laurie, is a portion of the mesenteron.
The excretory organs are the Malpighian tubes and the coxal glands. The former are generally described as entering the anterior portion of the stercoral pocket, but according to Laurie they pass along its ventral surface, attached to it by connective tissue, and really enter at the posterior end. The coxal glands are well developed, and lie beneath the endosternite, opening near the first coxae.
The nervous system is much concentrated and of the usual Arachnid type. The median abdominal nerve has a ganglion towards its extremity, supplying, according to Bernard,[[251]] the muscles which move the tail. The heart is extremely long, and varies little in width. It has nine pairs of ostia[[252]]—two in the thorax and seven in the abdomen. The generative glands are paired, and in the male there are large seminal vesicles. In the most ventral portion of the abdominal cavity lies a remarkable asymmetrically-situated gland, the “stink-gland.” It consists of a number of secretory tubules communicating with two elongated sacs, one of which lies beneath the nerve-cord, and therefore medially, while the other lies far to the left. Their ducts proceed to the anus or its vicinity.
The caudal organs, or white spots which, as already mentioned, are usually found on the last of the three post-abdominal segments of Thelyphonus, are of doubtful function. They have been variously explained as the stink-gland orifices, and as organs sensitive to light (“ommatoids”). Laurie[[253]] was unable to find any pore in this region, nor was there any of the pigment so characteristic of organs of sight. The histological structure indicated a sense-organ rather than a gland, but the use of these organs is entirely conjectural.
Classification.—The order Pedipalpi is divided into three families—Thelyphonidae, Schizonotidae and Tarantulidae. The first two are considered by some authors to form a sub-order, Uropygi, or tailed Pedipalpi, while the Tarantulidae constitute the remaining sub-order Amblypygi, the members of which are tailless.
Fam. 1. Thelyphonidae.[[254]]—This family comprises nine or more genera, differing chiefly in the position of the eyes, the structure of the genital operculum, the armature of the pedipalps, and the presence or absence of “ommatoids” in the anal segment.
The three following genera are among those most likely to be met with. Two ommatoids are present in each.
Thelyphonus has a spine on the second ventral plate, and a deep median impression on the male genital operculum, which is, however, absent from that of the female. There are about fifteen known species of this genus, inhabiting Southern Asia and the East Indies.
Typopeltis has ridges running forward from the lateral eyes. The middle third of the female operculum is raised and deeply impressed in the middle. This genus is represented in China and Japan. Mastigoproctus has a short and stout coxal apophysis of the pedipalp, without a tooth on its inner side. It is found in Mexico, Brazil, and the West Indies. Other genera are Thelyphonellus (Demerara), Labochirus (Ceylon), Hypoctonus (Burma), Mimoscorpius (Philippines), Uroproctus (Assam), Abalius (New Guinea), without ommatoids, and Tetrabalius (Borneo), with two pairs of ommatoids.
Fam. 2. Schizonotidae (= Tartaridae).—This family contains only two genera, Schizonotus (= Nyctalops, Pickard-Cambridge, nom. preocc. Aves) and Trithyreus[[255]] (= Tripeltis, Thorell, nom. preocc. Reptilia). They are very small, pale-coloured forms (about 5 mm. in length), found in Burma and Ceylon.
Fam. 3. Tarantulidae, better known as Phrynidae. Pocock has shown that Fabricius established the genus Tarantula from the species T. reniformis in 1793, while there is no earlier record of Olivier’s Phrynus, established for the same species, than Lamarck’s citation of it in 1801. The family is divided into three sub-families, Tarantulinae, Phrynichinae, and Charontinae.
(i.) The Tarantulinae are new-world forms, represented by three genera, Tarantula, Acanthophrynus (Phrynopsis), and Admetus (Heterophrynus), in Central and South America and the West Indies.
(ii.) The Phrynichinae belong to the Old World, being found in Africa, India, and Ceylon. Phrynichus, Titanodamon and Nanodamon are genera of this sub-family.
(iii.) The Charontinae are natives of South-East Asia and the Pacific Islands. There are five genera and eight species.
CHAPTER XIII
ARACHNIDA EMBOLOBRANCHIATA (CONTINUED)—ARANEAE—EXTERNAL STRUCTURE—INTERNAL STRUCTURE.
Order III. Araneae.
(ARANEIDA,[[256]] ARANEINA.)
Arachnida breathing by tracheae and “lung-books.” Cephalothorax and pedicellate abdomen, the latter usually soft, and only very rarely showing any traces of segmentation. Two-jointed non-chelate chelicerae, the distal joint bearing the orifice of a poison-gland. The tarsal joint of the male pedipalp develops a sexual organ. The abdomen is furnished with spinning mammillae.
The true Spiders can readily be distinguished from allied Arachnid groups, with which they are often popularly confounded, by the presence of a narrow constriction or “waist” between the cephalothorax and abdomen, and of a group of “spinnerets” or external spinning organs beneath the hind portion of the body. Thus the so-called “Harvest-spider” or “Harvestman” is clearly not a Spider, for there is no constriction of its body into two parts, nor does it possess any spinnerets. It belongs to the Phalangidea. The same considerations will exclude the “Red Spider” of popular nomenclature, which must be referred to the Acarina or Mites.
The Araneae, even as at present known, form a very extensive and widely-distributed order of animals. Compared with certain insect orders, they have received little attention from the collector, and the number of known forms is certain to be very largely increased. They form an extremely compact and natural group, for though, within the order, there is an infinite variety of detail, their uniformity in essential points of structure is remarkable, and they are sharply marked off from the neighbouring groups of Arachnida.
Fig. [172].—Epeira angulata. ♀.
It is perhaps unfortunate that the obtrusiveness of particularly unattractive specimens of the race has always caused spiders to be regarded with more or less aversion. This prejudice can hardly fail to be modified by a wider acquaintance with these animals. There are certainly few groups which present points of greater interest in respect to their adaptation to special modes of life and the ingenuity displayed in the construction of their nests and the ensnaring of their prey.
Spiders are wingless, yet they may often be observed travelling through the air. They are air-breathing, yet many are amphibious in their habits, and one species at least spends the greater part of its existence beneath the surface of the water. On land they may be found in all imaginable localities which admit of the existence of that insect life on which they depend for food.
External Structure.—The spider’s body consists of two portions, the cephalothorax and the abdomen.
Cephalothorax.—Looked at dorsally (Fig. [173]), the cephalothorax is generally seen to have a depression near the middle, the “median fovea,” and from this certain lines, the “radial striae,” radiate towards the sides. These depressions indicate the attachment of internal muscles.
The head region or “caput” lies in front of the foremost of the radial striae, and is often clearly marked off from the thorax, and different from it in elevation. It bears the eyes, which, in the great majority of spiders, are eight in number. Many, however, are six-eyed, while in rare cases the number is reduced to four (Tetrablemma, see p. [404]), or even to two (Nops, see p. [395]). The number, relative size, and particular arrangement of these eyes are of considerable systematic importance. Their disposition varies very greatly, but it is generally possible to regard them as forming two transverse rows, an anterior and a posterior, each possessing a pair of median and a pair of lateral eyes.
Fig. [173].—Diagrammatic dorsal view of a Spider. ch, Chelicera; f, median fovea; n, normal marking; o, ocular area; p, pedipalp; st, stria. (The dotted line should reach the radial marking on the cephalothorax.)
In many spiders all the eyes have a dorsal aspect, but in some groups (Attidae, Lycosidae) the prevailing arrangement is to have the anterior eyes directed forwards and the posterior upwards. In other spiders, again, a dorsal view may only show the eyes in profile, all having their axes directed forwards or sideways, or they may be mounted on turrets, and thus command a wide range of view. The rows are described as straight, “procurved” (with the convexity backwards), or “recurved” (with the convexity forwards). Thus, in Fig. [177], the anterior row is slightly, and the posterior row considerably “recurved.”
Sometimes there is a marked difference in the colour of the eyes, two or more being black, while the remainder are pearly white. In other cases they are homogeneous, either of the black or the white type. Simon considers the black eyes to be diurnal and the white nocturnal, but the evidence for this is indirect and not altogether satisfactory. The portion of the caput occupied by the eyes is often alluded to as the “ocular area.” The space between the ocular area and the chelicerae, well shown in Fig. [177], is known as the “clypeus.” It is usually more or less vertical, but in the Aviculariidae (see p. [386]) it is horizontal and dorsal.
The under surface of the cephalothorax is protected by the “sternum” or “plastron,” a large plate of variable shape, usually notched at either side for the reception of the legs, and having in front a small plate, generally hinged, but sometimes soldered to it, known as the “labium.” This has no homology with the labium of insects, but is a true sternite, more correctly described as “pars labialis sterni.”
The labium and the maxillary lobes of the palpi more or less conceal the under surface of the caput. The shape of the sternum and of the labium, and the contour and degree of inclination towards one another of the maxillae, are important considerations in the taxonomy of Spiders.
Fig. [174].—Diagrammatic ventral view of a Spider. Cephalothorax—l, Labium; m, maxilla; p, paturon of chelicera; st, sternum; u, unguis of chelicera. Abdomen—a.t, Anal tubercle; c, colulus; ep, epigyne; s, stigma; sp, spinnerets; tr, tracheal opening.
The appendages of the cephalothorax, which are the chelicerae or jaws, the pedipalpi or feelers, and the four pairs of ambulatory legs, will be treated separately.
Pedicle.—The chitinous investment of the narrow stalk which unites the thorax with the abdomen is for the most part thin and flexible, with only slight indurations of various patterns on the dorsal surface, where it is in most cases more or less protected by the forwardly-projecting abdomen. Beneath, it is usually quite membranous, guarded only by a sort of collar formed by the raised border of the anterior portion of the abdomen at the point of insertion. In some Spiders, however (Dysderidae), there is a posterior sternal plate, the “plagula,” closely corresponding with the labium in front, which partly embraces the pedicle. In Hermippus (Zodariidae) the plagula is detached from the sternum, and is succeeded posteriorly by two smaller paired plates.
Abdomen.—The abdomen differs remarkably in shape in the different groups of Spiders. In some families the prevailing shape is more or less globular, and in others cylindrical, while it may be diversified to almost any extent by prominences or spines. Ordinarily no sign of segmentation is observable, but in Liphistius it is covered dorsally by seven well-marked chitinous plates.
In most Spiders the integument of the abdomen is uniformly soft and flexible all over, but it is not rare to find portions of it thickened and hardened to form “scuta.” In the Gasteracanthinae and the Phoroncidinae there is a great dorsal scutum armed with spines, while in several families there are species characterised by the possession of a smooth dorsal scutum; and in some a ventral scutum is present.
Fig. [175].—Spider profiles. 1, Poltys ideae; 2, Phoroncidia 7–aculeata; 3, Ariamnes flagellum; 4, Stegosoma testudo; 5, Formicinoides brasiliana.
That these scuta are sometimes indicative of an obsolete segmentation would seem likely from the study of the remarkable species, Tetrablemma medioculatum (Fig. [176]), described by Pickard-Cambridge, from Ceylon. In addition to large dorsal and ventral scuta, the sides and posterior extremity are guarded by smaller scuta, the disposition of which is well seen in the figure.
Fig. [176].—Tetrablemma medioculatum, much enlarged. A, Posterior view; B, profile, showing the scuta. (After Cambridge.)
The normal smooth abdomen presents dorsally no very striking features. In species of variegated coloration there is very generally noticeable a median dentated band (Fig. [173]), the “normal marking” of some writers, which would appear to have some correlation with the underlying dorsal vessel. Beneath the abdomen are to be seen the orifices of the breathing and genital organs, the spinnerets, and the anal aperture upon its tubercle.
The breathing organs are, as will be explained later, of two kinds, lung-books and tracheae. The great majority of Spiders possess only two lung-books, and their transverse, slit-like openings (“stigmata” or “spiracles”) may be seen on either side of the anterior part of the abdomen. Where, as in the Theraphosae, there are four lung-books, the second pair open by similar slits a short distance behind the first. According to Bertkau, pulmonary sacs are entirely lacking in the genus Nops.
The tracheae generally debouch by a single median stigma towards the posterior end of the abdomen, just in front of the spinnerets. This opening clearly results from the fusion of two stigmata, which in some species retain their paired arrangement.
On a level with the openings of the anterior lung-books or pulmonary sacs there is usually observable a slight transverse ridge, the epigastric fold (Fig. [174]), and in the centre of this is the genital opening. This is never visible until after the last moult, and in the male is always a simple inconspicuous aperture. This is also the case with the females of some groups (Theraphosae, Filistatidae, Dysderidae, etc.), but in most cases there is a more or less complicated armature, the “epigyne,” the special design of which is of great specific value. In its simplest form it is merely a plate, usually of dark colour, with one or two apertures (Fig. [174], ep), but in some families, notably the Epeiridae, it is more complicated, and is furnished with a hooked median projection, the “ovipositor” (“clavus” of Menge), which is often absurdly like a petrified elephant’s trunk in miniature.
The abdomen also presents on its under surface, usually towards the posterior end or apex, a group of finger-like mammillae or spinnerets. They are normally six in number, two superior (or posterior), two median, and two inferior (or anterior). The number is reduced, in most of the Theraphosae, to four, while a few spiders possess only a single pair of spinnerets. These organs are described more fully on p. 325.
A small papilla, the “colulus” (Fig. [174], c), is often observable, projecting between the anterior spinnerets. The “anal tubercle” (Fig. [174], a.t), on which the vent is situated, terminates the abdomen, and is generally in close juxtaposition with the posterior spinnerets.
Appendages.—The cephalothoracic appendages are the chelicerae, the pedipalpi, and the four pairs of ambulatory legs. Those of the abdomen are the mammillae or spinnerets.
Chelicerae.—These are two-jointed appendages, articulated immediately below or in front of the clypeus. They are the “mandibles” of many authors, but there is good reason for believing that they are not homologous with the mandibles of Insects. There is little agreement, moreover, with regard to the names given to the two joints of which they consist. The term “falx,” often applied to the basal joint, is much more appropriate to the sickle-like distal joint. Base and fang are tolerably satisfactory, or we may avoid ambiguity by adopting the terms “paturon” and “unguis” suggested by Lyonnet.[[257]]
The paturon is a stout joint of more or less cylindrical or conical shape. The unguis (the “crochet” of Simon) is hook-like, and can generally be folded back upon the paturon, which often presents a groove for its reception. The Theraphosid spiders are distinguished from all others by the fact that the plane of action of the chelicerae is vertical and longitudinal. The paturon projects forward in a line parallel with the axis of the body, and its distal end can be raised or depressed, but not moved laterally; while the unguis in action has the point directed downwards, and, at rest, is applied to the under surface of the paturon.
In other spiders the patura hang more or less vertically, and while to some extent mobile in all directions, their principal motion is lateral, and the ungues have their points directed towards each other in action, and are applied to the inner surfaces of the patura in repose. The plane of action in this case is also more or less vertical, but transverse.
Fig. [177].—Front view of Textrix denticulata. × about 10. 1, Caput; 2, eyes; 3, paturon; and 4, unguis of chelicera.
The paturon is always extremely hard and strong. In Theraphosae of burrowing habits the distal end is furnished with a group of powerful teeth, the “rastellus.” The groove for the reception of the unguis is often guarded on one side or on both by rows of teeth, the arrangement of which is frequently an important specific character. The inner anterior border is also often furnished with a group of stiff hairs or bristles. This powerful joint is of use in crushing and expressing the fluids of insects pierced by the ungues.
The crescent-shaped unguis is tapering and smooth, except for the presence, on the posterior surface, of one or two feebly dentated ridges. Near its free extremity there is a small orifice leading to the poison reservoir and gland.
In the genus Pholcus (see p. [401]) the chelicerae may almost be regarded as chelate, the unguis being met by a spiny projection from the inner anterior border of the paturon.
Rostrum.—On examining a spider, even under a dissecting microscope, it will not be easy at first to discover the mouth. Indeed, Lyonnet had almost come to the conclusion that Spiders, like some Myrmelionid larvae, imbibed the juices of their prey by way of the mandibles, before he found the orifice and gave a remarkably accurate description of the adjacent parts.
If a specimen be placed on its back, and the labium raised while the chelicerae are pushed forward, no orifice is visible, but on careful examination it will be found that what appears to be a thick and fleshy labium is, in reality, two organs. The labium is thin and flat, and closely opposed to its upper surface is a somewhat flattened cone. This is the “rostrum,” and when it is separated from the labium the buccal orifice is disclosed. In a few spiders (Archeidae) in which the chelicerae are far removed from the mouth, the rostrum is tolerably conspicuous, but in most it is so hidden as to have escaped the observation of the great majority of observers. Schimkewitsch considers it homologous with the labrum of insects, but Simon thinks that it represents all the insect mouth-parts reduced to an exceedingly simple form. It is more probable that a beak consisting of a simple labrum and labium was a primitive Arachnid characteristic. If the rostrum be removed and its inner (or posterior) surface examined, a lance-shaped chitinous plate, the “palate,” becomes visible. It is furrowed down the middle by a narrow groove, which is converted into a tube for the passage of fluids when the rostrum is opposed to the labium.
Fig. [178].—Pedipalp of Tegenaria domestica ♂, × 5. 1, Coxa; 2, maxilla; 3, trochanter; 4, femur; 5, patella; 6, tibia; 7, tarsus; 8, palpal organ.
Pedipalpi.—The pedipalpi are extremely leg-like feelers, and are six-jointed, the metatarsal joint of the ambulatory legs being absent. The joints, therefore, are the coxa, trochanter, femur, patella, tibia, and tarsus (Fig. [178]).[[258]]
In the Theraphosae the coxa resembles that of the ambulatory leg, but in other spiders it is furnished, on the inner side, with a blade-like projection, the “maxilla” (Fig. [178]). The shape of the maxillae and the degree of their inclination towards the labium are of considerable taxonomic importance. The inner border of the maxilla is furnished with a tuft of hairs, which assist in retaining the juices expressed by the chelicerae, and its anterior border presents a cutting edge with a finely dentated ridge called the “serrula.”
In the female, and in the immature male, the remaining joints differ little from those of the legs, except that the tarsal joint is either clawless or has a single claw, which is generally smooth, and is never much dentated.
At the last moult but one the male pedipalp appears tumid at the end, and after the last moult the tarsus is seen to have developed a remarkable copulatory apparatus, the “palpal organ,” comparatively simple in some families, but in others presenting an extraordinary complexity of structure.
Fig. [179].—Diagram of palpal organ. 1, Tarsus; 2, bulb; 3, receptaculum seminis; 4, its aperture; 5, style; 6, haematodocha; 7, alveolus; 8, tibia.
Palpal Organs.—Externally the essential parts of the palpal organ are three, the “haematodocha,” the “bulb,” and the “style.” The spines and projections, or “apophyses,” which often accompany the palpal organ proper, are of secondary importance, and in many spiders are entirely absent; nor is their function when present at all clear; but the infinite variety of design which they exhibit, and their singular uniformity in all the males of a species, render them of the utmost value as specific characteristics.
The “haematodocha” is the portion of the palpal organ attached to the tarsus, and often received into an excavation, the “alveolus,” on its under surface. It is a fibro-elastic bag, in its normal collapsed state usually somewhat spirally disposed round the base of the following portion, the “bulb.”
The bulb is generally the most conspicuous portion of the organ, and is a sub-globular sac with firm, though often semi-transparent, integument. Its base rests upon the haematodocha, and its apex is produced, often spirally, to a point which bears the seminal orifice. This external opening leads into a coiled tube within the bulb, ending in a blind sac, the “receptaculum seminis,” which projects into the haematodocha; and it is the aperture by which the sperm both enters and leaves the organ. How the sperm is conveyed to the receptaculum was long a matter for speculation, after the belief in a direct communication between the generative glands and the pedipalpi had been abandoned. The process has been actually observed in the case of a few spiders, which have been seen to deposit their sperm on a small web woven for the purpose, and then, inserting the styles of their palpal organs into the fluid, to suck it up into the receptacula seminis. This is probably the usual method of procedure, though it may be true, as some have asserted, that the palp is sometimes applied directly to the genital orifice.
The receptaculum and its tube being thus charged with sperm, it is the function of the haematodocha to eject it by exerting pressure on its base. For this purpose the haematodocha is in communication with the cavity of the tarsus, from which, in copulation, it receives a great flow of blood, and becomes greatly distended. Bertkau believes that he has detected very minute pores (meatus sanguinis) communicating between the haematodocha and the receptaculum, and allowing some of the blood-plasma from the former to mingle with the semen, but this appears to be very doubtful.
The Legs are uniformly eight in number, and are seven-jointed, the joints, counting from the body, being the coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus.[[259]] In a few cases, through the presence of false articulations, i.e. rings of softer chitin, this number appears to be exceeded. Some of the Palpimanidae (see p. [398]) were at first thought to have only six joints on their anterior legs, but the tarsus is present, though very small.
In the case of most spiders, the legs take a general fore and aft direction, the first pair being directed forwards, the second forwards or laterally, and the third and fourth backwards. In the large group of “Crab-spiders” (Thomisidae), and in many of the Sparassinae, all the legs have a more or less lateral direction, and the spider moves with equal ease forwards, backwards, or sideways. The legs are usually more or less thickly clothed with hairs, but in some genera the clothing is so sparse that they appear glossy, while in others they have a positively shaggy appearance. Stouter hairs or “bristles” are often present, and some of the joints are also often furnished with “spines,” which in many cases are erectile.
The tarsi of all spiders are furnished with terminal claws, usually three in number, though in some families (Drassidae, Thomisidae, etc.) there are only two. The two principal claws are paired and usually dentated, though the number of their teeth may be unequal. The third claw, when present, is always smaller, median, and inferior.
Fig. [180].—Spider tarsi. 1, Tarsus of Epeira showing three claws and supplemental serrate hairs (a); 2, tarsus of a Thomisid Spider, with two claws; 3, 3a, lateral and dorsal view of tarsus of an Attid Spider, showing scopula at b.
In many spiders of climbing habits the place of the third claw is taken by a remarkable tuft of club-like hairs termed a “scopula” (Fig. [180], b), by means of which they are able to cling to smooth surfaces where claws would be able to obtain no hold. In some species there is a special false articulation—the “onychium”—at the end of the tarsus to bear the claws.
In the Cribellatae the metatarsus is always furnished with a comb-like organ, the “calamistrum,” correlated with an extra spinning apparatus, the “cribellum,” but this will be dealt with when we reach the systematic portion of the subject.
The general direction taken by the legs, the comparative length of the different joints, their armature of hairs, bristles, and spines, and the number and conformation of the tarsal claws, are points of great importance in the classification of Spiders.
Under considerable magnification the legs of all Spiders exhibit a number of minute organs, arranged with absolute uniformity throughout the Araneae, and known as the “lyriform organs.” They consist of little parallel ridges of thickened chitin, the slit between them being covered by thinner chitin. They are eleven on each leg, and are distributed near the distal extremities of each of the first six joints. Their function is unknown, though some authors consider them to be organs of hearing.
Fig. [181].—Spinnerets of Epeira diademata. A, Ventral view of Epeira; B, spinnerets magnified; C, profile.
The Spinnerets are normally six in number, and, except in rare instances, are placed beneath the abdomen, near its apex and immediately in front of the anal tubercle. Their arrangement varies greatly, but they can generally be recognised as comprising three pairs, a posterior (or superior) pair, a median pair, and an anterior (or inferior) pair.
In nearly all the Theraphosae the anterior pair are absent, while the posterior spinnerets are largely developed. In the Palpimanidae only the anterior spinnerets are present. When all six are found, the usual arrangement is in the form of a rosette, the median spinnerets being hidden by the others in repose, but this disposition is widely departed from. In Hahnia (Agelenidae), for instance, they are ranged in a transverse row at the end of the abdomen, the posterior spinnerets occupying the extremities of the row, and the median ones the centre.
These spinnerets are highly mobile appendages, and additional play is given to their action by the presence of articulations, much resembling the “false” joints sometimes found on the legs, on the posterior and anterior pairs. They are always at least bi-articulate, and sometimes present three or four joints. They are movable turrets on which are mounted the “fusulae” or projections where the tubes from the spinning glands open. These are often very numerous, especially in the orb-weaving spiders, where the spinning powers are most highly developed. They consist of two portions, a cylindrical or conical basal part, succeeded by a very fine, generally tapering tube.
In some spiders the fusulae are all much alike, but usually a few very much larger than the rest are noticeable under the microscope, and these are often alluded to as “spigots.” The smaller ones are also divisible into two kinds, a few short conical fusulae being noticeable amongst the much more numerous cylindrical tubes. We shall treat of the functions of the various fusulae later (see pp. [335] and 349).
Simon remarks that though the battery of fusulae is most complicated in those spiders which possess the greatest spinning powers, it is by no means among them that extremely long spinnerets are developed. The posterior spinnerets of some of the Hersiliidae are of great length, but these spiders spin very little except in forming their egg-cocoons.
Fig. [182].—A, Spinnerets of Amaurobius similis ♀. Much enlarged. a, Anus; cr, cribellum; i.s, inferior spinneret; m.s, median spinneret; s.s, superior spinneret. B, Part of the 4th leg of the same Spider, showing the calamistrum (ca) on the metatarsus.
In addition to the six spinnerets, and just in front of them, there is to be found in some spiders an extra spinning organ in the form of a double sieve-like plate, the “cribellum.” This is always correlated with a comb of curved bristles on the metatarsi of the fourth pair of legs, the “calamistrum.” Such importance is assigned to these organs by Simon, that the Araneae Veraeare divided by him according to whether they are present or absent, into Cribellatae and Ecribellatae. This is probably an exaggerated view of the importance of these organs, and the spiders possessing them certainly do not seem to form a natural group.
Stridulating Organs.—When Arthropod animals are capable of producing a sound, the result is nearly always obtained by “stridulation,” that is, by the friction of two rough surfaces against each other. The surfaces which are modified for this purpose form what is called a “stridulating organ.” Such organs have been found in three very distinct Spider families, the Theridiidae, the Sicariidae, and the Aviculariidae. Hitherto they have only been observed in three positions—either between the thorax and abdomen, or between the chelicerae and the pedipalpi, or between the pedipalpi and the first legs.
In the Sicariidae and the Aviculariidae, the sounds have been distinctly heard and described. Those produced by the Theridiidae would appear to be inaudible to human ears.
Fig. [183].—Stridulating apparatus of Steatoda bipunctata, ♂. Much enlarged. A, Ridged and toothed abdominal socket; B, striated area on the cephalothorax; C, profile of the Spider, × 5.
Westring[[260]] was the first to discover (1843) a stridulating organ in the small Theridiid spider Asagena phalerata. The abdomen, where the pedicle enters it, gives off a chitinous collar, which projects over the cephalothorax, and has the inner surface of the dorsal part finely toothed. When the abdomen is raised and depressed, these teeth scrape against a number of fine striae on the back of the posterior part of the cephalothorax. A similar organ has been since found in various allied spiders, of which the commonest English species is Steatoda bipunctata. In this group it is generally possessed by the male alone, being merely rudimentary, if present at all, in the female.
In 1880 Campbell[[261]] observed that in some of the Theridiid Spiders of the genus Lephthyphantes, the outer surface of the chelicera and the inner surface of the femur of the pedipalp were finely striated at the point, where they were rubbed together when the palps were agitated, but though the appropriate motion was frequently given, he could hear no sound.
Fig. [184].—Chilobrachys stridulans in stridulating attitude. After Wood-Mason. Natural size.
Meanwhile the noise produced by a large Theraphosid spider in Assam (Chilobrachys stridulans) had attracted attention, and its stridulating apparatus was described in 1875 by Wood-Mason.[[262]] The sound resembled that obtained by “drawing the back of a knife along the edge of a strong comb.”
Subsequently certain Sicariid spiders of a genus confined to the southern hemisphere were heard to produce a sound like the buzzing of a bee by the agitation of their palps, and both sexes were found to possess a very perfect stridulating organ, consisting of a row of short teeth on the femur of the pedipalp, and a striated area on the paturon of the chelicera.
Pocock has recently discovered that all the large kinds of Theraphosidae in the countries between India and New Zealand are, like Chilobrachys, provided with a stridulating organ. In these spiders also it is between the palp and the chelicera, and consists of a row of teeth or spines constituting a “pecten,” and a series of vibratile spines or “lyra,” but whereas in Chilobrachys and its near relations the lyra is on the palp and the pecten on the paturon, in other spiders the positions are reversed. The lyra is a very remarkable organ, consisting of club-shaped, often feathery bristles or spines, which lie parallel to the surface to which they are attached, and which is slightly excavated for their reception.
Lastly, many African Theraphosids possess a similar organ, not between the palp and the chelicera, but between the palp and the first leg.
Various suggestions have been hazarded as to the use of these organs, but they partake largely of the nature of conjecture, especially in connexion with the doubt as to the possession of a true auditory organ by the Araneae. They may be summarised as follows. The Theridiid spiders are among those which show most indication of auditory powers, and the stridulating organs, being practically confined to the male, may have a sexual significance. Chilobrachys stridulates when attacked, assuming at the same time a “terrifying attitude,” and its stridulating organ may serve the purpose attributed to the rattle of the rattlesnake, and warn its enemies that it is best let alone. If this be the case, there is no need that it should itself hear the sound, and, indeed, there is no evidence that the Aviculariidae possess the power of hearing. In the inoffensive stridulating Sicariid spiders the sounds could hardly serve this purpose, and the presence of the organ in both sexes, and in immature examples, precludes the idea that its function is to utter a sexual call. Instead of trying to escape when disturbed, the spider starts stridulating, and Pocock suggests that the similarity of the sound produced to the buzzing of a bee may be calculated to induce its enemies to leave it in peace.
Internal Anatomy.
Alimentary System.—The alimentary canal of the Spider is divided into three regions, the “stomodaeum,” the mid-gut or “mesenteron,” and the hind-gut or “proctodaeum.”
The Stomodaeum consists of the pharynx, the oesophagus, and the sucking stomach. As we have said, the mouth is to be found between the rostrum and the labium. It opens into the pharynx, the anterior wall of which is formed by a chitinous plate on the inner surface of the rostrum, sometimes called the palate. As the inner surfaces of the rostrum and labium are practically flat, the cavity of the pharynx would be obliterated when they are pressed together, were it not for a groove running down the centre of the palate, which the apposed labium converts into a tube, up which the fluids of the prey are sucked. In the Theraphosidae there is a corresponding groove on the inner surface of the labium.
At the top of the pharynx, which is nearly perpendicular, the canal continues backwards and upwards as a narrow tube, the oesophagus, passing right through the nerve-mass, which embraces it closely on all sides, to the sucking stomach. At the commencement of the oesophagus is the opening of a gland, probably salivary, which is situated in the rostrum.
Fig. [185].—Diagram showing the anatomy of the cephalothorax of a Spider. The right alimentary diverticulum has been removed. a, Aorta; c, left diverticulum with secondary caeca; e, endosternite; oes, oesophagus, descending to the mouth; s, sucking stomach; sh, dorsal shield of sucking stomach.
We now reach the sucking stomach, which occupies the centre of the cephalothorax. It is placed directly over a skeletal plate, the “endosternite” (Fig. [185], e), to which its lower surface is connected by powerful muscles, while its upper wall is protected by a hard plate or “buckler,” which is similarly attached to the roof of the cephalothorax in the region of the “fovea media.” The walls of the stomach are not themselves muscular, but by the contraction of the muscles above mentioned its cavity is enlarged, and fluids from the pharynx are pumped up into it.
The canal thus far is lined by chitin, like the exterior of the body, and forms a sort of complicated mouth-apparatus.
The Mesenteron lies partly in the cephalothorax and partly in the abdomen. The thoracic portion, shortly behind the sucking stomach, sends forward on either side a large branch or “diverticulum,” from each of which five secondary branches or “caeca” are given off (Fig. [185]). Of these the anterior pair sometimes join, thus forming a complete ring; but usually, though adjacent, they remain distinct. The other four pairs of caeca curve downwards, protruding into the coxae of the legs, where they often terminate, but sometimes (Epeira) they continue their curve until they meet, though they never fuse, under the nerve-mass. Behind the origin of the diverticula the mesenteron continues as a widish tube, and shortly passes through the pedicle and enters the abdomen, where, curving slightly upwards, it proceeds along the middle line till it ends in the proctodaeum.
In the abdomen it is surrounded by a large gland, the so-called liver, and is dilated at one spot (Fig. [186]) to receive the ducts from this gland. The fluid elaborated by this large abdominal gland has been shown to have more affinity with pancreatic juice than with bile.
The Proctodaeum consists of a short rectum, from the dorsal side of which protrudes a large sac, the “stercoral pocket.” At its origin, the rectum receives the openings of two lateral tubes which reach it after ramifying in the substance of the liver. These have been called “Malpighian tubules,” but their function is unknown. Loman[[263]] has shown that they open into the mid-gut and not into the rectum, and there is reason to believe that true Malpighian tubules homologous to those of Insecta are absent in Arachnida, where their place seems to be taken by the coxal glands, which are considered to be the true excretory organs. In most spiders they open near the third coxae. Like the stomodaeum, the proctodaeum has a chitinous lining.
Vascular System.—The earlier investigations on the circulation of the blood in Spiders were made by direct observations of the movements of the blood corpuscles through the more or less transparent integuments of the newly-hatched young. Claparède’s[[264]] results were arrived at by this method. It is invaluable for demonstrating roughly the course taken by the blood, but in these immature spiders the blood-system has not attained its full complexity, and other methods of research have shown the spider to possess a much more elaborate vascular system than was at first suspected.
The tubular heart lies along the middle line in the anterior two-thirds of the abdomen, sometimes close up against the dorsal wall, but occasionally at some little distance from it, buried in the substance of the liver. It is a muscular tube with three pairs of lateral openings or “ostia,” each furnished with a simple valve which allows the entrance, but prevents the exit, of the blood. It is contained in a bag, the “pericardium,” into which the ostia open. Both heart and pericardium are kept in place by a complicated system of connective tissue strands, by which they are anchored to the dorsal wall of the abdomen. Eight arteries leave the heart, the principal one, or “aorta,” plunging downward and passing through the pedicle to supply the cephalothorax. Besides this, there is a caudal artery at the posterior end, and three pairs of abdominal arteries, which proceed from the under surface of the heart, and the ramifications of which supply, in a very complete manner, the various organs of the abdomen. The heart is not divided up into compartments. The anterior aorta passes through the pedicle, above the intestine, and presently forks into two main branches, which run along either side of the sucking stomach, near the front of which they bend suddenly downwards and end in a “patte d’oie,” as Causard[[265]] expresses it—a bundle of arteries which proceed to the limbs (Fig. [185]). Where the downward curve begins, a considerable artery, the mandibulo-cephalic, runs forward to supply the chelicerae and the head region. We have omitted certain minor branches from the main trunks which supply the thoracic muscles. The nerve-mass receives fine vessels from the “patte d’oie.”
Fig. [186].—Diagram of a Spider, Epeira diademata, showing the arrangement of the internal organs, × about 8. 1, Mouth; 2, sucking stomach; 3, ducts of liver; 4, so-called Malpighian tubules; 5, stercoral pocket; 6, anus; 7, dorsal muscle of sucking stomach; 8, caecal prolongation of stomach; 9, cerebral ganglion giving off nerves to eyes; 10, sub-oesophageal ganglionic mass; 11, heart with three lateral openings or ostia; 12, lung-sac; 13, ovary; 14, acinate and pyriform silk-glands; 15, tubuliform silk-gland; 16, ampulliform silk-gland; 17, aggregate or dendriform silk-glands; 18, spinnerets or mammillae; 19, distal joint of chelicera; 20, poison-gland; 21, eye; 22, pericardium; 23, vessel bringing blood from lung-sac to pericardium; 24, artery.
There are no capillaries, but the blood is delivered into the tissues and finds its way, by irregular spaces or “lacunae,” into certain main venous channels or “sinuses.” There are three such in the cephalothorax, one median and the others lateral, considerably dilated in front, in the region of the eyes, and connected by transverse passages. By these the blood is brought back through the pedicle to the lung-books. In the abdomen also there are three main sinuses, two parallel to one another near the lower surface, and one beneath the pericardium. These likewise bring the blood to the lung-books, whence it is conducted finally by pulmonary veins (Fig. [186]) back to the pericardial chamber, and thus, by the ostia, to the heart.
The Spider’s blood is colourless, and the majority of the corpuscles are “amoeboid,” or capable of changing their shape.
Generative System.—The internal generative organs present no great complexity, consisting, in the male, of a pair of testes lying beneath the liver, and connected by convoluted tubes, the “vasa deferentia,” with a simple aperture under the abdomen, between the anterior stigmata.
The ovaries are hollow sacs with short oviducts which presently dilate to form chambers called “spermathecae,” which open to the exterior by distinct ducts, thus forming a double orifice, fortified by an external structure already alluded to as the “epigyne.” The eggs project from the outer surface of the ovary like beads, connected with the gland by narrow stalks, and it was not at first clear how they found their way into the interior cavity, but it has been ascertained that, when ripe, they pass through these stalks, the empty capsules never presenting any external rupture.
The palpal organs have already been described. The spermatozoa, when received by them, are not perfectly elaborated, but are contained in little globular packets known as “spermatophores.”
Nervous System.—The Spider’s central nervous system is entirely concentrated in the cephalothorax, near its floor, and presents the appearance of a single mass, penetrated by the oesophagus. It may, however, be divided into a pre-oesophageal portion or brain, and a post-oesophageal or thoracic portion.
The brain supplies nerves to the eyes and chelicerae, while from the thoracic mass nerves proceed to the other appendages, and through the pedicle to the abdomen. The walls of the oesophagus are closely invested on all sides by the nerve-sheath or neurilemma.
Sense-Organs.—Spiders possess the senses of sight, smell, and touch. Whether or not they have a true auditory sense is still a matter of doubt. Since sounds are conveyed by vibrations of the air, it is never very easy to determine whether responses to sounds produced near the animal experimented upon are proofs of the existence of an auditory organ, or whether they are only perceived through the ordinary channels of touch. In any case, the organs of hearing and of smell have not yet been located in the Spider. M‘Cook considers various hairs scattered over the body of the spider to be olfactory, but from Gaskell’s researches upon allied Arachnid groups it would seem that the true smelling organ is to be sought for in the rostrum.
Eyes.—Spiders possess from two to eight simple eyes, the external appearance and arrangement of which have already been briefly explained. They are sessile and immovable, though often so placed as to command a view in several directions. In structure they are essentially like the ocelli of Insects. Externally there is a lens, succeeded by a mass of transparent cells, behind which is a layer of pigment. Then come the rods and cones of the retina, to which the optic nerve is distributed. A comparison of this with the arrangement in the Vertebrate eye will show a reversal of the positions of the retina and the pigment-layer. The lens is part of the outside covering of the animal, and is cast at the time of moulting, when the spider is temporarily blind. It is stated, however, that the eyes do not all moult simultaneously. There is often a considerable difference between the various eyes of the same spider, especially with regard to the convexity of the lens and the number of rods and cones.
Though most spiders possess eight eyes, the number is sometimes smaller, and in some groups of eight-eyed spiders two of the eyes are sometimes so reduced and degenerate as to be practically rudimentary. As might be expected, Cave-spiders (e.g. Anthrobia mammouthia) may be entirely sightless.
Touch.—The sense of touch would appear to be extremely well developed in some spiders, and there is reason for believing that the Orb-weavers, at all events, depend far more upon it than upon that of sight.
Among the hairs which are distributed over the spider’s body and limbs, several different forms may be distinguished, and some of them are undoubtedly very delicate sense-organs of probably tactile function.
Spinning Glands.—Spiders vary greatly in their spinning powers. Some only use their silk for spinning a cocoon to protect their eggs, while others employ it to make snares and retreats, to bind up their prey, and to anchor themselves to spots to which they may wish to return, and whence they “drag at each remove a lengthening chain.”
All these functions are performed by the silk-glands of the Orb-weavers, and hence it is with them that the organs have attained their greatest perfection. We may conveniently take the case of the common large Garden-spider, Epeira diademata. The glands occupy the entire floor of the abdomen. They have been very thoroughly investigated by Apstein,[[266]] and may be divided into five kinds.
Fig. [187].—Spinning glands. A, Aciniform; B, tubuliform; C, piriform gland.
On either side of the abdomen there are two large “ampullaceal” glands debouching on “spigots,” one on the anterior, and one on the middle spinneret; there are three large “aggregate” glands which all terminate on spigots on the posterior spinneret; and three “tubuliform” glands, two of which have their orifices on the posterior, and one on the middle spinneret. Thus, in the entire abdomen there are sixteen large glands, terminating in the large fusulae known as spigots. In addition to this there are about 200 “piriform” glands whose openings are on the short conical fusulae of the posterior and anterior spinnerets, and about 400 “aciniform” glands which debouch, by cylindrical fusulae, on the middle and posterior spinnerets. Thus there are, in all, about 600 glands with their separate fusulae in the case of Epeira diademata.
The great number of orifices from which silk may be emitted has given rise to the widespread belief that, fine as the Spider’s line is, it is woven of hundreds of strands. This is an entire misconception, as we shall have occasion to show when we deal with the various spinning operations.
A few families are, as has already been stated, characterised by the possession of an extra spinning organ, the cribellum, and the orifices on this sieve-like plate lead to a large number of small glands, the “cribellum glands.”
Respiratory Organs.—Spiders possess two kinds of breathing organs, very different in form, though essentially much alike. They are called respectively “lung-books” and “tracheae.” The Theraphosae (and Hypochilus) have four lung-books, while all other spiders, except Nops, have two. Tracheae appear to be present almost universally, but they have not been found in the Pholcidae.
The pulmonary stigmata lead into chambers which extend forwards, and which are practically filled with horizontal shelves, so to speak, attached at the front and sides, but having their posterior edges free. These shelves are the leaves of the lung-book. Each leaf is hollow, and its cavity is continuous, anteriorly and laterally, with the blood-sinus into which the blood from the various parts of the Spider’s body is poured.
The minute structure of the leaf is curious. Its under surface is covered with smooth chitin, but from its upper surface rise vast numbers of minute chitinous points whose summits are connected to form a kind of trellis-work. The roof and floor of the flattened chamber within are connected at intervals by columns. The pulmonary chamber usually contains from fifteen to twenty of these leaves, and the two chambers are always connected internally between the stigmata.
The tracheae are either two or four (Dysderidae, Oonopidae, Filistatidae) in number, and their stigmata may be separate or fused in the middle line. Each consists of a large trunk, projecting forwards, and giving off tufts of small tubes which lose themselves among the organs of the abdomen, but do not ramify. In the tracheae of Argyroneta[[267]] a lateral tuft is given off immediately after leaving the stigma, and another tuft proceeds from the anterior end. Histologically the main trunk of the trachea is precisely like the general chamber of the pulmonary sac, and differs greatly from the trachea of an insect.
Cephalothoracic Glands.—In addition to the generative glands and the so-called “liver” which occupy so large a portion of the abdomen, there are, in Spiders, certain glandular organs situated in the cephalothorax which call for some notice. These are the coxal glands and the poison-glands.
The COXAL GLANDS are two elongated brownish-yellow bodies, situated beneath the lateral diverticula of the stomach, and between it and the endosternite. They present four slight protuberances which project a short distance into the coxae of the legs. The glands appear to be ductless, but their function is thought to be excretory. They were first observed in the Theraphosae.
All Spiders possess a pair of POISON-GLANDS, connected by a narrow duct with a small opening near the extremity of the fang of the chelicerae. The glands are sac-like bodies, usually situated in the cephalothorax, but sometimes partially (Clubiona) or even entirely (Mygale) in the patura, or basal joints of the chelicerae. Each sac has a thin outer layer of spirally-arranged muscular and connective tissue fibres, and a deep inner epithelial layer of glandular cells. The cavity of the gland acts as a reservoir for the fluid it secretes. The virulence of the poison secreted by these glands has been the subject of much discussion, and the most diverse opinions have been held with regard to it. The matter is again referred to on p. 360.
CHAPTER XIV
ARACHNIDA EMBOLOBRANCHIATA (CONTINUED)—ARANEAE (CONTINUED)
HABITS—ECDYSIS—TREATMENT OF YOUNG—MIGRATION—WEBS—NESTS—EGG-COCOONS—POISON—FERTILITY—ENEMIES—PROTECTIVE COLORATION—MIMICRY—SENSES—INTELLIGENCE—MATING HABITS—FOSSIL SPIDERS
EARLY LIFE OF SPIDERS.
Ecdysis or Moulting.—Spiders undergo no metamorphosis—that is to say, no marked change of form takes place, as is so often the case among Insects, in the period subsequent to the hatching of the egg. This fact, by the by, is a great trouble to collectors, as it is generally extremely difficult, and sometimes quite impossible, to identify immature specimens with certainty.
But although unmistakably a spider as soon as it leaves the egg, the animal is, at first, in many respects incomplete, and it is only after a series of moults, usually about nine in number, that it attains its full perfection of form.
Until the occurrence of its first moult it is incapable of feeding or spinning, mouth and spinning tubes being clogged by the membrane it then throws off. It is at first pale-coloured and less thickly clothed with hairs and spines than it eventually becomes, and the general proportions of the body and the arrangement of the eyes are by no means those of the adult in miniature, but will be greatly modified by unequal growth in various directions. It speedily, however, attains its characteristic shape and markings, and after one or two ecdyses little alteration is to be noticed, except increase in size, until the final moult, when the spider at length becomes sexually mature.
The first moult takes place while the newly-hatched spider is still with the rest of the brood either in or close to the “cocoon” or egg-bag. M‘Cook[[268]] thus describes the conclusion of the operation in the case of Agelena naevia:—
“While it held on to the flossy nest with the two front and third pairs of legs, the hind pair was drawn up and forward, and the feet grasped the upper margin of the sac-like shell, which, when first seen, was about half-way removed from the abdomen. The feet pushed downwards, and at the same time the abdomen appeared to be pulled upward until the white pouch was gradually worked off.”
The later moults are generally accomplished by the spider collecting all its legs together and attaching them with silk to the web above, while the body, also attached, hangs below. The old skin then splits along the sides of the body, and the animal, by a series of violent efforts, wriggles itself free, leaving a complete cast of itself, including the legs, suspended above it. For a day or two before the operation the spider eats nothing, and immediately upon its completion it hangs in a limp and helpless condition for a quarter of an hour or so, until the new integument has had time to harden. It is not unlikely that the reader has mistaken these casts for the shrivelled forms of unlucky spiders, and has had his sympathies aroused, or has experienced a grim satisfaction, in consequence—an expenditure of emotion which this account may enable him to economise in future.
Limbs which the animal has accidentally lost are renewed at the time of moulting, though their substitutes are at first smaller than those they replace. On the other hand, the struggle to get rid of the old skin sometimes results in the loss of a limb, and the spider is doomed to remain short-handed until the next ecdysis.
Until the last moult the generative apertures, which are situated under the anterior part of the abdomen, are completely sealed up. Their disclosure is accompanied, in the case of the male, by a remarkable development of the last joint of each pedipalp, which becomes swollen and often extremely complicated with bulbs, spines, and bristles. A mature male spider may at once be distinguished by the consequent knobbed appearance of its palps; and the particular form they assume is highly characteristic of the species to which the spider belongs.
The number of moults, and the intervals at which they occur, no doubt vary with different species. In the case of Argiope aurelia, Pollock[[269]] has found that the female moults nine times after leaving the cocoon, the first ecdysis occurring after an interval of from one to two months, according to the abundance or scarcity of food. The subsequent intervals gradually increase from about a fortnight to something over three weeks.
Behaviour of the Newly-hatched Spider.—The mode of life of a spider just freed from the cocoon will of course vary greatly according to the Family to which it belongs.
The Epeiridae are the builders of the familiar wheel or orb-web. Spiders of this Family usually remain together on friendly terms for a week or more after leaving the nest. Most of the time they are congregated in a ball-like mass, perhaps for the sake of warmth, but upon being touched or shaken they immediately disperse along the multitudinous fine lines which they have spun in all directions, to reassemble as soon as the panic has subsided. Such a ball of the yellow and black offspring of the large Garden-spider, Epeira diademata, is no uncommon sight in the early autumn, and the shower of “golden rain” that results from their disturbance is not likely to be forgotten if it has ever been observed by the reader. This harmonious family life only continues as long as the young spiders are unable to feed—a period which, in some of the larger species, is said to extend to ten days or a fortnight.
Individual life then commences, and each member of the dispersed group sets up housekeeping on its own account, constructing at the first attempt a snare in all respects similar, except in size, to those of its parent.
Of course the young Spiders have not migrated far, and a bush may frequently be seen covered by the often contiguous nets of the members of a single brood. This, as Dr. M‘Cook thinks, is the true explanation of some of the cases of “gregarious spiders” which Darwin[[270]] and other naturalists have occasionally described, though social spiders exist (see Uloborus, p. 411).
Very similar habits obtain among the Theridiidae, or line-weaving spiders, a familiar example of which is the pretty little Theridion sisyphium, whose highly-irregular snare may be found on any holly bush during the summer months.
Fig. [188].—A, Pardosa sp. ♀, with young on the abdomen; B, young Pardosa detached; C, outline of the Spider with young removed. (From the living specimen.)
The Lycosidae, or Wolf-spiders, which chase their prey instead of lying quietly in ambush to ensnare it, are exceedingly interesting in their treatment of their young. The cocoon, or bag of eggs, is carried about on all their expeditions, attached beneath the abdomen, or held by the jaws, and the young spiders, on escaping from it, mount on the mother’s back, and indulge vicariously in the pleasures of the chase from this point of vantage. The empty egg-bag is soon discarded, but the brood continues to ride on the mother’s back for about a week, dismounting only to follow her as she enters her little silk-lined retreat in the ground.
During this time they appear to require no food, but they at length begin to disperse, the mother gently but firmly removing such individuals as are disposed to trespass upon her maternal solicitude longer than she considers desirable.
Many young spiders of various Families proceed immediately to seek new hunting-grounds by the aid of the wind, and become for the time being diminutive aeronauts. This habit was observed by the earliest British araneologist, Martin Lister,[[271]] as long ago as 1670, and has been alluded to by many writers since his time.
The topmost bar of an iron railing in spring or early autumn will generally be found peopled with minute spiders, and if the day be fair and the wind light, the patient observer may be rewarded by a curious and interesting sight.
The spider seeks the highest spot available, faces the wind, and straightens its legs and body, standing, so to speak, upon its toes, its abdomen with its spinning tubes being elevated as much as possible. Streamers of silk presently appear from the spinnerets and float gently to leeward on the light current of air. The spider has no power to shoot out a thread of silk to a distance, but it accomplishes the same result indirectly by spinning a little sheet or flocculent mass which is borne away by the breeze.
Fig. [189].—Young Spider preparing for an aerial voyage. (After Emerton.)
When the streaming threads pull with sufficient force the animal casts off, seizes them with its legs, and entrusts itself to the air, whose currents determine the height to which it is carried and the direction of its journey. The duration, however, is not quite beyond the spider’s control, at all events in calm weather, for it can furl its sail at will, hauling in the threads “hand-over-hand,” and rolling them up into a ball with jaws and palps.
This curious ballooning habit of young Spiders is independent of the particular family to which they belong, and it is remarkable that newly-hatched Lycosidae and Aviculariidae, whose adult existence is spent entirely on or under the ground, should manifest a disposition to climb any elevated object which is at hand.
The “Gossamer,” which so puzzled our forefathers, is probably no mystery to the reader. It is, of course, entirely the product of Spider industry, though not altogether attributable to the habit of ballooning above described. Only a small proportion of gossamer flakes are found to contain spiders, though minute insects are constantly to be seen entangled in them. They are not formed in the air, as was supposed long after their true origin was known, but the threads emitted by multitudes of spiders in their various spinning operations have been intermingled and carried away by light currents of air, and on a still, warm day in spring or autumn, when the newly-hatched spider-broods swarm, the atmosphere is often full of them.
They rise to great heights, and may be carried to immense distances. Martin Lister relates how he one day ascended to the highest accessible point of York Minster, when the October air teemed with gossamer flakes, and “could thence discern them yet exceeding high” above him. Gilbert White describes a shower, at least eight miles in length, in which “on every side, as the observer turned his eyes, he might behold a continual succession of fresh flakes falling into his sight, and twinkling like stars as they turned their sides toward the sun.” The ascent of a hill 300 feet in height did not in the least enable him to escape the shower, which showed no sign of diminution.
The mortality among very young spiders must be exceedingly great; indeed, this is indicated by the large number of eggs laid by many species, an unfailing sign of a small proportion of ultimate survivors. We shall have, by and by, to speak of some of their natural enemies, but apart from these their numbers are sadly reduced by the rigours of the weather, and appreciably also by their tendency to cannibalism. A thunderstorm will often destroy a whole brood, or they may perish from hunger in the absence of an adequate supply of insects minute enough for their small snares and feeble jaws. In the latter case they sometimes feed for a time on one another, and it is even said that two or three of a brood may be reared on no other food than their unfortunate companions.
The large and handsome Garden-spider, Epeira diademata, has been known, when well fed, to construct six cocoons, each containing some hundreds of eggs, and some species are even more fertile, while their adult representatives remain stationary, or even diminish in number.
Spider-Webs.—Some account has already been given of the external and internal spinning organs of Spiders. Within the body of the animal the silk is in the form of a gummy fluid; and this, being emitted in exceedingly fine streams, solidifies as it meets the air. It cannot be shot out to any distance, but the animal usually draws it out by its hind legs, or attaches it to a spot, and moves away by walking or allowing itself to drop. It has some power of checking the output, and can stop at will at any point of its descent; but the sphincter muscles of the apertures are but weak, and by steady winding the writer has reeled out a hundred yards of the silk, the flow of which was only then interrupted by the spider rubbing its spinnerets together and breaking the thread.
There is, of course, no true spinning or interweaving of threads in the process, but parallel silken lines are produced, varying in number according to the special purpose for which they are designed, and sometimes adhering more or less to one another on account of their viscidity and closeness.
The silk is utilised in many ways, serving for the construction of snares, nests, and cocoons, as well as for enwrapping the captured prey, and for anchoring the spider to a spot to which it may wish to return.
Spiders may be roughly distinguished as sedentary or vagabond, the former constructing snares, and the latter chasing their prey in the open. We will first consider the various forms of snare, beginning with that characteristic of the Epeiridae.
The Circular Snare.—This familiar object, sometimes spoken of as the orb-web or wheel-web, is always the work of some spider of the Family Epeiridae.
The accuracy and regularity of form exhibited by these snares has caused their architects to be sometimes called the geometric spiders. The ingenuity displayed by them has always excited the admiration of the naturalist, and this is increased on closer observation, for the snares are in reality even more complex than they appear at first sight.
The first care of the spider is to lay down the foundation threads which are to form the boundary lines of its net. If the animal can reach the necessary points of attachment by walking along intervening surfaces the matter is comparatively simple. The spinnerets are separated and rubbed against one of the points selected, and the spider walks away, trailing behind it a thread which it keeps free from neighbouring objects by the action of one of its hind legs. On reaching another desirable point of attachment the line is made taut and fixed by again rubbing the spinnerets against it. By a repetition of this proceeding a framework is presently constructed, within which the wheel or orb will ultimately be formed.
The process of fixing and drawing out a line can be conveniently watched in the case of a Spider imprisoned in a glass vessel, and it will be seen, by the aid of a lens, that a large number of very fine lines starting from the point of attachment seem to merge into a single line as the Spider moves away. This has given rise to the prevalent and very natural idea that the ordinary spider’s line is formed or “woven” of many strands. This, however, is not the case,[[272]] for the fine attachment-lines are not continued into the main thread, but only serve to anchor it to the starting-point.
As has been said, the spider can throw into play a varying number of spinning tubes at will, and in point of fact those used in laying down these foundation-lines are either two or four in number. The spider, however, often finds it necessary to strengthen such a line by going over it afresh.
Every one must have noticed that orb-webs frequently bridge over gulfs that are clearly quite impassable to the spider in the ordinary way. They often span streams—and Epeirid spiders cannot swim—or they are stretched between objects unattainable from each other on foot except by a very long and roundabout journey. When this is the case, the animal has had recourse to the aid of the wind. A spider of this family placed on a stick standing upright in a vessel of water is helpless to escape if the experiment be tried in a room free from draughts. With air-currents to aid it, silken streamers will at length find their way across the water and become accidentally entangled in some neighbouring object. When this has happened, the spider hauls the new line taut, and tests its strength by gently pulling at it, and if the result is satisfactory, it proceeds to walk across, hand-over-hand, in an inverted position, carrying with it a second line to strengthen the first. This is exactly what happens in nature when a snare is constructed across chasms otherwise impassable, and it may be imagined that the spider regards as very valuable landed property the foundation lines of such a web, for, if destroyed, the direction or absence of the wind might prevent their renewal for days. They are accordingly made strong by repeated journeys, and are used as the framework of successive snares, till accident at length destroys them.
A single line which finds anchorage in this way is sufficient for the purposes of the spider. It has only to cross over to the new object, attach a thread to some other point of it, and carry it back across the bridge to fix it at a convenient spot on the surface which formed the base of its operations. Between two such bridge-lines the circular snare is constructed in a manner to be presently described. Sometimes the tentative threads emitted by the spider travel far before finding attachment. In the case of the English Epeira diademata the writer has measured bridge-lines of eleven feet in length; and with the great Orb-weavers of tropical countries they frequently span streams several yards in width.
Two stout bridge-lines thus constructed will form the upper and lower boundaries of the net. The lateral limits are easily formed by cross lines between them at a convenient distance apart. The spider chooses a point, say, on the upper bridge-line, fixes its thread there, and carries it round to the lower line, where it is hauled taut and firmly attached. Two such cross lines give, with the bridge-lines, an irregular four-sided figure within which to stretch the snare, and now the work is perfectly straightforward, and can proceed without interruption.
Attention is first paid to the radii of the circular web. The first radii are formed by drawing cross lines within the framework in the same manner as before, but the spider carefully attaches these where they intersect by a small flossy mass of silk, and this central point or hub becomes the basis of its subsequent operations. It is a simple matter to add new radial lines by walking from the centre along one of those already formed and fixing the thread to some new point of the circumference. They are not laid down in any invariable order, but with a kind of alternation which has the general effect of keeping the strain on every side fairly equal. Almost every time the spider reaches the centre it slowly revolves, uniting the radii afresh at their point of junction, and increasing the strength and complexity of the hub. It also occasionally digresses so far as to stretch the whole structure by bracing the framework at additional points, so that it loses its four-sided form and becomes polygonal. We have now a number of spokes connecting a central hub with an irregular circumference.
The hub is next surrounded by what Dr. M‘Cook calls a “notched zone,” consisting of a few turns of spiral thread which serve to bind more firmly the spokes of the wheel. The most important part of the work is still to be performed. The lines hitherto laid down are perfectly dry and free from viscidity, so that an entangled insect would easily be able to free itself. A viscid spiral line remains to be spun, and the snare will be complete. The precise method of laying this down will vary somewhat according to the species, but, to refer again to the large Garden-spider, the proceeding is as follows:—Commencing at a point somewhat outside the notched zone, the creature rapidly works in a spiral thread of ordinary silk with the successive turns rather far apart. This forms a kind of scaffolding, by clinging to which the spider can put in the viscid spiral, which it commences at the circumference.
Its action now becomes exceedingly careful and deliberate, though by no means slow, and so great is its absorption in the work that it may be observed quite closely with a hand-lens without fear of interrupting it. The proceeding consists in drawing out from its spinnerets with one (or both) of its hind legs successive lengths of a highly elastic line, which it stretches just at the moment of fixing it to a radius, and then lets go with a snap. There is no hesitation or pause for consideration, but there is a peculiar deliberateness in drawing out each length of the thread which, together with stretching and sudden release, require explanation. Now, it has already been mentioned that the framework and radii of the snare are not at all moist or adhesive. This, however, is not the case with the spiral, upon which the spider chiefly relies in capturing its prey. A close examination of it—even with the naked eye—will show it to be beaded over with little viscid globules which, under a low magnifying power, are seen to be arranged with remarkable regularity.
Fig. [190].—A, B, C, D, Stages in the formation of the viscid globules of the web.
A very convenient method of investigation is to carry off a newly-constructed web—or, better still, one not quite finished—on a piece of plate glass, to which it will adhere by reason of the viscid spiral, and on which it may be examined at leisure. Immersion in a staining fluid will colour the viscid spiral, and show its structure in a striking manner. It will appear to consist of a thread strung with beads of two sizes, occurring with pretty uniform alternation, though two of the larger beads are often separated by two or more of the smaller.
Until recently it was supposed that the deposition of these beads upon the spiral line was a subsequent operation, and, in view of their vast numbers and regularity, the circumstance naturally excited great wonder and admiration. Blackwall[[273]] estimated that, in a fourteen-inch net of Epeira cornuta, there were at least 120,000 viscid globules, and yet he found that its construction occupied only about forty minutes! The feat, from his point of view, must be allowed to be rather startling.
As a matter of fact, the thread, on being slowly drawn out, is uniformly coated with viscid matter which afterwards arranges itself into beads, the change being assisted by the sudden liberation of the stretched line.
Boys[[274]] has shown their formation to be quite mechanical, and has obtained an exact imitation of them by smearing with oil a fine thread ingeniously drawn out from molten quartz. The oil arranged itself into globules exactly resembling the viscid “beads” on the spider’s line. If the web be carried bodily away on a sheet of glass, as above described, while the spider is engaged upon the spiral line, the experimenter will have permanent evidence of the manner in which the globules are formed. The last part of the line will be quite free from them, but uniformly viscid. Tracing it backwards, however, the beads are soon found, at first irregularly, but soon with their usual uniformity. The thread which the spider has thus “limed” for the capture of prey is really two-stranded—the strands not being twisted, but lying side by side, and glued together by their viscid envelope.
The snare is now practically complete, and the proprietor takes up her position either in the centre thereof, or in some retreat close at hand, and connected with the hub by special lines diverging somewhat from the plane of the web. Notwithstanding the possession of eight eyes—which, in sedentary spiders, are by no means sharp-sighted—it is mainly by the sense of touch that the spider presently becomes aware that an insect is struggling in the net. She immediately rushes to the spot, and suits her action to the emergency.
If the intruder is small, it is at once seized, enveloped in a band of silken threads drawn out from the spinnerets, and carried off to the retreat, to be feasted on at leisure. If it seems formidable it is approached carefully—especially if armed with a sting—and silk is deftly thrown over it from a safe distance till it is thoroughly entangled, and can be seized in safety by the venomous jaws of its captor. Sometimes the insect is so powerful, or the spider so sated with food, that the latter hastens to set free the intruder by biting away the threads which entangle it before much havoc has been wrought with the net.
The viscid matter on the spiral line dries up after some hours, so that, even if the web has not been destroyed by insects and stress of weather, this portion of it must be frequently renewed. Commencing a new web is, as has been seen, a troublesome matter, and it will readily be understood that the spider prefers, where practicable, to patch up the old one. This is done by biting away torn and ragged portions and inserting new lines in their place.
The part played by the various spinning glands in the construction of the orb-web may be briefly stated.[[275]] The ampullaceal glands furnish the silk for the foundation lines and radii. The spiral has a double ground-line proceeding from the middle spinnerets, but it is not quite certain whether it proceeds from the ampullaceal or the tubuliform glands. The chief function of the latter, in the female, is to furnish silk for the egg-cocoon. The viscid globules are the products of the aggregate glands. The aciniform and piriform glands provide the multitudinous threads by which the spider anchors its various lines and enwraps its prey.
Some Orb-weavers always decorate their snares with patches or tufts of flossy silk. In the snare of the North American Argiope cophinaria the hub is sheeted, and from it extends downwards a zigzag ribbon of silk stretched between two consecutive radii. Vinson[[276]] discovered a remarkable use for similar zigzag bands in the web of the Mauritian spider, Epeira mauritia. It furnished a reserve supply of silk for enveloping partly entangled insects whose struggles were too vigorous to succumb to the rather feeble threads which the spider was able to emit at the moment of capture. The spider was able to overcome a grasshopper much more powerful than itself by dexterously throwing over it with one of its hind legs a portion of the ribbon of silk which it had thus stored up for emergencies.
Fig. [191].—A, Snare of Hyptiotes cavatus; B, enlarged view of the Spider, showing the “slack” of the hauled-in line. (After Emerton.)
Many orb-webs are defective, a sector of the circle being uniformly omitted in the structure. The genus Hyptiotes does not belong to the Epeiridae but to the cribellate Uloboridae, but its defective orb-web is so curious that it deserves a special mention. A single foundation-line is laid down, and from it four radii are drawn and are connected with cross lines, the snare constituting about one-sixth of a circle. From the centre of the incomplete circle a thread proceeds to some more or less distant object, and on this the spider takes up its position, inverted, and hauls in the line till the snare is taut. When the trembling of the line shows the spider that an insect has struck the net, it lets go with its fore-legs, and the web, springing back to its normal position, entangles the intruder more thoroughly by its vibrations. When large insects are in question the spider has been observed to “spring” the net several times in succession. H. cavatus is common in the pine woods of Pennsylvania, but the only English species, H. paradoxus, is extremely rare.
A remarkable spider has been discovered in Texas by M‘Cook, which, after building a horizontal orb-web, converts it subsequently into a dome (Fig. [192]) of exceedingly perfect form. It is named Epeira basilica, and has been the object of careful study by Dr. Marx, who observed the whole process of web-construction. Threads are attached at various points on the upper surface of the horizontal wheel, the central portion of which is gradually pulled up until the height of the dome is nearly equal to the diameter of its base. But the snare of this spider does not consist of the dome alone. A sheet of irregular lines is stretched below, while above there is a maze of threads in the form of a pyramid. Several other Orb-weavers, as, for instance, E. labyrinthea and E. triaranea, supplement their typical webs by an irregular structure of silk, and thus form connecting links, as regards habit, between the group of which we have been speaking and the Theridiidae or Line-weavers, which may now briefly be dealt with.
Fig. [192].—Snare of Epeira basilica. (After M‘Cook.)
The Irregular Snare.—The great majority of British Spiders belong to the family of the Theridiidae, or Line-weavers. Some of these are among the handsomest of our native species, and are in other respects highly interesting, but their snares lack the definiteness of structure exhibited by the orb-web, and little need be said about them.
For the most part they consist of fine irregular lines running in all directions between the twigs of bushes or among the stems of grass and herbage. One large and important genus, Linyphia, always constructs a horizontal sheet of irregular threads with a maze of silk above it. Such snares may be seen in myriads in the wayside hedges during the summer, and they are especially notable objects when heavily laden with dew. Insects impeded in their flight by the maze of threads drop into the underlying sheet, and are soon completely entangled. The spider usually runs beneath the sheet in an inverted position.
The sheet or hammock of silk is absent in the case of most of the other genera of this family, their snares being innocent of any definite method in their structure. They are frequently quite contiguous, and it is no uncommon thing to find a holly bush completely covered with a continuous network of threads, the work of a whole colony of the pretty little spider Theridion sisyphium.
As might be imagined from the simplicity or absence of design in the structure of the net, there appears to be very little complexity in the nature of the silk used. It is interesting, however, to find that viscid globules, not unlike those which stud the “spiral line” of the Epeiridae, are sometimes present in the snares of the Line-weavers,[[277]] and in these, too, aggregate glands are present. There is a large spider of this family, Theridion tepidariorum, which may be found to a certainty in almost any hothouse in this country. In its snare, which is of the ordinary irregular type, F. Pickard-Cambridge has observed little patches of flocculent silk, calculated to render more certain the entanglement of prey, and he has further described a curious comb-like structure on the hind leg of the animal which is probably used in the production of this phenomenon. It is by no means unlikely that a more careful study of these apparently simple snares will lead to the discovery of further complexity of structure.
Fig. [193].—Snare of Uloborus sp., some of the lines being thickened with threads from the cribellum. (After M‘Cook.)
Uloborus, a cribellate genus which has an Epeirid-like, orbicular snare, decorates some of the lines with the produce of the cribellate glands, but viscid globules are absent.
Sheet-Webs.—The webs which are such familiar—and, by association, unpleasant—objects in unused rooms and outhouses are usually the work of spiders belonging to the Agelenidae and the Dictynidae. To the first belongs the common House-spider, Tegenaria civilis, and its larger congener, T. parietina. These spiders are not attractive in appearance, and the last-named species especially, with the four-inch span of its outstretched legs, is a formidable object, and a terror to domestic servants. An obscure tradition connecting it with Cardinal Wolsey and Hampton Court has caused it to be known as the Cardinal Spider. An out-door example of the Agelenidae is the very abundant Agelena labyrinthica, whose sheet-web, with its tubular retreat, is to be sought on the banks of ditches, or in the hedges of our country lanes.
The snares of these spiders are exceedingly closely woven of very fine silk, and take a long time to complete. The process of their construction may be watched by keeping an Agelena labyrinthica confined in a box with a glass front, and the web, kept free from dust, is a beautiful object, as its fine texture gradually becomes visible as a delicate transparent film which develops by imperceptible stages into an opaque white sheet. The excessive fineness of the silk makes it difficult at first to see what is taking place. The animal is seen to be busily moving about, but the result of its labours only gradually becomes visible. A few delicate foundation-lines are first stretched across the compartment in which it is confined, and upon these the spider walks to and fro incessantly with a serpentine motion, and by and by a muslin-like floor of silk comes into view.
An examination of the spinnerets throws some light upon the operation. The posterior pair are very long and mobile, and the hair-like spinning-tubes are distributed on their under surface. The cephalothorax and abdomen are far more rigidly connected in Agelena than in the Orb-weavers, but its length of leg and the length and mobility of its posterior spinnerets enable it to give a wide lateral sweep as it walks along, strewing fine silken threads upon the foundation-lines already laid down. Some hours elapse before even a moderately stout web is constructed, and for long afterwards the spider devotes odd moments of leisure in going over the ground again and strewing new silk upon the gradually thickening web. At one corner a silken tube of similar structure is formed, and in this the spider awaits the advent of any insect which may alight upon the sheet, when it immediately rushes forth and seizes it.
The webs of the Dictynidae are very similar in general appearance to those of the Agelenidae, consisting of a closely-woven sheet with a tubular nest. They are to be found, moreover, in similar situations, stretching across the angles of walls in cellars or outhouses, though some species prefer an out-door existence. Crannies in rock form convenient sites for such snares, but the family is not without its representatives in still more open situations. The web, though so similar to that of Agelena, is, however, constructed in a different manner. In the Dictynidae neither the legs nor the spinnerets are unusually long, and they do not strew the foundation-lines by a swinging motion of the body, but the operation is effected by a special apparatus. These spiders are cribellate, and in front of the six ordinary spinnerets there are a pair of perforated plates connected with a large number of additional minute spinning glands (see Fig. [182], p. 326). In conjunction with this, the female possesses on the last joint but one of each hind leg a curious comb-like arrangement of spines, the “calamistrum.” The animal constructs a sort of skeleton web by means of its ordinary spinnerets, and when this is completed it combs out silk from the cribellum by means of the calamistrum, using each hind leg alternately, and distributes it with a curling motion upon the scaffolding prepared for it, a nearly opaque web being the result. The silk from the cribellum is of an adhesive nature, and renders escape from the web very difficult.
Spiders’ Nests and Retreats.—All Spiders construct some description of nest, and often display great ingenuity in building them. Perhaps none are more curious than those of the burrowing Aviculariidae, a family which includes the interesting “Trap-door Spiders.” They are nocturnal in their habits, about which, consequently, little is known, but their nests have been carefully studied, especially by Moggridge, who found them in considerable abundance in various districts of the Riviera.
The jaws of these spiders are especially adapted for digging, and with them a hole is excavated in the ground to the depth of several inches, and wide enough to allow the animal to turn. This is carefully lined with silk which the spider throws against the sides from its long and upturned posterior spinnerets. But the chef d’œuvre of the whole structure is a lid or door which protects the entrance to the tube. There are two types of door which find favour with different species—the wafer and the cork type, as Moggridge has named them. The former consists of a thin circular or oval sheet of silk which flaps down loosely over the tube-entrance, with which it is connected by a hinge-like attachment. A trap-door of the cork type is a more complicated structure, being of considerable thickness and having a bevelled edge, so that it fits into the tube like a plug. Like the wafer door, it possesses a silken hinge.
To form the wafer door, the spider covers the entrance to the tube with a closely-woven layer of silk, which it afterwards bites away at the edge, except at the point where the hinge is to be. Doors of the cork type consist of alternate layers of silk and earth. After weaving a covering of silk, the creature brings earth in its jaws and lays it on the top, binding it down with a second layer of silk, and the process is repeated until the requisite thickness is attained.
The nests are exceedingly difficult to detect, as the spiders take the precaution of attaching leaves, moss, or small twigs to the outer surface of the doors. This does not appear to be the result of intelligence, but a mere instinctive habit; for if a door be removed and the surrounding earth denuded of moss, the spider will render the new door conspicuous by bringing moss from a distance, and thus making a green spot in the bare patch of earth.[[278]]
The cork doors fit with great exactness, and there is always to be found on their under surface a notch by which they are held down by the fore-legs of the spider against any attempt to open them from without.
Many nests with trap-doors of the wafer type are found to have a second and more solid door within the tube. This serves to shut off the lower part of the nest as a still more secure retreat. This second door opens downwards, and the Spider, getting beneath it, is effectually shielded from an enemy which may have mastered the secret of the outer barrier. The nests of some species present still further complications in the way of lateral branches from the main tube. In one case (Nemesia congener) the burrow becomes Y-shaped, and the second door hangs at the fork of the Y in such a manner as to connect the bottom chamber either with the entrance or with the branch, which does not reach the surface, but ends blindly.
Trap-door Spiders are greatly attached to their tubes, which they enlarge and repair at need. They begin burrowing very early in life, and their tiny tubes resemble in all respects those of their parents. Their habits are nocturnal, and little is known of them; an observation, however, on a species inhabiting the island of Tinos in the Grecian Archipelago (Cteniza ariana), by Erber,[[279]] must not be omitted. This spider leaves its tube at night and spins a web near at hand and close to the ground. It carries captured insects into its tube, and in the morning clears away the net, adding the material of it, M. Erber believes, to the trap-door.
No true trap-door Spider has as yet been found in this country, but the allied Atypidae are represented by at least one species, Atypus affinis, which has been discovered in colonies in some localities in the south of England, notably near Ventnor in the Isle of Wight, and on Bloxworth Heath in Dorsetshire. This spider, like its continental cousins, excavates a hole in the earth, generally near the edge of a heathery bank, and lines it with a tube of silk of such firm texture that it may be removed intact from the earth in which it is embedded. The silken tube projects some two inches above the ground, either erect among the roots of the heather, or lying loosely upon the surface. Its extremity is always found to be closed, whether from its own elasticity or by the deliberate act of the proprietor is uncertain, and it seems probable that the animal spends almost the whole of its existence in the tube. Simon believes that it feeds almost entirely upon earth-worms which burrow into its vicinity, and which it, therefore, need not leave its nest to catch; but the remains of beetles and earwigs have been found in the tubes at Ventnor.
Fig. [194].—Funnel of Cyrtauchenius elongatus. (After M‘Cook.)
This description of nest seems common to all species of the genus Atypus. The American “Purse-web Spider,” A. abboti, burrows at the foot of a tree, against the trunk of which it rears the projecting portion of its silken tube. At the bottom of the nest the cavity is enlarged, and blind processes project in different directions.
Another burrowing spider, Cyrtauchenius elongatus, surmounts its silk-lined burrow by a funnel-shaped structure of pure white silk, about three inches in height and two or three inches in width. There is no attempt at concealment, and the white funnels are conspicuous among the thin grass, presenting the appearance of fungi.
The burrowing habit is also common to the Wolf-spiders or Lycosidae, but beyond a very slight lining of silk there is usually little spinning work about their nests. Occasionally there is a certain amount of superstructure in the shape of a silken funnel (Lycosa tigrina, M‘Cook), or of an agglomeration of twigs and pebbles, as in the case of the “Turret-spider” (Lycosa arenicola, Scudder).
Fig. [195].—Turret of Lycosa carolinensis. (After M‘Cook.)
A colony of our handsome species, Lycosa picta, is an interesting sight to watch. Their favourite habitat is a sandy soil, variegated with many-coloured patches of moss and lichen, among which their own markings are calculated to render them inconspicuous. The observer, by lying perfectly still, may see them silently stealing forth from their burrows in the bright sunshine, and hunting diligently in the neighbourhood, ready to dart back on the faintest alarm, or if the sun should be temporarily obscured by a passing cloud. So closely do they resemble their surroundings, that it is only when in motion that they can readily be detected. It is very curious to see them popping out their heads to ascertain that the coast is clear before venturing forth, and the utter silence of their operations adds to the eeriness of the effect. The tubes of these spiders, though without a trap-door, and only slightly lined with silk, are Y-shaped like those of Nemesia congener, the main tunnel giving off a blind branch about half-way down.
The nest of the Water-spider, Argyroneta aquatica, must not be passed over without mention. This spider, though strictly an air-breathing animal, spends almost the whole of its existence beneath the water. That it can live in such a medium is due to the fact that the long hairs which clothe its abdomen retain a bubble of air as it swims beneath the water, so that it carries with it its own atmosphere. The air-bubble which invests its body gives it a strong resemblance to a globule of quicksilver, and renders it a pretty object in an aquarium as it swims about in search of food or in prosecution of its spinning operations.
Of these the most interesting is the building of its nest. Working upon a water plant some distance below the surface, it forms a silken dome of closely-woven threads, which it next proceeds to fill with air. To do this the spider rises in the water, raises its abdomen above the surface, and jerks it down again quickly, so as to carry with it a bubble of air which it helps to retain with its hind legs. With this it swims back to its tent, into which it allows the imprisoned air-globules to escape. By degrees the dome or bell is filled, and the creature has a dry and snug retreat beneath the water. In this it passes the winter in a torpid condition. The young of this species appear to be fond of utilising the empty shells of water-snails, which they float by filling them with air, and thus save themselves the trouble of nest-construction.
Fig. [196].—Egg-cocoons. A, Epeira diademata, nat. size. B, Theridion pallens × 4, attached to a leaf. C, Agroeca brunnea, nat. size, attached to a weed, and not yet coated with mud. D, Ero furcata × 4, attached to a log.
Cocoon.—The last important spinning operation which remains to be described is the building of the so-called cocoon. This must be distinguished from the cocoon of insects, which is a protective covering of silk within which the larva assumes the pupa form. In the case of the Spider, the term is applied to the structure which serves to protect and conceal the eggs. It is often of considerable complexity, and is highly characteristic of the particular species which constructs it.
All egg-bags are commenced in very much the same way. A small sheet of silk is woven, and against this, sometimes upon the upper and sometimes on the under surface, the eggs are deposited, and then covered in with a second silken layer. The compact silk-covered ball of eggs is then, in many cases, enclosed in a small compartment which the spider builds with infinite care and unfailing uniformity, after the pattern peculiar to its kind. A considerable number of the Orb-weavers are content with a simple silken case closely investing the eggs, and by its thickness and the non-conducting quality of the material, sufficient protection is afforded against inclement weather.
The egg-bag of the large Garden-spider (E. diademata) may be recognised by its great size and its yellow colour, which is deepened by the still more yellow tint of the eggs within. Those of Zilla x-notata and of many other English Epeirids are of similar structure, but of white silk. The mother generally avails herself of some natural shelter, hiding her cocoon beneath loose bark, in the crannies of masonry, or under the copings of walls.
Many species, on the contrary, boldly expose their cocoons in their snares, sometimes as many as fourteen being constructed in succession and strung in a chain. The American species Epeira caudata and E. bifurca are good examples of this habit, stringing a chain of characteristic cocoons upon the line connecting the retreat with the web.
The sedentary Theridiid spiders usually suspend their cocoons in the neighbourhood of their irregular snares. The green cocoon of Theridion sisyphium is generally more or less concealed by an accumulation of débris. The minute species T. pallens constructs a cocoon of peculiar shape on the under surface of a leaf (Fig. [196], B). It is a conical structure of white silk, considerably larger than the spider itself, attached at its broad end,[[280]] and having several curious lateral projections near the middle.
Among the Lycosidae or “Wolf-spiders” the prevailing habit of the mother is to carry the egg-bag attached beneath her abdomen upon all her hunting excursions. It is spheroidal in shape, made up of an upper and a lower half, with a seam-like junction at the equator, so to speak. The lower half is first woven, and the eggs are deposited within it. The upper hemisphere is then spun, and the edges gathered in and finished off, the seam or suture being always discernible. The bag is now attached by silken threads to the spinnerets, and bumps merrily over the ground as the animal hurries along in search of prey. If deprived of it she evinces the greatest distress, and frequently will not try to escape without it.
Attempts to utilise Spider Silk.—It is long since the web of the House-spider, taken internally, was considered a specific for the ague, though its value as a styptic has been recognised in quite recent times. It is, however, with other uses of Spider silk that we are here concerned.
Spider silk has been extensively used in the micrometer eyepieces of telescopes where very fine intersecting lines are required. For this purpose the radial or scaffolding lines of the circular snare were selected, the spiral being unsuited on account of its row of viscid beads. Professor C. V. Boys has, however, discovered in his quartz fibres a material better adapted for this purpose.
Several attempts have been made to weave the silk of Spiders as a substitute for that of the silk-worm. Web silk is, of course, far too fine to furnish a durable material, but the cocoons are usually formed of coarser silk, and it is with them that the experiment has been tried. About the beginning of the eighteenth century certain stockings and mittens made of Spider silk from the cocoons of Epeira diademata, by M. Bon of Languedoc, attracted so much attention that the Academy desired M. Réaumur to investigate the matter. His report was unfavourable to the commercial utility of Spider silk. The cocoon threads, though eighteen times stronger than those of the web, were but one-fifth of the strength of those obtained from the silk-worm, and the lustre was inferior. A still more fatal objection, however, was founded upon the cannibalistic habits of the spider, and the difficulty of furnishing it with acceptable food.
M. Vinson has recorded that some of the spiders of Madagascar, especially Epeira madagascarensis, are far better adapted than any of our English species to a commercial use. They furnish silk of a beautiful clear yellow colour; they are accustomed to live harmoniously together in families; and the range of climate in which they can thrive is very considerable. The Creole ladies of this island, under the administration of General Decaen, wove a magnificent pair of gloves from spider silk, with their own hands, for presentation to the French Empress.
Poison of Spiders.—All spiders possess poison-glands, which have their openings on the fangs of the chelicerae. The action of the chelicera in striking does not express the venom, but the poison-bag itself is covered with a muscular coat by which the contained fluid is expelled. It is highly probable, therefore, that the venom is under the control of the animal’s will, and is economised when the simple wound is sufficient for the purpose—a supposition which may partially explain the very divergent opinions held with regard to the effect of the spider’s bite. The reputation of the “Tarantula” Spider is well known, but what particular species, if any, was intended by the name is quite uncertain. The name is derived from the town Tarentum, and was certainly applied to a Lycosid spider. Probably the common south European species, Lycosa narbonensis, has as good a claim to the honour as any. The confusion has been increased by extending the name to spiders of quite a different family. Eurypelma hentzii, one of the Aviculariidae, is commonly known as the Tarantula in America.
The superstition of the tarantula dance is well known. The bite of the spider was supposed to induce a species of madness which found its expression—and its cure—in frantic and extravagant contortions of the body. If the dance was not sufficiently frenzied, death ensued. In the case of survivors, the symptoms were said to recur on the anniversary of the bite. Particular descriptions of music were supposed to incite the patient to the excessive exertion necessary for his relief; hence the “Tarantella.”
In the Middle Ages epidemics of “tarantism” were of frequent occurrence, and spread with alarming rapidity. They were seizures of an hysterical character, analogous to the ancient Bacchic dances, and quite unconnected with the venom of the spider from which they took their name. The condition of exaltation and frenzy was contagious, and would run through whole districts, with its subsequent relapse to a state of utter prostration and exhaustion. The evil reputation of the Tarantula appears to have exceedingly little basis in fact.
Baglivi relates how the country people capture the Tarantula by imitating the buzzing of an insect at the mouth of its hole. “Quo audito, ferox exit Tarentula ut muscas, quorum murmur esse putat, captet; captatur tamen a rustico insidiatore.”
Fabre[[281]] acted the part of the “insidious rustic” with slight success; but by other stratagems he enticed the creatures from their holes, and made some interesting observations upon the effects of their bite. He found that bees and wasps were instantaneously killed by them. This immediately fatal effect he found to be due to the fact that the spider invariably struck the insect in a particular spot, at the junction of the head with the thorax. Bees must often wander into Tarantula’s holes, and a prolonged contest, though it might end in the death of the insect, would be certain also to result fatally for the spider. It has, therefore, acquired the habit of striking its foe in the one spot which causes instant death. When Fabre presented a bee to a Tarantula in such a manner that it was bitten in some other region, the insect survived several hours.
A young sparrow, just ready to leave the nest, was bitten in the leg. The wound became inflamed, and the limb appeared to be paralysed, but the victim did not at first suffer in general health, and fed heartily; death resulted, however, on the third day. A mole died in thirty-six hours after the bite.
From these experiments, Fabre came to the conclusion that the venom of the Tarantula was at all events too powerful to be entirely negligible by man.
Fig. [197].—Latrodectus mactans, ♂, natural size.
Trifling causes may have a fatal effect upon a man in ill health, and it is quite possible that death has sometimes resulted from the Tarantula’s bite. Its effect upon a healthy subject, however, is certainly not serious. Goldsmith, in his Animated Nature, entirely discredits the current stories about this animal, saying that the Italian peasants impose upon credulous travellers by allowing themselves, for money, to be bitten by the Tarantula, and then feigning all the symptoms which are traditionally supposed to ensue.
There is a genus of the Theridiidae, by name Latrodectus, whose poisonous reputation almost rivals that of the Tarantula. It is remarkable, moreover, that it is regarded as particularly dangerous in such widely separated portions of the world as Madagascar, New Zealand, Algeria, the West Indies, and North America. These spiders, strangely enough, are by no means particularly large or formidable in appearance.
There are two species in Madagascar, known to the natives by the names of Mena-vodi and Vancoho. Vinson[[282]] describes the terror which is locally inspired by the first-named species, whose bite is believed to be fatal unless measures are promptly taken to counteract the poison. They sometimes cauterise the wound, but the usual treatment consists in inducing profuse perspiration—a method of cure which recalls the Tarantula dance of Southern Europe. Flacourt[[283]] mentions the Vancoho as the most dangerous animal of Madagascar, and more formidable than the scorpion. He relates cases among his own negroes where the bite was followed by a condition of syncope which lasted two days.
A New Zealand species is known by the natives as the Katipo. It is of about the size of a pea, and almost black in colour. Mr. Meek of Waiwera gives a most circumstantial account of the effect of its bite upon his son.[[284]] During the four days which followed the bite he suffered excruciating pain, which spread from his leg to the spine, arms, and chest, and he lost twelve pounds in weight. Relief was obtained by frequent doses of brandy and the use of a liniment.
The natives of New Zealand have a great horror of this spider, but hold the curious belief that its death will ensure the cure of any one it may have bitten. If unable to find it, they will burn the house down rather than allow it to escape. Their dread, however, is confined to a variety which lives among the sedge of the sea-beach, and they carefully avoid sleeping in such places.
Two of the best authenticated cases of serious results ensuing from the bite of a spider of this genus come from North Carolina.[[285]]
A farm labourer in the employ of Mr. John Dick of Greensborough was bitten by Latrodectus mactans about half-past eight in the morning, and died between ten and eleven o’clock at night. Small pimples were raised in the neighbourhood of the bite, but no puncture was discernible. Intermittent pains and spasms ended in a comatose condition from which he did not rally. The man appeared previously to be in perfect health.
Another man on Mr. Dick’s farm was bitten by the same species of spider. He resumed work, but a spasm of pain caused him to mount his horse and endeavour to ride home, but he fell off, and lay in a state of unconsciousness. He was found in this condition by a fellow-workman, and taken home. Large quantities of whisky were administered without any intoxicating effect, and this afforded some relief from the constantly-recurring spasms. The paroxysms continued for three weeks, and two months elapsed before he was able to resume work. On the ankle where he was bitten pimples appeared as in the previous case, and these broke out again, long after the occurrence, whenever he became overheated in his work.
These accounts are sufficiently circumstantial and well authenticated, but the fact of the actual bite depends upon the statement of the victims alone, and they may possibly have mistaken the cause of their trouble.
Southern Europe possesses a congener of this spider in Latrodectus 13–guttatus, the well-known “Malmignatte,” which is also considered extremely poisonous. The Royal Academy of Medicine and Surgery at Barcelona appointed Dr. Graells, in 1833, to inquire into the effects of the bite of this spider, cases of which had become exceedingly frequent. He found a curious correspondence between the frequency of these cases and the advent of migratory locusts, which the spider successfully attacked. In his report[[286]] he details the symptoms in certain unquestionably authentic cases. There was a double puncture, surrounded by red circles, the region of the wound afterwards swelling greatly. The pain and swelling extended over the whole limb, and often to the body, and convulsions occurred, followed by great prostration and collapse. All the patients eventually recovered, their cure being heralded by profuse perspiration.
It must be mentioned, however, that the eminent Arachnologist M. Lucas states that he has several times allowed himself to be bitten by this identical spider without any ill effects.
The testimony is thus conflicting in this case also. It is impossible, however, to believe that there is no basis in fact for the poisonous reputation of a comparatively insignificant-looking spider in so many widely separated parts of the world, supported as it is by certain well-substantiated cases. The variable effects of its bite may find a partial explanation in a variation in the strength of its venom at different seasons, and it has already been mentioned that the injection of poison into its victim is a voluntary act, and does not necessarily accompany its bite. Among the species regarded as especially venomous must be mentioned Phidippus morsitans, one of the larger of the Attidae.
It is exceedingly likely that the bite of the large tropical Aviculariidae is really formidable. They appear, however, more anxious to escape than to show fight, and we possess little reliable information with regard to them. Doleschall shut up small birds with two West Indian species, and death followed their bite almost immediately. Ten days’ starvation appeared to weaken the venom, for a bird bitten by a spider fasting for that period recovered after an indisposition of six hours.
Most Arachnologists have recorded experiments with regard to the venom of the commoner European species, with equally conflicting results. Blackwall came to the conclusion that loss of blood, and not poison, caused the death of spider-bitten insects. He could not himself distinguish a spider bite from the prick of a needle inflicted upon his hand at the same time. Bees, wasps, and grasshoppers survived the bite about as long as other insects of the same species outlived a needle-prick in the same part of the body. Walckenaer’s experience was of the same nature. Bertkau, however, when bitten in the hand, felt clear indications of an irritant poison in the wound. The hairs of some of the large hairy species of the Aviculariidae possess poisonous properties. They are readily parted with, and when the animal is touched by the hand considerable irritation is set up.
Fertility of Spiders.—Spiders vary greatly in the average number of eggs laid by different species, and within the limits of each species there is a very considerable variation in fertility. As a rule it appears that the large and vigorous spiders are more prolific than the smaller and weaker members of the order. Were all the facts before us, however, we should no doubt find that the number of eggs laid bore a direct proportion, not to the size of the species, but to the dangers to which the young of that species are exposed. Where the total numerical strength of a species is fairly stationary, such a proportion must of course exist. Some species, no doubt, are tending to become extinct, while others are increasing in numerical importance. As a general rule, however, it is safe to infer that, if a species is especially prolific, special dangers attend the rearing of the young. The largest of North American Epeirids, Argiope cophinaria,[[287]] constructs a cocoon containing, on an average, 1150 eggs. As many as 2200 have been counted in exceptional cases. Even this number is exceeded in the case of some of the great Aviculariidae. Theraphosa leblondi deposits as many as 3000 eggs. The large European Epeirids, E. quadrata and E. diademata, lay about 600 eggs, those of Lycosa narbonensis reaching about the same number. Those American spiders which have been described as stringing up a series of cocoons in their webs usually attain about the same aggregate, the eggs being less numerous in each cocoon.
These are examples of fairly large and fertile spiders. In the case of other species the number of eggs laid is exceedingly small. Ero furcata makes a single cocoon containing six eggs. Synageles picata, an ant-like Attid, lays only three. Oonops pulcher constructs several cocoons, but each contains only two eggs. The eggs of Cave-spiders, and such as live in dark and damp places, are generally few in number. Anthrobia mammouthia, for example, an inhabitant of the great American caves, deposits only from two to five eggs.
Our knowledge of the special perils which beset particular species is so incomplete that we are often at a loss for the reason of this great inequality in fertility. For instance, how does Synageles picata maintain its numerical strength by laying only three eggs, when, as M‘Cook points out, its resemblance to the ant, though advantageous to the adult spider, affords no protection to the egg? Our knowledge must be greatly extended before we are able to account for particular cases. Many influences hostile to spiders as a group are, however, well known, and we may here enumerate them.
Natural Enemies.—The precautions taken by the mother in constructing the cocoon render the inclemency of the weather very much less destructive to the eggs than to the newly-hatched young. Nevertheless, among spiders inhabiting swampy regions great havoc is wrought by the occasional wholesale swamping of the cocoons by floods. Professor Wilder considers the great fertility of Nephila plumipes necessary to counterbalance the immense destruction worked by the heavy rains upon their cocoons, which are washed in great numbers from the trees, to the leaves of which they are attached. But such exposed situations are avoided by many species, and their eggs, enclosed in their silken envelope, are well protected against the severities of the weather.
A more universal enemy to the egg is found in Ichneumon flies. On examining the cocoons of almost any species of spider, a large proportion are almost certain to be found to contain Ichneumon larvae. Mr. F. Smith, in the Transactions of the Entomological Society for 1860, describes two species, Hemeteles fasciatus and H. formosus, which are parasitic on the eggs of Agelena brunnea. They are figured in Mr. Blackwall’s book on British Spiders. Pezomachus gracilis attacks the cocoons of many kinds of American spiders, appearing to have no special preference for any particular species, while Acoloides saitidis seems to pay special attention to the eggs of certain of the Jumping-spiders, and particularly of Saitis pulex.
The Ichneumons which thus regard the Spider’s eggs as convenient food for their own larvae are probably very numerous. Nor are they themselves always free from parasites. Occasionally the larvae of minute Hymenopterous insects are found to be parasitic upon the eggs of an Ichneumon which have been laid in a Spider’s cocoon.
It sometimes happens that the development of the young spiders has so far advanced at the time of the Ichneumon’s intrusion that the latter’s intention is frustrated, and its offspring, instead of devouring, are themselves devoured. Again, some few of the eggs in an infested cocoon occasionally escape the general destruction and reach the adult condition, but there can be no doubt that Ichneumons are largely instrumental in keeping down the numbers of most species of spiders. The perils which attend the Spider after leaving the cocoon are no less formidable, and much more numerous. The whole newly-hatched brood may be destroyed by a heavy rain-storm. If there is not a sufficient supply of food suitable to their feeble digestive powers they perish of inanition, or eat one another. This cannibalistic propensity is a considerable factor in the mortality among young spiders, and the adult animals frequently prey upon one another.
Argyrodes piraticum, in California, invades the webs of larger spiders of the family Epeiridae, which it seizes and devours. A. trigonum, common in the eastern states of North America, has the same habit.[[288]] Hentz found in Alabama a spider, which he named Mimetus interfector, of still more ferocious and piratical habits. Its special quarry is Theridion tepidariorum. Sometimes the Theridion overcomes the invader, and one case was observed in which a second Mimetus was devouring a Theridion beside the dead body of its predecessor, who had come off the worse in the combat.
The eggs of Theridion tepidariorum are also sometimes devoured by this spider, and a similar propensity has been observed in some English species, for Staveley[[289]] states that it is common to see certain spiders of the genus Clubiona feeding upon the eggs which have been laid by their neighbours. The larvae of some Hymenopterous insects are parasitic upon Spiders themselves, and not upon their eggs. Blackwall found this to be the case with the larvae of Polysphincta carbonaria, an Ichneumon which selects spiders of the genera Epeira and Linyphia on which to deposit its eggs.[[290]] The spider thus infested does not moult, and is soon destroyed by the parasite which it is unable to dislodge from its back. Menge, in his Preussische Spinnen, enumerates several cases of parasitism in which the larva, as soon as it has developed from the egg, enters the spider’s body, there to continue its growth. Spiders are also subject to the attack of a parasitic worm, Gordius (cf. vol. ii. p. 173).
Some of the most deadly foes of Spiders are the Solitary Wasps. There are many species of Pompilus (vol. vi. p. 101), which, having excavated holes in clay banks, store them with spiders or other creatures which they have paralysed by their sting. They then deposit an egg in the hole, and immediately seal up the orifice. This habit is found to characterise the solitary wasps of all parts of the world. Belt[[291]] relates the capture of a large Australian spider by a wasp. While dragging its victim along, it was much annoyed by the persistent presence of two minute flies, which it repeatedly left its prey to attempt to drive away. When the burrow was reached and the spider dragged into it, the two flies took up a position on either side of the entrance, doubtless with the intention of descending and laying their own eggs as soon as the wasp went away in search of a new victim. Fabre[[292]] gives an interesting account of one of the largest European Pompilidae, Calicurgus annulatus, which he observed dragging a “Tarentula” to a hole in a wall. Having with great difficulty introduced its burden into the cavity, the wasp deposited an egg, sealed up the orifice, and flew away. Fabre opened the cell and removed the spider, which, though completely paralysed, lived for seven weeks.
The same indefatigable observer describes the method adopted by the comparatively small Pompilus apicalis in attacking the formidable Wall-spider, Segestria perfida. The combatants are well matched, and the issue of the battle would be doubtful if the wasp did not have recourse to stratagem. Its whole energies are directed towards forcing the spider away from its web. At home, it is confident and dangerous; when once dislodged, it appears bewildered and demoralised. The wasp darts suddenly towards the spider and seizes it by a leg, with a rapid effort to jerk it forth, releasing its hold before the enemy has had time to retaliate. The spider, however, as well as being anchored by a thread from its spinnerets, is clinging to its web with its hind legs, and if the jerk is not sufficiently energetic, it hastily scrambles back and resumes its defensive position. Before renewing the attack the wasp gives the spider time to recover from the excitement of the first onset, seeking, meanwhile, the retreats of other victims. Returning, it succeeds, by a more skilful effort, in drawing the spider from its retreat and hurling it to the ground, where, terrified and helpless, it falls an easy prey. Should the insect bungle in its first attack and become entangled in the web, it would itself become the victim. Certain wasps thus appear to seek out particular species of spiders as food for their larvae. Others are less discriminate in their tastes. Again, some, as in the cases cited above, store their egg-nest with a single spider, while others collect many for the purpose.
The American “blue digger wasp” (Chlorion caeruleum) excavates its nest in the ground, and inserts a single large spider of any species.[[293]] Another wasp, of the genus Elis, selects the Wolf-spiders, and especially Lycosa tigrina, for the use of its larvae, while Priocnemus pomilius shows a preference for the Crab-spiders, or Thomisidae.
One of the most remarkable instances is that of Pepsis formosa, which preys upon the gigantic spider Eurypelma hentzii, wrongly styled in America the “tarantula,” but really belonging to an entirely different family, the Aviculariidae.
Fabre’s most interesting researches have established the fact that the instinct of the wasp leads it to sting the spider in a particular spot, so as to pierce the nerve-ganglion in the thorax. The precision with which this is effected is absolutely necessary for the purpose of the insect. If stung elsewhere, the spider is either incompletely paralysed, or it is killed outright, and thus rendered useless as food for the future larvae of the wasp. On the one hand, therefore, the Tarantula has acquired the habit of striking the wasp in the only point where its blow is instantaneously fatal, while on the other the wasp, with a different object in view, has been led to select the precise spot where its sting will disable without immediately destroying the spider. The latter case is, if anything, the more extraordinary, as the insect can hardly have any recollection of its larval tastes, and yet it stores up for progeny, which it will never see, food which is entirely abhorrent to itself in its imago state.
Spiders taken from the egg-nests of wasps by M‘Cook survived, on the average, about a fortnight, during which period they remained entirely motionless, and would retain any attitude in which they were placed.
There are many animals which either habitually or occasionally feed upon spiders. They are the staple food of some hummingbirds, and many other birds appear to find in them a pleasing variation on their customary insect diet. These creatures, moreover, are destructive to spiders in another way, by stealing the material of their webs, and especially the more closely textured silk of their egg-cocoons, to aid in the construction of their nests. M‘Cook has observed this habit in the case of Vireo noveborocensis, and he states, on the authority of others, that the “Plover” and the “Wren” are addicted to it. The smaller species of monkeys are extremely fond of spiders, and devour large numbers of them. They are said, moreover, to take a mischievous delight in pulling them in pieces. Armadillos, ant-eaters, snakes, lizards, and indeed all animals of insectivorous habit, draw no distinction between Insecta and Arachnida, but feed upon both indiscriminately. The army ants, so destructive to insect life in tropical countries, include spiders among their victims. These formidable insects march along in vast hordes, swarming over and tearing in pieces any small animal which lies in their path. They climb over intervening obstacles, searching every cranny, and stripping them bare of animal life. Insects which attempt to save themselves by flight are preyed upon by the birds, which are always to be seen hovering above the advancing army. The spider’s only resource is to hang from its thread in mid-air beneath the branch over which the ants are swarming, for the spider line is impracticable to the ant. Belt[[294]] has observed a spider escape the general destruction by this means.
Protective Coloration.—Examples are numerous in which the spider relies upon the inconspicuousness not of its nest, but of itself, to escape its natural foes. Its general hues and markings are either such as to render it not readily distinguishable among its ordinary surroundings, or the principle has been carried still further, and a special object has been “mimicked” with more or less fidelity.
This country is not rich in the more striking mimetic forms, but the observer cannot fail to notice a very general correspondence in hue between the spiders of various habits of life and their environment. Those which run on the ground are usually dull-coloured; tree-living species affect grey and green tints, and those which hunt their food amongst sand and stones are frequently so mottled with yellow, red, and grey, that they can scarcely be recognised except when in motion.
A few of our indigenous species may be mentioned as especially protected by their colour and conformation. Tibellus oblongus is a straw-coloured spider with an elongated body, which lives among dry grass and rushes. When alarmed it clings closely to a dry stem, remains motionless, and escapes observation by its peculiarity of colour and shape. Misumena vatia, another of the Thomisidae or Crab-spiders, approximates in colour to the flowers in which it is accustomed to lurk on the watch for prey. It is of a variable hue, generally yellow or pink, and some observers believe that they have seen it gently waving its anterior legs in a way which made them easily mistaken for the stamens of the flower stirred by the breeze. Its purpose appears to be to deceive, not its enemies, but its victims. It seems to be partial to the blooms of the great mullein (Verbascum thapsus), and Pickard-Cambridge has more than once seen it seize and overcome a bee which had visited the flower in search of honey. He has also observed it in the blossoms of rose and furze bushes.[[295]]
An Epeirid (Tetragnatha extensa) resembles Tibellus in its method of concealing itself when alarmed. It also possesses an elongated abdomen, of a grey-green tint, which it closely applies to one of the twigs among which it has stretched its net, at the same time extending its four long anterior legs straight before it, and in this position it lies perdu, and is very easily overlooked. Another Orb-weaver, Epeira cucurbitina, is of an apple-green colour, which is admirably calculated to conceal it among the leaves which surround its snare.
Most of our English Attidae, or Jumping-spiders, imitate closely the prevailing tone of the surfaces on which they are accustomed to hunt. This will be recognised in the familiar striped Wall-spider, Salticus scenicus, and we may also mention the grey Attus pubescens, which affects stone walls, and the speckled Attus saltator, which is hardly distinguishable from the sand which it searches for food.
Examples may also be found among the Lycosidae or Wolf-spiders. Of the prettily variegated Lycosa picta, Pickard-Cambridge says: “Much variation exists in the extent of the different portions of the pattern and in their depth of colouring, these often taking their prevailing tint from the colour of the soil in which the spider is found. The best marked, richest coloured, and largest examples are found on sandy and gravelly heaths, where there is considerable depth and variety of colouring.... But on the uniformly tinted greyish-yellow sandhills between Poole and Christchurch I have found a dwarf, pale yellow-brown variety, with scarcely any dark markings on it at all, the legs being of a uniform hue, and wholly destitute of dark annuli.”[[296]]
Mimicry.—In the island of Portland, a locality remarkable for the number of species peculiar to itself, there is found a spider, Micaria scintillans, very closely resembling a large blackish ant which frequents the same neighbourhood. Its movements, moreover, are exceedingly ant-like, as it hurries along in a zigzag course, frequently running up and down grass stems after the manner of those insects. It is a great lover of sunshine, and disappears as soon as the sun is obscured by a passing cloud.
Such resemblances, obvious enough in nature, and heightened by the behaviour of the mimetic form, are often by no means striking in the cabinet. In some American species of spiders, however, imitation of the ant has passed beyond the stage of a general resemblance as regards size and colour and method of progression. The head of the ant is well marked off from the body, and the thorax is frequently divided into distinct regions. These peculiarities are imitated by constrictions in the cephalothorax of mimetic spiders. The resemblance, moreover, is much increased by their habit of using but six legs for locomotion, and carrying the second pair as ants do their antennae. The best known examples of these spiders are Synageles picata and Synemosyna formica (see Fig. [215], C, p. 420), and even more striking resemblances have been observed among some undescribed South American species.
The object of such mimicry seems to vary in different cases. Sometimes the spider preys upon the ant which it resembles. Sometimes, again, by its disguise, it escapes the notice of the ant which would otherwise feed upon it. More often spider and ant are neutral as regards each other, but, under cover of its resemblance, the Arachnid is enabled to approach an unsuspecting victim to which the ant is not a terror. Again, the unpleasantly acid taste of ants is unpalatable to most birds, though not to all, and the increased danger from specially ant-eating birds may be more than counterbalanced by the immunity they acquire from other birds.
There is quite a large class of Spiders of nocturnal habits, whose only precaution by day is to sit perfectly still and be mistaken for something else. We have referred to the adaptation in colour of our English species, Misumena vatia, to the flowers in which it lies in wait for prey. Bates[[297]] mentions exotic examples of the same family which mimic flower-buds in the axils of leaves. Herbert Smith says of a spider which sits upon a leaf waiting for prey: “The pink three-lobed body appears just like a withered flower that might have fallen from the tree above; to the flies, no doubt, the deception is increased by the strong sweet odour, like jasmine.”
Trimen[[298]] describes a Cape Town species which is of the exact rose-red of the flower of the oleander. “To more effectually conceal it, the palpi, top of the cephalothorax, and four lateral stripes on the abdomen are white, according remarkably with the irregular white markings so frequent on the petals of Nerium.”
The same observer, approaching a bush of the yellow-flowered Senecio pubigera, noticed that two of the numerous butterflies settled upon it did not fly away with their companions. Each of these he found to be in the clutches of a spider, whose remarkable resemblance to the flower lay not only in its colour, but in the attitude it assumed. “Holding on to the flower-stalk by the two hinder pairs of legs, it extended the two long front pairs upward and laterally. In this position it was scarcely possible to believe that it was not a flower seen in profile, the rounded abdomen representing the central mass of florets, and the extended legs the ray florets; while, to complete the illusion, the femora of the front pair of legs, adpressed to the thorax, have each a longitudinal red stripe which represents the ferruginous stripe on the sepals of the flower.”
Cambridge found in Palestine some species of Thomisidae which, when at rest, were indistinguishable from bits of coarse fleecy wool, or the rough seeds of some plant.
There is perhaps no more curious case of mimicry than that of a spider, Phrynarachne (= Ornithoscatoides) decipiens, which Forbes discovered in Java while butterfly-hunting. It appears that butterflies of the Family Hesperidae have a custom of settling, for reasons best known to themselves, upon the excreta of birds, dropped upon a leaf. Forbes noticed one in this position. Creeping up, he seized the butterfly, but found it mysteriously glued by the feet. On further investigation the “excreta” proved to be a spider. So accurate was the mimicry that he was again completely deceived by the same species in Sumatra. Its habit is to weave upon a leaf a small white patch of web, of a shape which greatly assists the deception, and in the midst of this it lies on its back, holding on by the spines with which its legs are furnished. It then folds its legs over its thorax, and waits for some insect to settle upon it.
In rare cases spiders have come to resemble their enemies the Ichneumon flies. A frequent habit of these insects is to deposit their eggs in the newly-formed cocoon of the spider. The Ichneumon eggs are the first to hatch, and the larvae have a convenient food-supply at hand. Sometimes, however, they adopt another method, and insert their eggs into the body of the spider itself. It is probably in order to avoid this unpleasant contingency that the spider has evinced towards the Ichneumon the sincerest form of flattery.
The Senses of Spiders.
Sight.—Though, as has been shown, spiders are well provided with eyes, their power of vision, in most cases, is by no means remarkable. As might be expected, it is less developed in those of sedentary than in those of nomadic habit.
It is noticeable that, in most spiders, some of the eyes are of a pearly grey colour, and others of a much darker hue. Simon designates the former nocturnal and the latter diurnal eyes, according to the special use which he believes them to subserve.
This view of the matter cannot be regarded as at all established, and has not found general acceptation. Moreover, Pillai[[299]] has shown that certain Attid spiders can change the colour of their eyes by a movement of the internal mechanism. The Epeiridae, spinners of the round web, are certainly, as a rule, very dim-sighted creatures. A fly may be held within an inch of them, but, unless it buzz, it will excite no notice whatever. A careful observation of the performances of the large Garden-spider in securing her prey will soon convince the onlooker that she is guided almost entirely by appeals to her sense of touch communicated along the tremulous lines of her snare. Interpreting these too hastily, she will sometimes rush straight past the entangled fly, and wait for it to renew its struggles before making sure of its whereabouts. Keen sight would be of little utility to such spiders, as they are concerned with nothing beyond the limits of their snare, and within its range they are furnished with the equivalent of complete telegraphic communication.
That most of the vagabond spiders can see well within the range of several inches there is no doubt, though some observers have been misled by the result of certain experiments on the Lycosidae, or “Wolf-spiders.” It will be remembered that the female Lycosid carries her egg-bag about with her, attached usually to her spinnerets. If it be removed and placed close at hand, the spider experiences the greatest difficulty in finding it again. Lubbock attributed this to defective sight, whereas it merely arises from unfamiliarity with the appearance of the egg-bag, which, since its construction, has been so situated as to be out of the view of the spider. Peckham found that spiders of the genus Theridion, accustomed to the sight of their cocoons, readily recognised them by that sense when removed to a distance.
The most keen-sighted of the spider tribe are undoubtedly the Attidae, or Leaping-spiders. The little black and white striped Wall-spider, Salticus scenicus, is probably a familiar object to most of our readers, and a very little observation of its movements, like those of a cat stalking a bird, will convince the observer that its visual powers are wonderfully keen and accurate. Its attitude of “attention” on sighting its prey, its stealthy manœuvring to approach it unobserved, and the unerring certainty of its final leap, are very interesting to witness.
It is somewhat noticeable that both in the Epeiridae and in the Attidae the two portions of the body, cephalothorax and abdomen, have more than the usual freedom of independent motion. In the Orb-weavers this gives play to the spinnerets in binding up a captured insect, but in the Leaping-spiders it allows of the rapid directing of the large anterior eyes towards the quarry, as it continually alters its position.
Professor and Mrs. Peckham of Wisconsin[[300]] performed some interesting experiments to ascertain the sensitiveness of the spider’s eye to colour. Freely communicating compartments of differently coloured glass were constructed, and spiders were confined in them, when it was found that red was the most and blue the least attractive hue. This agrees well with what Lubbock found to be the case with ants, but those insects displayed a greater antipathy for blue and not so marked a preference for red.
Hearing.—Most of our knowledge about the auditory sense of spiders is due to experiments performed by C. V. Boys,[[301]] and repeated by Professor and Mrs. Peckham.
The spider usually responds to the stimulus in one of two ways; it either raises its front legs, extending them in the direction of the sound, or it allows itself to drop suddenly, as though in alarm. It was only in the case of the Epeiridae that any results were obtained, and these spiders were more sensitive to low than to high notes. Now, as M‘Cook points out, it is exceedingly strange that the nomadic and hunting spiders, to which the sense of hearing might be expected to be extremely useful, should be deficient in this faculty, while the sedentary spiders, to which it would appear comparatively unimportant, should possess it in a tolerably developed form. That writer may possibly be correct in supposing that the sense, as possessed by spiders, is hardly differentiated from that of ordinary touch, and that the web-making species are only aware of sounds by the vibrations communicated to their feet by the medium of the web. However this may be, we must reluctantly but sternly reject the numerous and seemingly authentic stories, often connected with historic personages, which credit the spider with a cultivated taste for music.
We have seen that among the spiders which possess a stridulating apparatus it is confined, in certain groups, to the male, or if present in the female it exists only in a rudimentary form. If in these cases stridulation has been rightly interpreted as a sexual call, the power of hearing, at least in the female, is of course connoted. The spiders in question are members of the Theridiidae, a family closely allied to the Epeiridae, and therefore more likely than most groups to possess the power of hearing.
Theraphosid spiders show no response to the stimulus of sound, and among them stridulation is not confined to one sex. If, as is generally believed, the organ is used to warn off enemies, it is not necessary that the sound produced should be audible to the spider itself. If there be any true hearing organ in spiders its location is quite uncertain. Some have supposed the so-called lyriform organs in the legs to have an auditory function, while others have supposed the power of hearing to reside in certain hairs, of which there are several different types distributed over the body and limbs of the animal.
Spider Intelligence.—The experiments performed by the Peckhams clearly proved that spiders have short memories—a sure indication of a low state of intelligence. Members of the Lycosid or “Wolf-spider” group, when deprived of their cocoons, recognised them again after a few hours, but in most instances they refused to resume them after a lapse of twenty-four hours, and in every case an absence of two days sufficed to prevent any sign of recognition on their restoration. Moreover, when, after a shorter interval, the cocoons of other spiders, even of different genera, were offered to them, they appeared equally satisfied, and attached them in the orthodox manner, beneath the abdomen. The same treatment was even accorded to pith balls, which, if of the right size, seemed to be a perfectly satisfactory substitute. The contents of one cocoon were replaced by a shot three or four times their weight, but the spider accepted it with alacrity, spending half an hour in refixing it, when its weight caused it to fall from its attachment.
The habit of “feigning death,” which seems to be especially characteristic of the Epeiridae or orb-weaving spiders, probably arises from no desire to deceive its adversary as to its condition, but from an instinct to remain motionless, and therefore inconspicuous. Where a nomadic spider seeks safety in flight, a sedentary species finds a greater chance of escape in dropping a certain distance, and, while still attached by its silken line, giving as little evidence of its whereabouts as possible—trusting, in many cases, to its protective colouring. This method, moreover, has the advantage of facilitating its return to the web when the danger is past—a feat of which it would be quite incapable were it once to relinquish its clue.
All the remarkable and apparently intelligent actions of these creatures seem to be done in obedience to a blind instinct, which is obeyed even when there is no longer any object to be served. We have seen how the Trap-door spiders decorate the lids of their nests with moss even when the surrounding ground is bare, and Agelena labyrinthica has been observed to go through the whole lengthy and laborious operation of constructing its egg-cocoon though all its eggs were removed immediately on being laid.[[302]]
Mating Habits.—The sex of a mature spider can readily be recognised by the palpus which, as we have seen, is furnished in the male with a “palpal organ.” After the last moult but one the palp appears tumid, but it is only at the last moult that the organ is fully formed, and that the genital orifice is visible under the anterior part of the abdomen.
No alteration takes place in the female palp at maturity, but it is only after the last moult that the “epigyne” is distinguishable.
Fig. [198].—Argiope aurelia, ♂ and ♀, natural size.
That the palpal organs are used in the fertilisation of the female has long been established. How they came to contain the sperm matured in the abdomen was a problem which has only been solved comparatively recently. No direct connection could be found by way of the palpus with the abdominal organs, which, indeed, were seen to have an orifice between the lung-sacs. It is now known that some spiders at all events spin a slight web upon which they deposit a drop of spermatic fluid, which they afterwards absorb into their palpal organs for transference to the female. Secondary sexual differences are often very marked, the male being almost invariably the smaller in body, though its legs are frequently longer and more powerful than those of the female.
Among some of the sedentary spiders the disparity in size is excessive. The most striking examples are furnished by the Epeirid genera Argiope and Nephila, the male in some instances not attaining more than the thousandth part of the mass of the female. The coloration of the sexes is frequently quite dissimilar, the male being usually the darker, though in the Attidae he is in many cases the more strikingly ornamented.
In the minute Theridiid spiders of the group Erigoninae (see p. 404), the male cephalothorax often presents remarkable and characteristic excrescences not observable in the female. Some curious examples of this phenomenon may be seen in Fig. [209].
To the ordinary observer male spiders will appear to be comparatively rare, and to be greatly outnumbered by the females. This is probably to some degree true, but the unsettled habits of the males and the shorter duration of their life are calculated to give an exaggerated impression of their rarity. They only appear in considerable numbers at the mating season, shortly after which the males, in the case of many species, may be sought for in vain, as, after performing their functions, they quickly die. The snares they spin are often rudimentary, their capabilities in this direction appearing to deteriorate after the adult form is attained. Young spiders of indistinguishable sex make perfect snares on a small scale, while such as eventually develop male organs will often thereafter be content with a few straggling lines made with very slight regard to symmetry. They become nomadic in their habits, wandering off in search of the females, and pitching a hasty tent by the way.
The relations between the sexes in the Spider tribe present points of extreme interest, but in this connexion the various groups must be separately treated on account of their very different habits of life.
In no group are these relations more curious than in the Epeiridae, the constructors of the familiar wheel-like web. Love-making is no trifling matter here. If the female is not in the mood for the advances of the male she will probably regard him as a desirable addition to her larder. Even if his wooing is accepted, he has to beat a precipitate retreat after effecting his purpose, or he may fall a victim to his partner’s hunger.
This strange peril braved by the male in courting the female, which has, as far as is known, no parallel in any other department of the animal kingdom, is frequently mentioned as universal among spiders. It unquestionably exists, and may be verified by any patient observer in the case of the large Garden-spider Epeira diademata, but it has only been observed among certain species of the Epeiridae and Attidae. It will be remembered that in the Epeiridae the males are sometimes absurdly small in comparison with the females, and this diminution of size is thought to have a direct connection with the danger undergone at the mating season. Small active males stand a better chance of escape from ferocious females, so that natural selection has acted in the direction of reducing their size as far as is compatible with the performance of their functions.
Pickard-Cambridge[[303]] cites an extreme case. He says: “The female of Nephila chrysogaster, Walck. (an almost universally distributed tropical Epeirid), measures 2 inches in the length of its body, while that of the male scarcely exceeds ⅒th of an inch, and is less than ¹⁄₁₃₀₀th part of her weight.”
During the mating season the males may be looked for on the borders of the snares of the females. Their action is hesitating and irresolute, as it well may be, and for hours they will linger on the confines of the web, feeling it cautiously with their legs, and apparently trying to ascertain the nature of the welcome likely to be extended to them. If accepted, they accomplish their purpose by applying their palps alternately to the epigyne of their mate. If repulsed, they do their best to make their escape, and wait for a more auspicious moment. Emerton[[304]] says: “In these encounters the males are often injured; they frequently lose some of their legs; and I have seen one, that had only four out of his eight left, still standing up to his work.”
Among the other groups of sedentary spiders the relations between the sexes seem to be more pacific, and there is even some approach to domesticity. Males and females of Linyphia may be found during the mating season living happily together in their irregular snares. The same harmony seems to exist among the Tube-weavers, and Agelena labyrinthica lingers for days unmolested about the web of the female, though it is perhaps hardly correct to say that they have their home in common.
Among the wandering spiders the male usually seeks out the female and leaps on her back, from which position his sperm-laden palps can reach their destination. This is the habit of the Thomisidae or Crab-spiders, and of the quick-running Wolf-spiders, or Lycosidae.
Fig. [199].—Male Astia vittata dancing before the female. (After Peckham.)
The sexual relations of the Leaping-spiders, or Attidae, are so remarkable as to deserve a longer notice. This Family includes the most beautiful and highly ornamented examples of spider life. Their headquarters are the tropics, and their brilliant colouring led Wallace to speak of those he saw in the Malay Archipelago as “perfect gems of beauty.”
Now among these spiders the male is almost always more highly decorated than the female, and Peckham’s observations would lead to the conclusion that the female is influenced by the display of these decorations in the selection of her mate.
The so-called “love-dances” of certain tropical birds are known to all readers of natural history, but it was hardly to be expected that their counterpart would exist among spiders. Yet the antics by which male Attidae endeavour to attract the attention of the females afford an almost exact parallel.
The following extract from the account of Professor and Mrs. Peckham[[305]] of their observations on Saitis pulex will make this abundantly clear: “When some four inches from her he stood still, and then began the most remarkable performances that an amorous male could offer to an admiring female. She eyed him eagerly, changing her position from time to time, so that he might be always in view. He, raising his whole body on one side by straightening out the legs, and lowering it on the other by folding the first two pairs of legs up and under, leaned so far over as to be in danger of losing his balance, which he only maintained by sidling rapidly towards the lowered side.... Again and again he circles from side to side, she gazing towards him in a softer mood, evidently admiring the grace of his antics. This is repeated until we have counted a hundred and eleven circles made by the ardent little male. Now he approaches nearer and nearer, and when almost within reach whirls madly around and around her, she joining with him in a giddy maze. Again he falls back and resumes his semicircular motions, with his body tilted over; she, all excitement, lowers her head and raises her body so that it is almost vertical; both draw nearer; she moves slowly under him, he crawling over her head, and the mating is accomplished.”
Fig. [200].—Dancing attitude of male Icius mitratus. (After Peckham.)
A similar but not exactly identical performance was gone through by the male of several different species, but it was noteworthy that the particular attitudes he adopted were always such as to display to the best advantage his special beauties, whether they consisted in crested head, fringed palpi and fore-legs, or iridescent abdomen. Sometimes even such exertions failed to captivate the female, and she would savagely attack the male, occasionally with fatal effect.
In the case of some species, when the male had won the consent of his mate, he would weave a small nuptial tent or web, into which he would partly lead and partly drive the female, who no longer offered serious resistance.
Fossil Spiders.
About 250 species of fossil spiders have been discovered. Of these about 180 are embedded in amber, a fossil resinous substance which exuded from ancient coniferous trees, and quantities of which are annually washed up from the Baltic upon the shores of northern Prussia.
The most ancient fossil spider known was obtained from the argillaceous slate of Kattowitz in Silesia, and belongs, therefore, to the Carboniferous strata of the Palaeozoic epoch. It has been named Protolycosa anthrocophila. There is some doubt as to the affinities of this spider. Roemer, who described it, placed it among the Citigradae, while others have thought it to belong rather to the Territelariae. Thorell, on account of its agreement in certain important points with the very curious recent Malay spider Liphistius, has placed them both in a separate sub-family, Liphistioidae. To the same epoch belongs the American fossil spider Arthrolycosa antiqua, which was found in the Coal measures of Illinois.
The other localities from which fossil spiders have been obtained are the Swiss Miocene at Oeningen, the Oligocene deposits at Aix, the Oligocene of Florissant, Colorado, Green River, Wyoming, and Quesnel, British Columbia.
Many of the spiders from the rocks are so fragmentary that it is impossible to decide with certainty on their systematic position, but a considerable number of them—more than half—have been assigned to recent genera.
The amber spiders are mostly well preserved, and can be classified with more certainty. Many of them are surprisingly like existing forms, though others, like Archaea paradoxa, differ greatly from most spiders now extant, though they show some affinities with one or two remarkable and aberrant forms.
CHAPTER XV
ARACHNIDA EMBOLOBRANCHIATA (CONTINUED)—ARANEAE (CONTINUED)—CLASSIFICATION
The systematic study of Spiders has hitherto presented very great difficulties. There is an extensive literature on the subject, but the more important works are costly, not commonly to be found in libraries, and written in diverse languages. Moreover, the nomenclature is only now emerging from a condition of chaos. Able and diligent Arachnologists have done admirable work in studying and describing the Spider fauna of their various countries, and occasional tentative suggestions have been put forth with a view to reducing to some sort of order the vast mass of heterogeneous material thus collected. Most schemes of classification, based chiefly upon a knowledge of European forms, have proved quite inadequate for the reception of the vast numbers of strange exotic species with which recent years have made us acquainted. The number of described species is very large, and is rapidly increasing; but though we are very far indeed from anything like an exhaustive knowledge of existing forms, it may now be said that almost every considerable area of the earth’s surface is at least partially represented in the cabinets of collectors, and it is possible to take a comprehensive view of the whole Spider fauna, and to suggest a scheme of classification very much less likely than heretofore to be fundamentally deranged by new discoveries.
The first to apply the Linnaean nomenclature to Spiders was Clerck, in his Araneae Suecicae (1757), which gives an account of seventy spiders, some of which are varieties of the same species. A few new species were added by Linnaeus, De Geer, Scopoli, Fabricius, etc., but the next work of real importance was that of Westring (1861), who, under the same title, described 308 species, divided among six families. Blackwall’s beautiful work, the Spiders of Great Britain and Ireland, was published by the Ray Society in 1864. He divides spiders into three tribes, Octonoculina, Senoculina, and Binoculina, according to the number of the eyes, and describes 304 British species, distributed among eleven families.
His successor in this country has been Pickard-Cambridge, whose work, under the modest title of The Spiders of Dorset (1879–81), is indispensable to British collectors.
Blackwall’s division of the order into tribes was evidently artificial, and has not been followed by later Arachnologists. Dufour (1820) founded two sub-orders, Dipneumones and Tetrapneumones, based on the presence of two or four pulmonary sacs. Latreille (1825) established, and many Arachnologists adopted, a division into tribes based upon habits, Orbitelariae, Retitelariae, Citigradae, Latigradae, etc., and this method of classification was followed in the important work of Menge, entitled Preussische Spinnen, which was published between 1866 and 1874.
Since 1870 determined efforts have been made to grapple with the difficult subject of Spider classification, notably by Thorell and Simon. The latter, undoubtedly the foremost living Arachnologist, writes with especial authority, and it is inevitable that he should be largely followed by students of Arachnology, who cannot pretend to anything like the same width of outlook.
It is indicative of the transition stage through which the subject is passing that Simon in his two most important works,[[306]] propounds somewhat different schemes of classification, while in the Histoire naturelle, where his latest views are to be found, he introduces in the course of the work quite considerable modifications of the scheme set forth in the first volume.
In that work the order is divided into two sub-orders, Araneae theraphosae and Araneae verae, the first sub-order containing Liphistius and the Mygalidae or Theraphosidae of other authors, while all other spiders fall under the second sub-order. The Araneae verae are subdivided into Cribellatae and Ecribellatae, according to the presence or absence of “cribellum” and “calamistrum” (see p. [326]) in the female. Important as these organs doubtless are, the Cribellatae do not appear to form a natural group, some of the families having apparently much closer affinities with certain of the Ecribellatae than with one another. This is especially evident in the case of the cribellate Oecobiidae and the ecribellate Urocteidae (see p. [392]), which most authors unite in a single family.
After all, the larger divisions of the order are not of great importance, and in the present chapter Simon’s linear arrangement of families will in the main be followed, except for the distribution of the eight families which constitute his Cribellatae[[307]] to the positions which a more general view of their structure would seem to indicate.
Fam. 1. Liphistiidae.—Spiders with segmented abdomen, as shown by the presence of a series of tergal plates. Eight spinnerets in the middle of the ventral surface of the abdomen, far removed from the anal tubercle. Sternum long and narrow. Eight compact eyes on a small eminence. Four pulmonary stigmata.
Fig. [201].—Profile (nat. size) and ocular area (enlarged) of Liphistius desultor.
This Family includes a single genus and two species of large spiders (about two inches in length), one from Penang and one from Sumatra. Very few examples have been found, and these are more or less defective and in bad condition. In some respects, especially the distinct segmentation of the abdomen, this genus much more nearly approaches the Pedipalpi than do any others of the order. No other spider possesses more than six spinning mammillae, but it is possible that eight was the more primitive number, and that the “cribellum” (see p. [326]) of the so-called Cribellate spiders is derived from the pair now possessed by Liphistius alone.
Some Arachnologists consider the genus Liphistius so different from all other spiders as to constitute in itself a sub-order, for which, on account of the position of its spinnerets, the name MESOTHELAE has been suggested, all other forms falling into the sub-order OPISTHOTHELAE.
Fam. 2. Aviculariidae. (Mygalidae).[[308]]—Spiders with independent chelicerae, the paturon directed forward and the unguis or fang articulating in a vertical plane. The eyes are eight (except Masteria, six), usually compact, and situated on an eminence. Pedipalpi very leg-like, and palpal organs of male simple. No maxillae. Four pulmonary stigmata. Spinnerets normally four. No colulus.
The Aviculariidae inhabit the warmer portions of the world, and are entirely unrepresented in this country. The monster spiders which excite wonder in zoological collections belong to this group, as do the moderate-sized “Trap-door” Spiders which are found abundantly in the Mediterranean region.
The Family has been divided into about a hundred and fifty genera, nearly half of which, however, contain only a single species.
They have been grouped by Simon[[309]] into seven sub-families, Paratropidinae, Actinopodinae, Miginae, Ctenizinae, Barychelinae, Aviculariinae, and Diplurinae, of which the first three may be dealt with very briefly.
(i.) The Paratropidinae include only two American species, Paratropis scrupea from the Amazon, and Anisaspis bacillifera from St. Vincent. They have thick, rugose integuments, and the internal angle of the coxa of the pedipalp is produced. The labium is fused with the sternum, which is very broad. Nothing is known of their habits, but as they do not possess a “rastellus” (see p. [320]) they are probably not burrowing spiders.
(ii.) The Actinopodinae comprise three genera, Stasinopus represented by a single South African species, S. caffrus; Eriodon, of which about ten species inhabit Australia; and Actinopus, of which about ten species are found in Central and South America. They have the coxae of the pedipalps very short and broad, and somewhat produced at the internal angle. The eyes are not in the usual compact group, but are somewhat extended across the caput. Actinopus burrows a deep cylindrical hole lined with silk, and furnished with a round, bevelled trap-door.
(iii.) The sub-family Miginae is established for the reception of three genera, Moggridgea (South Africa), Migas (Australia and South-West Africa), and Myrtale, whose single species, M. perroti, inhabits Madagascar. They are chiefly characterised by their very short and downwardly-directed chelicerae. They are not terricolous, but inhabit trees, either boring holes in the bark, or constructing a sort of silken retreat fortified by particles of wood.
(iv.) The Ctenizinae form a large group, including some forty genera. All the “Trap-door” Spiders of the Continent fall under this sub-family, which, moreover, has representatives in all the tropical and sub-tropical regions of the world. A rastellus is always present, and the eyes form a compact group on an eminence. The coxae of the pedipalps are longer than in the groups previously mentioned, and there is no production of the internal angle. The labium is generally free.
The commonest European genus is Nemesia, of which about thirty species inhabit the Mediterranean region. The cephalothorax is rather flat, and the central fovea is recurved (◠). The burrow is sometimes simple and sometimes branched, and the trap-door may be either thin, or thick with bevelled edges.
Allied genera are Hermacha and Rachias in South America, Spiroctenus in South Africa, Genysa in Madagascar, Scalidognathus in Ceylon, and Arbanitis in New Zealand. The genus Cteniza (fovea procurved ◡) possesses only a single species (C. sauvagei), found in South-East France and Italy.
Pachylomerus is a widely-distributed genus, being represented in North and South America, Japan, and North Africa. The tibiae of the third pair of legs are marked above by a deep impression near the base. A closely allied genus, Conothele. inhabits Southern Asia and New Guinea.
The widely-distributed genus Acanthodon, which has representatives in all the sub-tropical countries of the world, together with the South American genera Idiops and Pseudidiops, and the Indian genus Heligmonerus, present a peculiar arrangement of the eyes, one pair being situated close together in the middle of the front of the caput, while the remaining six form a more or less compact group some distance behind them.
Among the many other genera of the Ctenizinae may be mentioned Cyrtauchenius, of which many species inhabit North-West Africa, and its close ally Amblyocarenum, represented on both shores of the Mediterranean, and in North and South America. They differ from Cteniza chiefly in the possession of strong scopulae on the tarsi and metatarsi of the first pair of legs, and in the double row of teeth with which the tarsal claws are furnished. Their burrows are often surmounted by a sort of turret raised above the level of the ground.
(v.) The Barychelinae are burrowing forms which resemble Nemesia, but have only two tarsal claws. Leptopelma is the only European genus, and has close affinities with certain South American genera (Psalistops, Euthycoelus, etc.). Pisenor inhabits tropical Africa, and Diplothele, unique in possessing only two spinning mammillae, is an inhabitant of India.
(vi.) The Aviculariinae include all the large hairy spiders which are commonly called Mygale. The genus Phlogius, which inhabits Southern Asia, forms a lidless burrow, though it has no rastellus, but practically all the other members of the group are non-terricolous, living under stones or in holes in trees, where they weave a slight web. They are nocturnal in their habits. They all possess two tarsal claws, and the labium is free and spined at the tip. Of the four spinnerets the posterior pair are long and three-jointed, while the anterior are short and not very close together.
The particular form of the tarsi and the nature of the scopulae,[[310]] “claw-tufts,” and spines upon them are of great importance in distinguishing the members of this group.
The Aviculariinae comprise about sixty genera from all the tropical and sub-tropical regions of the world.
The genus Ischnocolus extends into the Mediterranean region, having representatives besides in Southern Asia and in Central and South America. All the tarsi have their scopulae divided longitudinally by a band of hairs. Chaetopelma inhabits Egypt, Syria, and Arabia, and Cyclosternum is found in West Africa as well as in Central and South America. In these genera the scopulae of the last two pairs of legs are alone divided. The largest known spider is Theraphosa leblondi, which is a native of Guiana. It measures 9 cm. (about three and a half inches) in length.
Eurypelma is a genus of large spiders entirely confined to the New World, where it possesses many species. The genus Avicularia is also American, and includes a number of large long-haired spiders with short and very strong legs, on which the scopulae and claw-tufts are well developed. Its nearest allies in the Old World are the Indian genus Poecilotheria, and the West African genus Scodra. The stridulating spider figured on p. 328 belongs to this group, Chilobrachys being a genus from Ceylon.
Fig. [202].—Ischnothele dumicola, ♀ × 2. (After Pocock.)
(vii.) The Diplurinae are a very aberrant group, including some twenty genera of Aviculariidae, usually of medium size, and possessed, as a rule, of very long posterior spinnerets. They do not burrow or live in holes or under stones, but weave webs of close texture, much resembling those characteristic of the Agelenidae (see p. [415]). The tarsal claws are three in number, and there are never any claw-tufts. The rastellus, of course, is absent.
Two genera have representatives in Europe, Brachythele inhabiting the East Mediterranean region (as well as many other parts of the world), while Macrothele is found in Spain as well as in the Malay Peninsula and New Zealand. Ischnothele dumicola is a native of Western India. Diplura is a South American genus. Trechona venosa, a large species remarkable for the orange bands which decorate its abdomen, is also a native of South America. The New Zealand genus Hexathele, and the genus Scotinoecus from Chili, possess six spinnerets. Masteria (Ovalan Island) and Accola (Philippines and South America) differ from the rest of the family in having only six eyes.
Fam. 3. Atypidae.—Spiders with anteriorly projecting and vertically articulating chelicerae, but with no trough on the paturon for the reception of the unguis, which is guarded when closed by a single row of teeth. The spinnerets are normally six, and the anal tubercle is above, and well removed from the posterior spinnerets.
Fig. [203].—Atypus affinis, ♀.
The Atypidae are a small family of six genera, rather closely related to the Aviculariidae, and by some Arachnologists incorporated with them. They may be regarded as the representatives of that family in sub-tropical and temperate regions. In form they are strongly built, with smooth integuments, and their legs are short and powerful. Of the twenty-four species hitherto described almost all belong to the northern hemisphere. Five are natives of Europe, and two are included in the English fauna. The best known is Atypus affinis, which has been found in several localities in the south of England, and which has occurred on the Devil’s Dyke, near Cambridge. The female measures about half an inch in length, the male being smaller. It burrows a deep cylindrical hole at the edge of a grassy or heathery bank and lines it with a loose tube of silk, which extends considerably beyond the orifice of the burrow, either lying flat on the ground, or raised up and attached to the neighbouring herbage. There is no lid, but the upper end of the tube is always found closed, whether by its elasticity or by the deliberate operation of the spider is not known. The animal is nocturnal in its habits. Another species, A. beckii, occurs very rarely in the south of England.
The genus Atypus has representatives in Central and South Europe, North Africa, Japan, Java, and North America. Of the other genera, Calommata inhabits Central and South-East Asia and Japan, Brachybothrium, Atypoides, and Hexura are peculiar to North America, while Mecicobothrium comprises a single species (M. thorelli) native to the Argentine.[[311]]
Fam. 4. Filistatidae.—Cribellate Spiders of moderate size, usually brown or yellow in colour, with smooth integuments and somewhat long tapering legs. The eight eyes are compactly arranged, and the palpal organs of the male are of simple structure. The six spinnerets are short, the anterior pair being thick and separated. Two pulmonary sacs, with two minute tracheal stigmata close behind them and widely separate.
There is but one genus, Filistata, in this family. About fifteen species have been described, five of which inhabit the Mediterranean region. Three are found in America, and others inhabit Central Asia, the Philippines, and Australia. The genus is not represented in this country, but one species, F. testacea, has an extremely wide distribution in the Old World, while F. capitata extends throughout the American continent.
The calamistrum of the female is short, only occupying a portion of the metatarsus of the fourth leg. The cribellum is divided. These spiders weave a web of close texture, of an irregular tubular form.
Fam. 5. Oecobiidae (Urocteidae).—Two very remarkable genera constitute this family, Oecobius and Uroctea.
The species of Oecobius, about fifteen in number, are small spiders, inhabiting sub-tropical countries—and especially desert regions—and spinning a slight web under stones, or in holes in walls. The female possesses a small transverse cribellum, the two halves of which are widely separated. The calamistrum is but feebly developed. No example has occurred in this country, but nine species have been described in the Mediterranean region.
Fig. [204].—A, Oecobius maculatus, much enlarged; B, Uroctea durandi, slightly enlarged. (After Simon.)
The three species of Uroctea are rather large spiders, two being native to Africa, while the third inhabits China and Japan. They are ecribellate. These two genera very closely resemble each other, not only superficially, but in certain structural details—notably the remarkably developed and two-jointed anal tubercle—and their close affinity supplies the strongest argument against separating the spiders which possess cribellum and calamistrum into a group by themselves. In both genera the cephalothorax is very broad and rounded at the sides. The eight eyes are compactly arranged. The sternum is broad and heart-shaped. The legs are nearly of equal length, and the posterior spinnerets have very long terminal joints.
Fam. 6. Sicariidae (Scytodidae).—The Sicariidae are a small group of six-eyed spiders, usually with weak legs and slow halting movements; they live under stones or in outhouses. The cephalothorax is generally smooth and devoid of the median fovea, and the palpal organs of the male are extremely simple. The best known genus is Scytodes, one species of which (S. thoracica) has on rare occasions been found in outhouses in the south of England, in Dorsetshire, and Kent. This is a remarkable spider, about one-third of an inch long, with a pale yellow ground-colour, marked with black spots and patches. The cephalothorax is smooth and dome-shaped, and highest near the posterior end.
All the other members of the family are exotic. Loxosceles is found in the Mediterranean region and all over America, as well as in Japan. The median fovea is present in this genus. Sicarius is a native of America and South Africa. It is of stouter build than Scytodes, and the legs are stronger. Drymusa belongs to South Africa. The peculiar New Zealand species Periegops hirsutus is placed by Simon in this family, as is also the North American genus Plectreurys, notwithstanding its possession of eight eyes.
Fam. 7. Hypochilidae.—Two species only are included in this family, Hypochilus thorelli of North America, and Ectatosticta davidi, a native of China. They have four pulmonary sacs, though they possess little else in common with the “Theraphosae.” The pedipalpus of the male is very remarkable, the tarsus being almost unmodified, and the very small palpal organ being inserted at its extremity. These spiders are cribellate.
Fam. 8. Leptonetidae.—The Leptonetidae are small spiders with smooth and usually dull-coloured integuments. Most of them are cave-living, but some are found amidst vegetable débris in damp spots in forests. The eyes are six in number, and the legs are generally long and thin. There are five genera. Leptoneta has about ten species living in caves in the Pyrenees. The single species of Telema (T. tenella) has the same habitat. Ochyrocera has representatives in tropical Asia and America, and is somewhat more ornate than most members of the group. Usofila has a single species, inhabiting North America, while Theotina is found in caves in the Philippines and in Venezuela.
Fam. 9. Oonopidae.—The Oonopidae are very small spiders, seldom exceeding 2 mm. in length (the largest 4 mm.), living among vegetable débris. Oonops pulcher, the only English representative of the family, is not rare under stones or in the débris at the bottom of hedges. It is a small brick-red spider, easily recognised by its six comparatively large oval eyes, which are pale-coloured, and occupy the whole of the caput.
The minute spiders of this family were until recently overlooked by collectors in foreign countries, but now more than a hundred species have been described, belonging to some eighteen genera. Thirteen species inhabit the Mediterranean region, occurring especially on the African side. In several genera there is a “scutum” or hard plate on the abdomen. This is the case with Dysderina, which has a wide distribution, as have also Ischnyothyreus and Opopaea, and the non-scutate genus Orchestina.
Fam. 10. Hadrotarsidae.—This family contains only two species, Hadrotarsus babirusa from New Guinea, and Gmogala scarabeus from Sydney. In general appearance they resemble the scutate Oonopidae, but they have eight eyes, curiously arranged, two large, somewhat triangular eyes being situated near the middle of the cephalothorax, and two groups of three small eyes on either side of the front part of the caput. These spiders are very minute.
Fam. 11. Dysderidae.—Six-eyed spiders, with long free labium, and long maxillae provided with a well-developed scopula. The cephalothorax is rather flat, and the abdomen is oval or cylindrical, the integument being smooth and usually rather soft. The palpal organ of the male is of simple structure.
The Dysderidae are divided into two sub-families, Dysderinae and Segestriinae, for the most part confined to temperate regions.
(i.) The Dysderinae are easily recognised by a peculiarity of the sternum. Instead of being merely excavated along its border for the reception of the legs, its edge is folded round the coxae to meet the carapace, and thus forms a series of collars or sockets in which the limbs are articulated in perfect isolation from each other. These spiders vary considerably in size, and are generally of a somewhat uniform coloration, never marked with vivid patterns. There are eight genera of this sub-family, two of which are represented in England.
Dysdera cambridgii is not a rare spider under stones in rocky localities, such as the Isle of Portland, and occurs, though less commonly, all over the country in similar situations, and under the loose bark of trees. It is half an inch in length, with a chestnut-coloured cephalothorax and legs, and dull yellow abdomen. A closely allied species, D. crocota, also occurs more rarely.
Harpactes hombergii is common in vegetable débris and under decaying bark. It is about a quarter of an inch in length, of slender form, with black-brown cephalothorax and clay-coloured abdomen. The legs are yellowish and annulated. More than forty exotic species of Dysdera and twenty-four of Harpactes have been described. Another genus of the Dysderinae is Stalita, which comprises three species, inhabiting the caves of Dalmatia and Carniola.
(ii.) The Segestriinae include two genera, Segestria and Ariadna.
Segestria senoculata occurs in England in similar localities to those where Dysdera cambridgii is found. It is not much smaller than that spider, and has a dark brown cephalothorax and legs and a dull yellow abdomen, with a series of adder-like diamond-shaped black markings along the middle. Two other species have occurred on rare occasions in England, and twelve more are recorded from the various temperate regions of the world.
Ariadna is the only Dysderid genus which invades the tropical regions. It includes about twenty species.
Fam. 12. Caponiidae.—This is a small family of three genera and about twelve species, remarkable in having no pulmonary sacs but five tracheal stigmata,[[312]] and in the peculiar arrangement of their six spinnerets, those which are ordinarily median being in the same transverse line with the anterior ones.
The single species of Caponia (C. natalensis) inhabits South Africa, while Caponina has two species in South America. These spiders are eight-eyed, but the two median posterior eyes are much the largest, and these alone are present in the remarkable genus Nops, of which several species inhabit South America and adjacent islands.
Fam. 13. Prodidomidae.—This small family includes about twenty species of minute spiders from sub-tropical regions. They are eight-eyed, with short smooth legs, terminated by two claws not dentated. The spinnerets are especially characteristic.
Prodidomus (Miltia) includes fifteen species from the Mediterranean region, Africa, and America. Zimris is an Asiatic genus. The single species of Eleleis (E. crinita) is from the Cape.
Fig. [205].—Drassid Spiders. 1. Drassus lapidosus. 2. Clubiona corticalis. 3. Zora spinimana. 4. Micaria pulicaria.
Fam. 14. Drassidae.—Elongate spiders with low cephalothorax. Legs usually rather long, strong, and tapering, terminated by two pectinate claws, armed with spines, and scopulate. The body is smooth or short haired and frequently unicolorous and sombre-coloured, seldom ornate. The eyes, normally eight, are in two transverse rows. The mouth-parts (labium and maxillae) are long. Spinnerets as a rule terminal, and visible from above.
This important family includes a large number of species from all parts of the world, fifty-six being natives of the British Isles. There are familiar examples in the brown or mouse-coloured spiders which scurry away when stones are raised, or when loose bark is pulled off a tree.
The family may be divided into seven sub-families, of which four, Drassinae, Clubioninae, Liocraninae, and Micariinae, are represented in this country.
(i.) The Drassinae include more than twenty genera, some of which possess numerous species and have a wide distribution. The following may be mentioned:—
Drassus contains twelve British species. The commonest is D. lapidosus, a large dull brown spider, more than half an inch in length, which lives beneath stones in all parts of the country. At least a hundred species of this genus have been described.
Melanophora (= Prosthesima)[[313]] includes a large number of species. They are dark-coloured active spiders, many of them jet black and glossy. Seven are recorded from the British Isles, the average size being about a quarter of an inch. They are found under stones. A closely allied genus is Phaeocedus, whose single species (P. braccatus) has occurred, though very rarely, in the south of England. Gnaphosa has fifty-five species, of which twenty-eight are European, and four are British.
(ii.) The Clubioninae have the anterior spinnerets closer together, and the eyes more extended across the caput than in the foregoing sub-family. Nearly thirty genera have been established, of which three claim special attention. Clubiona includes more than 100 species, chiefly inhabiting temperate regions. Fifteen are included in the British list. They are mostly unicolorous, and yellow or brown in colour, but a few (C. corticalis, C. compta, etc.) have a distinct pattern on the abdomen. Cheiracanthium is a large and widely spread genus, counting three English species. There are more than a hundred species of the genus Anyphaena, of which one only (A. accentuata) occurs in this country, where it is common upon bushes and trees in the south.
(iii.) The Liocraninae include about twenty-four genera, of which Zora, Liocranum, Agroeca, and Micariosoma are sparingly represented in this country.
(iv.) The Micariinae are a remarkable group of Spiders containing numerous ant-like mimetic forms. Two species of Micaria alone are English, but that genus is abundantly represented on the Continent, where the species mount up to forty. They are mostly small, dark, shining spiders, which, though not particularly ant-like in form, recall those insects both by their appearance and movements. Some of the exotic genera, and particularly the South American genus Myrmecium, possess remarkable instances of mimetic resemblance to ants. Micaria pulicaria is a very pretty little spider, about a sixth of an inch in length, black, with iridescent hairs, and some white marks on the abdomen. It runs about in a very active ant-like fashion and does not object to the sunshine. It is fairly abundant in England.
Fam. 15. Palpimanidae.—This family includes a few genera of exotic spiders. They are especially characterised by the great development of their anterior legs, which are not much used for locomotion, but are frequently raised as the spider moves along, generally somewhat slowly, by means of the other three pairs. The best known genera are Metronax and Stenochilus from India, Huttonia from New Zealand, and Palpimanus from the Mediterranean region, Africa, and South Asia.
Fam. 16. Eresidae.—The Eresidae are a small family of cribellate spiders whose systematic position has been the subject of much discussion. In general appearance they resemble the Attidae (vide infra), but this resemblance is quite superficial. On the whole they seem more nearly allied to the following family than to any other. They are stoutly built, with thick, strong legs, and live either in the ground or on bushes, where they weave a close-textured web. One species, Eresus cinnaberinus, has occurred on rare occasions in the south of England, and the male, which is a third of an inch in length, is perhaps the most striking member of our Spider fauna, the abdomen being scarlet, with four (or sometimes six) black spots edged with white hairs. The cephalothorax is black, with red on the postero-lateral borders. The abdomen of the female is black.
Fam. 17. Dictynidae.—Cribellate spiders, with oval cephalothorax and broad convex caput, with the eyes, normally eight, ranged across it in two straight or slightly curved transverse rows. Basal joints of chelicerae long and strong, often bowed. Legs rather strong. Tarsi three-clawed and devoid of scopula.
The Dictynidae are sedentary spiders which weave a web of irregular strands, covered by the close weft which is the product of the cribellum. Some live under stones or in holes in walls, while others spin their webs in bushes or herbage. There are about sixteen genera, of which Dictyna and Amaurobius are the most important.
Nearly a hundred species of Dictyna have been described. They are small spiders, usually living in grass and herbage. Thirty species inhabit Europe and the neighbouring coast of Africa, and eight of these are natives of Britain. D. arundinacea is very abundant, especially in heather. It is about an eighth of an inch in length. D. uncinata is also often met with. Amaurobius, of which about eighty species are known, includes some species of much larger size. Three species are native to this country, A. ferox, A. similis, and A. fenestralis. A. ferox is a large and rather formidable-looking spider, more than half an inch in length, with powerful chelicerae. It is found under stones and bark, and in cellars and outhouses. A. similis is the commonest species in England, though A. fenestralis somewhat replaces it in the north. They are smaller than A. ferox, but are found in similar situations.
Fam. 18. Psechridae.—This is a small family of cribellate spiders, consisting only of two genera, Psechrus and Fecenia, and some eight species, all natives of Southern Asia and the adjacent islands. The two species of Psechrus are large spiders. They make large domed webs, which they stretch between trees or rocks, and beneath which they hang in an inverted position.
The calamistrum of these spiders is short, about half the length of the fourth metatarsus.
Fam. 19. Zodariidae (Enyoidae).—In this family are included a number of remarkable exotic spiders, most of them somewhat Drassid-like in appearance, but generally with three-clawed tarsi. The group appears to be a somewhat heterogeneous one, the twenty genera of which it consists presenting rather a wide range of characteristics.
Cydrela is an African genus of moderate sized spiders, containing five species of very curious habits. They scramble about and burrow in the sand, in which, according to Simon,[[314]] they appear to swim, and their chief burrowing implements are their pedipalpi, which are specially modified, the tarsi in the female bristling with spines, and being armed with one or more terminal claws.
Laches (Lachesis) includes some larger pale-coloured spiders found in Egypt and Syria, under stones in very hot and dry localities.
Fig. [206].—Hermippus loricatus, ♂ × 2½. (After Simon.)
Storena has representatives in all the tropical and sub-tropical parts of the world, and numbers about fifty species. They are of moderate size, with integuments smooth and glossy or finely shagreened, usually dark-coloured, with white or yellow spots on the abdomen. Hermippus (Fig. [206]) is also African. Zodarion (Enyo) includes about thirty-five species of rather small, generally unicolorous spiders, very active and fond of the sunshine. They spin no web, but have a retreat under a stone. Their chief prey appear to be ants. Most of the species are native to the Mediterranean region, the others belonging to Central and Southern Asia.
Simon includes in this family the remarkable genus Cryptothele, found in Ceylon, Malacca, New Guinea, and various Oceanic islands. They are moderate sized brownish spiders, with hard integuments rugged with tubercles and projections. Their most curious characteristic is their power of retracting their spinnerets within a sort of sheath, so that they become entirely invisible.
Fig. [207].—Hersilia caudata, ♀. (After Pickard-Cambridge.)
Fam. 20. Hersiliidae.—This is a very distinct family of spiders, with broad cephalothorax, with well-marked fovea and striae, and small, well defined caput. The eyes, usually eight, are black except the median anterior pair. The legs are long and thin, and the tarsi three-clawed. The abdomen is oval or sub-globular, short haired, and generally of greyish coloration. The spinnerets supply the chief characteristic, the posterior pair being long—often excessively long—and two-jointed, the terminal joint tapering and flexible. The colulus is large. They are very active spiders, living on tree trunks or walls, or under stones, but spreading no snare. Some of them are of considerable size.
Hersilia includes nine species native to Africa and Asia. Tama is the only genus represented in the New World, two of its species being found in South America, while others inhabit Africa, Asia, and Australia. Another genus, Hersiliola, is principally African, but extends into Spain.
Fam. 21. Pholcidae.—This is another very well-marked family. The most striking peculiarity of its members is the possession of extremely long and thin legs, the metatarsi being especially elongated, and the tarsi furnished with several false articulations.
The eyes are also very characteristic. They are usually eight in number, the two anterior median eyes being black, while the other six are white, and arranged in lateral groups of three, sometimes on prominences or stalks. The abdomen is sometimes nearly globular, but more often long and cylindrical. Most of the genera, which, including several new genera lately established by Simon, number more than twenty, are poor in species, but enjoy a very wide distribution. This is explained by the fact that many of them live in cellars and outhouses. This is the case with the genus Pholcus, of which the sole English species Ph. phalangioides is a perfect nuisance in buildings in the most southern parts of the country, “spinning large sheets of irregular webs in the corners and angles, and adding to them year by year.”[[315]] Other genera are Artema (Africa, South Asia, Polynesia, America), which includes the largest examples, and Spermophora, a six-eyed genus whose few species are widely distributed.
Fam. 22. Theridiidae.—Sedentary spiders, usually with feeble chelicerae and relatively large abdomen. Snare irregular.
The Theridiidae, as here understood, are a very extensive family, and more than half the British spiders (about 270 species) are included within it. This family and the next present unusual difficulties of treatment, and there is great divergence of opinion as to the most satisfactory way of dealing with them. This is chiefly due to the fact that, notwithstanding an infinite variation of facies, important points of structure are wonderfully uniform throughout both the two groups, while any differences that do occur are bridged over by intermediate forms which merge into each other.
Simon[[316]] has become so impressed with the difficulty of drawing any clear line between certain groups which he previously classed under the Theridiidae and the spiders commonly known as Epeiridae, that he has recently removed them from the Theridiidae and united them with the orb-weaving spiders to form the Family Argiopidae, the family name Epeiridae being discarded. The groups which, in his view, belong to the Argiopidae will be indicated below. This view has not met with universal acceptance, and notwithstanding the undoubted difficulty of clearly distinguishing between the two families, it is more convenient in the present work to maintain as a separate family a group of spiders nearly all of whose members possess the easily recognised characteristic of spinning a circular snare.
The Theridiidae and the Epeiridae form the great bulk of the sedentary spiders. They do not wander in search of prey, but sit in snares of various structure and wait for their victims to entangle themselves. The spinnerets, organs whose peculiarities are often strongly marked in other families, are here wonderfully constant in their arrangement and general appearance, forming a compact rosette-like group beneath the abdomen. Their eyes, normally eight in number, present an infinite variety of arrangement. Their chelicerae and mouth-parts vary considerably, but no abruptness of variation is distinguishable. This is unsatisfactory from a systematic point of view, and the necessary result is that certain groups might with equal propriety be classed with the Theridiidae or the Epeiridae. The latter family will here be taken as including all the orb-weaving spiders and a few groups which appear inseparable from them.
We shall consider the Theridiidae as comprising the seven sub-families, Argyrodinae, Episininae, Theridioninae, Phoroncidiinae, Erigoninae, Formicinae, and Linyphiinae, and shall briefly deal with them in this order.
(i.) The Argyrodinae are very curious spiders with very long and often flexible abdomen. They are commonly parasitic on the circular snares of Epeirid spiders, between the rays of which they spin their own irregular webs. There are three genera, Argyrodes, Ariamnes, and Rhomphaea, which are distributed in the tropical and sub-tropical regions all over the world.
(ii.) The Episininae hardly conform to the character of sedentary spiders, being frequently found outside their webs. In most species the abdomen is narrow in front and broader behind, where it is abruptly truncated or bluntly pointed. The genus Episinus is widely distributed, and one species, E. truncatus, is one of our most peculiar English spiders. It occurs occasionally under ledges of grassy or heathery banks. The genus Tomoxena is an inhabitant of tropical Asia. Janulus is found in the same regions, and in tropical America.
(iii.) The Theridioninae are a large group of spiders, often very ornate, and spinning snares of irregular threads running in all directions. The abdomen is usually more or less globular. The chelicerae are small and weak, and the paturon is transversely (not obliquely) truncated for the reception of the small unguis or fang. The somewhat long thin legs are almost or entirely destitute of spines.
We may consider certain genera as typical of the various groups into which this sub-family naturally falls. Theridion is the richest genus of the entire order, numbering some 320 species, of which seventeen inhabit the British Isles. During the summer months nearly every bush is studded with the irregular webs of these little spiders, generally prettily coloured, and with globular abdomen. The commonest is T. sisyphium, which swarms on hollies and other bushes all over the country. One of the handsomest is T. formosum, a rather local species, about a sixth of an inch in length, with the abdomen beautifully marked with oblique lines of white, yellow, red, and black. T. tepidariorum, common in conservatories, is like a large and plainer edition of T. formosum. T. riparium is remarkable for the curious earth-encrusted tube which it forms for the reception of its egg-cocoon. T. bimaculatum may often be seen among coarse herbage, holding on to its ridiculously large egg-cocoon; it is a small spider, and the sexes are more than usually unlike.
Latrodectus and Dipoena are associated exotic genera, including some of the largest species of the group. Latrodectus is peculiarly interesting on account of the great reputation for especially poisonous properties which some of its species have acquired. The New Zealand “Katipo” is L. scelio, while L. 13–guttatus enjoys an almost equally evil reputation as the “malmignatte” in Corsica. The American species L. mactans (Fig. [197], p. 362) is also considered highly venomous. These spiders form their irregular webs on low bushes, and it is curious that they are usually marked with red or yellow spots on the abdomen. They have been referred to in the section on the venom of spiders (see p. [362]).
The genus Steatoda possesses one English species (S. bipunctata) which is extremely common in buildings and in the angles of walls, and is a rather striking spider, with dark cephalothorax, and livid brown abdomen with a broken white stripe down the middle. Several closely allied genera are also sparingly represented in this country, among which may be mentioned Crustulina (two species), Asagena (one species), Teutana (two species), Lithyphantes (one species), Laseola (five species), and Euryopis (two species). In some of these the male is provided with a stridulating organ between the thorax and abdomen (Fig. [183], p. 327). The remarkable genus Tetrablemma (see p. [318]) is considered by Simon to have affinities with this group, though Pickard-Cambridge, who first described it, is inclined to rank it among the Dysderidae.
Fig. [208].—Trithena tricuspidata ♀. × 3½. (After Simon.)
(iv.) The Phoroncidiinae are a remarkable group of spiny Theridiids whose superficial resemblance to the Gasteracanthinae of the Epeiridae (see p. [409]) has often deceived Arachnologists as to their true affinities. There are eight genera, all exotic, inhabiting hot countries, and spinning a Theridion-like web on bushes. Phoroncidia has twelve species in South Asia and Madagascar. Trithena (Fig. [208]) is its American representative, five species being found in South America. Ulesanis has about twenty species, and extends from South America to Australia.
(v.) The Erigoninae are an immense group of minute, sober-coloured spiders, which include the “Money-spinners” of popular nomenclature, and are largely responsible for the gossamer which fills the air and covers every tuft of grass in the autumn. The number of species described is very large and constantly increasing, and more than a hundred are recognised as British.
Desperate efforts have been made of late years to grapple with this almost unmanageable group, but the multitude of genera which have been proposed can hardly as yet be considered to be finally established. The small size of these spiders, which renders the aid of a microscope necessary to make out their structural peculiarities, robs them of their attractiveness to any but the ardent Arachnologist, but they number among them some of our most remarkable English forms, and many of them well repay examination. The smallest English species, Panamomops diceros, measures about 1 mm. (about ¹⁄₂₅ inch) in length. Many of the groups are jet black, some with dull and others with shiny integuments. They are never greatly variegated in hue, but the glossy black of the cephalothorax, combined with red-brown or yellow legs, gives to some species a rather rich coloration.
Fig. [209].—Profile of cephalothorax of 1, Lophocarenum insanum; 2, Dactylopisthes digiticeps; 3, Walckenaera acuminata (+ abdomen); 4, Diplocephalus bicephalus; 5, Metopobractus rayi.
It is impossible here to deal with this sub-family in detail. Some of its members must be familiar enough to everybody, and the reader is recommended to spend an hour of a warm autumn day in watching them depart on the ballooning excursions, of which a description has been given (see p. [341]), from the knobs which surmount iron railings in a sunny spot. Among them he is pretty sure to find the genus Erigone—containing some of the largest members of the group—strongly represented.
In some species the male presents a remarkable difference from the female in the structure of its cephalothorax, which has the head region produced into eminences sometimes of the oddest conformation. An extreme example is seen in Walckenaera acuminata, a fine species in which the male caput is produced into a sort of spire, bearing the eyes, and nearly as high as the cephalothorax is long (Fig. [209], 3).
(vi.) The Formicinae include only two genera, Formicina (South Europe) and Solenysa (Japan). They are somewhat ant-like in appearance.
(vii.) The Linyphiinae are closely allied to the Erigoninae, but the legs are usually armed with spines, and very commonly the female has a dentated claw at the end of the pedipalp.
We include here about thirty genera of spiders of moderate or small size, living for the most part on bushes or herbage. The characteristic Linyphian web is a horizontal sheet of irregular strands, anchored to neighbouring twigs or leaves by cross threads in all directions, and the spider generally lurks beneath the web in an inverted position. Some of the larger species are very familiar objects, Linyphia triangularis being one of the most abundant English spiders, filling furze and other bushes with its extensive spinning work.
The sub-family may be roughly divided into three groups, of which the first is small, consisting of only three exotic genera of one species each. Donachochara may be taken as the type genus. They are moderate-sized spiders with rather short legs, found in France and Holland.
The second group consists of a number of genera of small spiders, sober-coloured, and generally more or less unicolorous in brown, yellow, or black, living in herbage. The sexes are much alike, the males never exhibiting the excrescences on the caput so often met with in the Erigoninae. The genus Tmeticus may be considered the type. It includes about forty species, of which about half are British. They are mostly dull yellow or brown spiders, averaging perhaps the eighth of an inch in length. Allied genera which are represented in England are Porhomma (twelve species), Microneta (twelve species), Sintula (twelve species). The American cave-genus Anthrobia comes here.
The third and last group is that including Linyphia and allied genera. They are moderate-sized or small spiders with long spiny legs and particularly long tarsi. The abdomen is generally decorated. The caput is frequently rather prominent and crowned with hairs.
Of the large number of spiders which have been described under the generic name of Linyphia, Simon[[317]] only admits about fifty species. Ten are included in the British list. L. triangularis has already been mentioned, but there are other common species, as L. montana, L. marginata, and L. clathrata. The members of most of the associated genera are rather small in size. We may briefly mention Bolyphantes, Bathyphantes, Lephthyphantes, and Labulla, all of which include English species.[[318]]
Fam. 23. Epeiridae.—This family includes all the spiders which spin circular or wheel-like snares, the highest form of spider industry, together with a few forms so closely allied in structure to orb-weaving species as to be systematically inseparable from them. It is practically co-extensive with the Argiopinae, Tetragnathinae, and Nephilinae of Simon’s Argiopidae in the Histoire naturelle des araignées.[[319]]
No one is unfamiliar with the orbicular snares, the structure of which has already been described with some minuteness (see p. 344), and some of the spiders which construct them are among the best known members of the order.
It is impossible here to deal with the multitudinous forms embraced by this family. We must mention those genera richest in species, and some others of special interest. It will be convenient to indicate eight sub-families or groups, which include most of the forms likely to be met with. These are the Theridiosomatinae, Tetragnathinae, Argiopinae, Nephilinae, Epeirinae, Gasteracanthinae, Poltyinae, and Arcyinae.
(i.) The Theridiosomatinae are a small group which might with equal propriety be classed with the Theridiidae or the Epeiridae. Theridiosoma argenteolum is a rare spider in Dorsetshire. It is a minute spider, one-twelfth of an inch in length, with silvery white globular abdomen variegated with reddish brown, and yellow cephalothorax with darker caput. Some allied spiders spin a roughly circular snare.
(ii.) The Tetragnathinae consist chiefly of two genera, Pachygnatha and Tetragnatha. The first consists of spiders which are not orb-weavers, but live in herbage, especially in swampy places. Two species, Pachygnatha clerckii and P. degeerii, are common in England, and a third, P. listeri, is sometimes met with. They are rather striking, prettily marked spiders, with strongly developed chelicerae.
The species of Tetragnatha are true orb-weavers, and may easily be recognised by their cylindrical bodies, elongated chelicerae, and long legs, stretched fore and aft along the rays of their webs. Five species have been recorded from England, and the genus contains at least a hundred species in all; almost every country in the world, regardless of its latitude, supplying examples.
Simon associates with these spiders the genus Meta, which includes perhaps our commonest Epeirid, Meta segmentata, a smallish and not very striking Orb-weaver, with a rather elongated or subcylindrical abdomen. Every garden is pretty sure to abound in it.
(iii.) The Argiopinae include many large and very striking members of the Epeiridae. There are about a hundred species of Argiope (Fig. [198], p. 379) spread over the tropical and sub-tropical countries of the world. They rarely invade the temperate regions, but A. bruennichi is found in South Europe, and A. trifasciata in Canada. The large spiders with transverse bars of yellow or orange on their abdomen, and often with a silvery sheen, belong to this genus. The species of the allied genus Gea are generally much smaller, and their abdomen more elongated. Both genera are found in tropical and sub-tropical regions all over the world. Argiope always sits in the middle of its circular web. There are invariably some flossy zigzag bands of silk stretched between two of the rays, and the web is generally accompanied by an irregular net on its border, where the much smaller male may be found.
(iv.) Among the Nephilinae are to be found the largest Epeirids. Indeed, the largest yield in size only to the Aviculariidae. Nephila is a tropical genus, numbering about sixty species. The abdomen is generally elongated and somewhat cylindrical, and is strikingly variegated. It is in this group that the disparity in size between the sexes is most marked (see p. [379]).
(v.) The Epeirinae[[320]] include the bulk of the Orb-weavers, and form a very extensive group. Five genera and twenty-eight species are in the British list.
No spider is more familiar than Epeira diademata (Fig. [181], p. 325), the Garden-spider, par excellence, which attains its greatest size and spreads its largest snares in the autumn. The smaller and much less conspicuous Zilla x-notata is sure to be found abundantly in the same locality. Several other Epeirids are to be found in this country, especially in the south, by sweeping heather or bushes with a net, or shaking the boughs of trees over an umbrella or other receptacle. The little apple-green species is Epeira cucurbitina. E. cornuta is extremely common in marshy places all over the country. In furze bushes, and often among sedge in swampy places, will frequently be found E. quadrata, one of the largest and handsomest species we possess. The ground-colour may vary from orange-red to green, and there are four conspicuous white spots on the abdomen. The tent-like retreat which this spider makes near its snare often catches the eye.
Fig. [210].—Epeira angulata, ♀.
E. umbratica is a dark flat, somewhat toad-like Epeirid of retiring habits, which stretches its snare usually on wooden palings, between the timbers of which it squeezes its flat body, and waits for insects to entangle themselves.
Two of our finest Epeiras, E. pyramidata and E. angulata (Fig. [210]), are seldom met with, and only in the south.
Our only Cyclosa (C. conica) is easily recognised by the peculiar form of its abdomen, which is greatly prolonged beyond the spinnerets. It is a small, rather dark species, which constructs a particularly perfect snare.
Five British Epeirids belong to the genus Singa. They are small creatures, not exceeding a sixth of an inch in length. They live in heathery and marshy localities.
(vi.) The Gasteracanthinae are a remarkable group of Epeirids, characterised by the hard and coriaceous integument covering the abdomen, which is usually furnished with a number of more or less formidable thorn-like spines, calculated to render these spiders by no means pleasant eating for insectivorous birds. An even more constant characteristic is the presence on the back of the abdomen of a number of “sigilla,” or somewhat seal-like impressions arranged symmetrically, four forming a trapezium in the middle, while the others are distributed round the border.
Fig. [211].—Gasteracantha minax, ♀.
There are about 200 species of Gasteracantha, all natives of tropical countries.
The spiders of the genus Micrathena (Acrosoma) have a more elongate cephalothorax, and sometimes the spines are exceedingly long, far exceeding the length of the body proper. Among the less spiny members of this group are some remarkable mimetic ant-like forms.
(vii.) The Poltyinae include some remarkable spiders, found in Africa and South Asia for the most part, though sparingly represented in America and Oceania. They are generally largish spiders, often with a very odd conformation of the abdomen, which is generally much raised. The type genus is Poltys.
(viii.) The Arcyinae, which are more characteristic of Australia and the neighbouring islands, are a small group of spiders, usually yellow with black markings, and with the somewhat square-shaped cephalothorax usually prominent at the angles. The type genus is Arcys.
Fam. 24. Uloboridae.—The Uloboridae are cribellate spiders, with rather elongate cephalothorax, devoid of median fovea. The cribellum is transverse and generally undivided. The first pair of legs are usually much the longest. The metatarsi of the fourth legs, in addition to the calamistrum, bear a number of generally regularly arranged spines. The eyes are often situated on tubercles. Three sub-families are recognised, Dinopinae, Uloborinae, and Miagrammopinae.
(i.) The Dinopinae are a small group comprising only two genera, Dinopis and Menneus. The calamistrum is short, occupying not more than half of the metatarsus. Twenty species of Dinopis and six of Menneus are scattered over the tropical regions of the world.
(ii.) The Uloborinae include a number of spiders which have been described under several generic names, but are now considered to fall into two genera, Sybota and Uloborus. Sybota has only two species, one in the Mediterranean region and one in Chili. There are about sixty species of Uloborus, some of which have a wide distribution, while many (e.g. U. republicanus, of Venezuela) are social. The type species, U. walckenaerius, is a very rare spider in England.
(iii.) The Miagrammopinae include two genera containing some very interesting forms. The genus Miagrammopes, of which twenty species have been described, though the number is probably far greater, is characterised by a very long cylindrical abdomen, and by the apparent possession of only four eyes, in a transverse row. These are really the posterior eyes; and the anterior eyes, or some of them, are present in a very reduced condition. Little is known of the habits of these spiders.
Fig. [212].—Hyptiotes paradoxus, ♀.
The other genus, Hyptiotes, though only boasting three species, possesses a special interest on account of the remarkable snare constructed by the spiders which belong to it. This has already been described in the section upon defective orb-webs (see p. [349]).
The type species, H. paradoxus, is very rare in England, and though small and inconspicuous, it is certainly one of the most curious members of our Spider fauna.
Fam. 25. Archeidae.—This small family includes certain remarkable fossil spiders from Baltic amber, and two rare recent forms, Archea (Eriauchenus) workmani from Madagascar, and Mecysmauchenius segmentatus from America. The chelicerae, which are extraordinarily long, are articulated far away from the mouth-parts. The caput is clearly marked off from the thorax, and is much raised. In several other respects these spiders are very distinct from all other members of the order.
Fam. 26. Mimetidae.—The Mimetidae form a small group in general appearance recalling the Theridiidae, with which family they were for a long time incorporated. The chief genera are Ero, Mimetus, and Gelanor. Ero furcata (= thoracica) is a pretty little spider, not rare among grass in England. The upper side of its very convex abdomen is marked with red, yellow, and black, and bears two little protuberances or humps near the middle. It is only about an eighth of an inch long. Its interesting egg-cocoon has already been alluded to (see p. [358]). E. tuberculata has been found on rare occasions in this country. There are about ten other species of Ero, all small spiders, and living in temperate regions. The genus Mimetus (in which is merged Blackwall’s Ctenophora) includes a number of larger, more strongly-built spiders, living for the most part in tropical countries.
The genus Gelanor (Galena) is the American representative of the group, its three species being rather large spiders, inhabiting Central and South America. The males of this genus have remarkably long and slender pedipalpi, much longer than the whole body.
Fam. 27. Thomisidae.—The Thomisidae are the Latigrade spiders of Latreille, and the “Crab-spiders” of popular nomenclature. Their legs are extended more or less laterally instead of in the normal fore and aft directions, and their progression is frequently strikingly crab-like. They form a very large group of more than 140 genera, including spiders of every size, and they are to be found in every quarter of the world. Forty-three species are British. Many strange forms are included in this group, and several of the sub-families into which it has been divided contain only one or two genera. The bulk of its members fall into the sub-families Thomisinae, Philodrominae, and Sparassinae.
(i.) The Thomisinae (Misumeninae of Simon’s Hist. Nat.) include what may be called the more normal members of the family, distributed among more than sixty genera. Six of these genera are represented in the British Isles. Our commonest Crab-spider is probably Xysticus cristatus, abundant everywhere in grass and herbage. Young specimens may often be seen upon iron railings in the autumn. Twelve other species of that genus are on the British list. They are of small or moderate size, rarely exceeding a quarter of an inch in length. A closely allied genus is Oxyptila, of which we have seven species. The more striking members of this sub-family to be found in England are our single representatives of the genera Misumena, Diaea, and Thomisus. Misumena vatia is a handsome species, the female measuring sometimes more than a third of an inch, and having its large yellow or green abdomen marked, in many specimens, with a pair of bright red bands, which, however, are not always present. The males are much smaller and darker. It is common in some parts of England, especially in the south, where it is to be sought for in bushes and trees.
Fig. [213].—Thomisid spiders. A, Micrommata virescens, ♀; B, Xysticus pini, ♀; C, Philodromus margaritatus, ♂; D, Tibellus oblongus, ♀.
Diaea dorsata is one of our prettiest British species, with light green legs and cephalothorax, and a yellow abdomen with a red-brown central marking. It is common in the New Forest and other southern localities. The female attains a quarter of an inch in length.
Thomisus onustus, a rare spider among heather, is recognisable by the shape of its abdomen, which is broadest behind and abruptly truncated. When adult the abdomen is a pale yellow, but the young are suffused with a pink hue closely corresponding with that of the heather blossom in which they are frequently found sitting.
(ii.) The Philodrominae have the cephalothorax more rounded in front, and the legs, especially the second pair, usually longer than in the Thomisinae. There are ten genera, of which the most important is Philodromus, which numbers about a hundred species. They are active spiders, living upon bushes and trees, and most of them are inhabitants of temperate regions. We have about twelve species in the British Isles. The commonest is Ph. aureolus, which is abundant on bushes in most parts of the country. Some species are very prettily marked, and one, Ph. margaritatus (Fig. [213], C) presents a very good example of protective coloration, being almost indistinguishable on the blue-grey lichen on tree trunks, where it lies in wait for insects.
Another important genus, including some fifty species, is Thanatus, extending from tropical to arctic regions, but very sparingly represented in England. Th. striatus (= hirsutus) occurs occasionally, and one example of the fine species Th. formicinus has been taken in the New Forest. The members of this genus as a rule affect dry and sandy habitats.
The genus Tibellus includes few species, but has a wide distribution. The type species T. oblongus (Fig. [213], D) is found in the temperate regions all over the world, and is common in England. It is a pale straw-coloured spider with a much elongated abdomen. It closely resembles the stems of dry grass in hue, and when alarmed it remains perfectly still with its legs embracing the stem and its abdomen closely applied to it.
(iii.) The Sparassinae[[321]] include most of the large Latigrade forms, and number about forty genera.
Heteropoda venatoria is a cosmopolitan species, and though proper to warm countries, is often introduced here on hothouse plants, and has been known to establish itself in the open air in botanical gardens. Our only indigenous member of this sub-family is Micrommata virescens (Fig. [213], A). This striking spider is found, though rarely, in the south of England. The female is half an inch in length and of a vivid green hue, while the more cylindrical abdomen of the male is yellow with three longitudinal scarlet lines. Other genera are Sparassus, Torania, and Delena.
(iv.) The Aphantochilinae include two curious genera which are exclusively American. The labium is much reduced and the sternum is shortened, terminating between the third pair of legs. The species of Aphantochilus are largish, glossy-black spiders, sometimes spotted with white. Some of them mimic ants of the genus Cryptocerus. The other genus is Bucranium.
(v.) The Stephanopsinae include about sixteen genera, of which the best known are Stephanopsis and Regillus. There are about fifty species of Stephanopsis, most of them Australian, while the eight species of Regillus belong to Africa and South Asia.
The mimetic form Phrynarachne decipiens has already been alluded to (see p. [374]).
(vi.) The Selenopinae consist of a single genus, Selenops, of which ten or twelve species are known, some of which are very widely distributed, though confined to hot regions. These spiders, which are all large, are easily recognised by their extremely flat bodies and the peculiar arrangement of their eyes, all eight of them being placed more or less in a single transverse line.
Fam. 28. Zoropsidae.—The Zoropsidae are cribellate spiders of large size, with well-developed scopulae on tarsi and metatarsi. The cribellum is divided, and the calamistrum, which is very short, is not well developed. Most are inhabitants of hot regions, where they live under stones or bark. Zoropsis has six species, chiefly inhabitants of North Africa, though representatives occur on the European side of the Mediterranean. Acanthoctenus has two species in South and Central America.
Fam. 29. Platoridae.—The Platoridae are Thomisid-like, medium-sized spiders, generally with a uniform yellow or brown coloration. The spinnerets are their most characteristic features. The median pair present a large flat surface studded with two parallel rows of large fusulae, while the anterior pair are situated outside them, and are thus widely separated. There are only three genera, and very few species of this family. Plator insolens is a Chinese species. Doliomalus and Vectius belong to South America.
Fam. 30. Agelenidae.—Sedentary spiders with slight sexual dimorphism; with three tarsal claws and devoid of scopulae.
The Agelenidae spin a more or less extensive web of fine texture, usually accompanied by a tubular retreat. Our commonest cellar spiders belong to this group, which may be divided into three sub-families, Cybaeinae, Ageleninae, and Hahniinae.
(i.) The Cybaeinae include some sixteen genera, of which two deserve special mention on account of the peculiar habits of the spiders belonging to them.
Desis is a genus of marine spiders, said to live on coral reefs below high-water mark, and to remain in holes in the rock during high tide, enclosed in cocoons impermeable to the sea-water. At low tide it is stated that they come forth and prey upon small crustaceans. Argyroneta has only one species, A. aquatica, spread throughout Europe and North and Central Asia. It is the well-known “Water-spider,” which is so often an object of interest in aquaria.
(ii.) The Ageleninae also contain sixteen genera, but it is a much larger group, some of the genera being rich in species. They are mostly moderate or large-sized hairy spiders, living in temperate or cold climates. There are about fifty species of Tegenaria, seven of which have been recorded as British.
Our commonest Cellar-spider is T. derhamii, but the very large long-legged species found in houses in the southern counties of England is T. parietina (= guyonii = domestica). There are not many species of Agelena, but one, A. labyrinthica, is a common object in this country, with its large, close-textured web and accompanying tube spread on grassy banks by the wayside. Coelotes atropos is a formidable-looking spider, found occasionally under stones in England and Wales. Another genus, Cryphoeca, has three British representatives.
(iii.) The Hahniinae are recognised at once by their spinnerets, which are arranged in a single transverse line, the posterior pair being on the outside, and generally much the longest. Hahnia contains several species of very small spiders, of which four or five are British, usually occurring among moss or herbage. The aberrant form Nicodamus (Centropelma), usually placed among the Theridiidae, is removed by Simon to the Agelenidae, forming by itself the sub-family (iv.) Nicodaminae.
Fam. 31. Pisauridae.—The Pisauridae are hairy, long-legged spiders, intermediate, both in structure and in habits, between the Agelenidae and the Lycosidae. Many new genera have recently been added to the group, but many of them only include one or two species.
Pisaura is spread throughout the temperate regions of the Old World, and P. (Ocyale) mirabilis is common in England, being found abundantly in woods and on commons. It is a striking spider, more than half an inch in length, and its elongate abdomen is marked on either side with a sinuous longitudinal white band.
There are some thirty species of Dolomedes scattered over the temperate regions of the world. D. fimbriatus is a rare species in marshy spots in the south of England, and is one of the largest British spiders. The ground-colour is deep brown, with two longitudinal yellowish stripes both on cephalothorax and abdomen.
The genus Dolomedes is replaced by Thaumasia in South America.
Fam. 32. Lycosidae.—These are what are popularly known as “Wolf-spiders.” They are vagabond hunting spiders, spinning no snare, but chasing their prey along the ground, and in the breeding season carrying their egg-bags with them, attached beneath the abdomen. Some of them burrow in the loose earth or sand, but others seem to have nothing in the way of a habitation.
Fig. [214].—Lycosid Spiders. 1, Lycosa fabrilis, ♀; 2, Lycosa picta, ♀; 3, Pardosa amentata, ♀.
The arrangement of the eyes is very characteristic. They are in three rows. The front row consists of four small eyes above the insertion of the chelicerae, and directed forwards. Two comparatively very large eyes form the next row, and occupy the upper angles of the facies, being also directed forwards. The third row consists of two medium-sized eyes placed dorso-laterally on the caput, some distance behind the rest, and looking upwards. The tarsi are three-clawed. The so-called “Tarantula” spiders belong to this group, though the name has been so abused in popular usage, and has passed through so many vicissitudes in scientific nomenclature, that it is difficult to tell what creature is intended by it. In America the Aviculariidae are commonly called Tarantulas.
The two chief genera of this extensive family are Lycosa and Pardosa.
The genus Lycosa includes about 400 species. It has been broken up from time to time into various genera (Trochosa, Pirata, Tarentula, etc.), but these glide into each other by imperceptible degrees, and are now discarded. They are large or moderate-sized spiders, found in every part of the world. About twenty species are British, some of them being fine and handsomely marked. One of the prettiest is Lycosa picta, common on the sandhills in some localities.
Some exotic species are very large, Lycosa ingens, from Madeira, measuring sometimes more than an inch and a half in length.
Pardosa (Fig. [188], p. 341) is not so rich in species, but the individuals of some species are wonderfully numerous. Hundreds of P. lugubris, for example, may be seen scampering over the dead leaves of a wood in the autumn. These spiders are generally sombrely coloured and well covered with hair. Perhaps the commonest and most widely spread species in this country is P. amentata.
Fam. 33. Ctenidae.—The Ctenidae are Lycosa-like spiders, having in certain points of structure close affinities with the Pisauridae and the Sparassinae of the Thomisidae. The limits of the family are not well defined, and many arachnologists place in it some of the genera allotted above to the Pisauridae, while others do not consider the group sufficiently marked off to constitute a separate family at all. As here understood they are equivalent to the Cteninae of the Clubionidae in Simon’s Histoire naturelle. The eyes are arranged in the Lycosa fashion, but the tarsi have only two terminal claws and well-developed “claw-tufts,” frequently accompanied by a scopula. There are strong, regularly-arranged spines under the tibiae and tarsi.
There are about fifteen genera. Uliodon numbers six species of large hairy spiders in Australia. Ctenus is rich in species, having about sixty, found in all hot countries, but especially in America and Africa. They are also of large size and usually of yellowish coloration, often diversified by a pattern on the abdomen. The fifteen species of Leptoctenus are proper to tropical Asia. Acantheis from South Asia and Enoplectenus from Brazil are more slender, elongate forms, recalling Tetragnatha. Caloctenus includes a number of Pardosa-like spiders found at a high elevation in South America.
The Ctenidae have the habits of the Lycosidae, and are wandering spiders, some forming a burrow in the ground.
Fam. 34. Senoculidae.—The South American genus Senoculus (Labdacus) alone constitutes this family. The species are probably numerous, but ten only have been described. They are moderate-sized spiders, spinning no web, but running with astonishing speed over the leaves and stems of plants. The generic name is really inapplicable, as there are eight eyes, but the anterior laterals are much reduced. The abdomen is long, and the legs are long and unequal, the first pair much the longest and the third much the shortest.
Fam. 35. Oxyopidae.—The Oxyopidae form a well-marked group, with oval cephalothorax somewhat narrowed in front, and lanceolate abdomen. The eight black eyes have a characteristic arrangement, and the anterior medians are always very small. The legs are long and tapering, and not very unequal, and are furnished with particularly long spines, which give these spiders a very characteristic appearance. There are eight genera, of which the most important are Pucetia and Oxyopes.
Pucetia contains a number of rather large spiders, generally bright green, often variegated with red. They affect particular plants. For instance, P. viridis, which occurs in Spain, is always found on Ononis hispanica. There are about thirty species of this genus distributed over the tropical and sub-tropical regions of the world. Oxyopes numbers many species, certainly more than fifty, and has a similar distribution, but some of its members invade colder regions. They are of rather small size. O. lineatus is a very rare spider in the south of England.
The Oxyopidae are diurnal spiders, running over plants in search of prey, and often leaping, after the fashion of members of the following family.
Fam. 36. Attidae (Salticidae).—Wandering spiders with cephalothorax broad anteriorly, and bearing eight homogeneous eyes in three rows. Four eyes, largely developed, are directed forward; the remaining four eyes are placed dorsally in two rows, the first pair being much reduced in size.
The Attidae or Jumping-spiders form the most extensive family of the whole order, the known species amounting to something like four thousand. It is only of late years that their vast numbers have begun to be realised, for their vagabond habits and great activity enabled them to a great extent to elude the earlier collectors, whose methods were not as thorough as those now in vogue. Their real home is in the tropical regions, temperate fauna being comparatively poor in Attid species. France boasts nearly 150, but only 37 are recorded for the British Isles, and 2 at least of these are recent introductions.
Some of the tropical forms are most brilliantly coloured, glowing with vivid colours and metallic hues, and they have frequently excited the admiration of travellers. The coloration is nearly always due to the hairs and scales with which the spiders are clothed, and is, unfortunately, almost incapable of preservation in the collector’s cabinet.
These spiders are all wanderers, spinning no snares, though they form a sort of silken cell or retreat, in which the female lays her eggs. Their habits are diurnal, and they delight in sunshine. They stalk their prey and leap upon it with wonderful accuracy. They invariably attach a thread at intervals in their course, and on the rare occasions when they miss their aim while hunting on a perpendicular surface, they are saved from a fall by the silken line proceeding from the spot whence the leap was made.
Fig. [215].—Attid Spiders. A, Salticus scenicus, ♂; B, Marpissa muscosa, ♀; C, Synemosyna formica, ♀; D, Ballus variegatus, ♀.
The movements of these spiders are sufficient to indicate their systematic position without entering upon structural details, but their eyes deserve a special mention. They are all dark-coloured and very unequal in size, and they occupy the whole area of the caput, usually forming a large quadrilateral figure. Four large eyes occupy the facies or “forehead,” the medians being especially large. Next come two very small eyes, behind the anterior laterals, and lastly two of medium size at the posterior corners of the caput.
This vast family does not lend itself easily to division into sub-families, and it will be impossible here to do more than indicate a very few of the multitudinous forms.
The most familiar British example is Salticus scenicus (Epiblemum scenicum), the little black and white striped spider to be seen hunting on walls and fences during the summer. Marpissa muscosa is the largest English species, measuring about half an inch. It has a brownish-yellow coloration, and is found, though not commonly, in similar situations. Attus pubescens affects grey stone walls, on which it is nearly invisible except when moving. The other British species are mostly to be found on trees and shrubs or among herbage, or hunting over bare sandy spots in the sunshine. A few (Marpissa pomatia, Hyctia nivoyi) are fen species. Hasarius falcatus is a handsome spider, common in woods in some localities.
The species differ much in their jumping powers; the Marpissas, for example, are not great leapers, but the little Attus saltator, found on sandhills, jumps like a flea, and the North American species Saitis pulex has a suggestive specific name.
Again, in this family there are mimetic forms resembling ants. Myrmarachne formicaria (Salticus formicarius) is found very rarely in England, but is not uncommon on the Continent.
Synageles and Synemosyna are allied genera. Phidippus is a genus well represented in America, and Ph. morsitans has already been mentioned (p. 365) in connexion with its poisonous reputation. Astia and Icius have American representatives (see pp. 381, 382), though the type species belongs to the Old World.
CHAPTER XVI
ARACHNIDA EMBOLOBRANCHIATA (CONTINUED)—PALPIGRADI—SOLIFUGAE = SOLPUGAE—CHERNETIDEA = PSEUDOSCORPIONES
Order IV. Palpigradi.
Minute Arachnids with three-jointed chelate chelicerae, and with the last two joints of the cephalothorax free. The abdomen consists of eleven segments with a fifteen-jointed flagellum.
In 1885 Grassi discovered, at Catania, a minute Arachnid which did not fall into any of the established orders of Arachnida. He named it Koenenia mirabilis. In 1893 Hansen collected several specimens in Calabria, near Palmi and Scilla, and carefully redescribed the species in conjunction with Sörensen.[[322]] It has been studied still more minutely by Börner.[[323]]
There is a “head” portion, covered by a carapace, and bearing the chelicerae, pedipalpi, and two pairs of legs. The two free thoracic segments bear the third and fourth pairs of legs, recalling the Schizonotidae (see p. [312]), where the portion of the thorax bearing these legs is separate, though covered by a single dorsal plate. There are no eyes, but two hair-structures, believed to be sensory, are present on the cephalothorax, and Börner has observed openings in the second joint of the first pair of legs which have all the appearance of “lyriform” organs, as found in Spiders (see p. [325]).
The last three abdominal segments narrow rapidly, the last bearing the anus. A fifteen-jointed caudal flagellum is carried, Scorpion-like, above the animal’s back. The body and tail are each about a millimetre in length, and the animal is of a translucent white colour.
The mouth is extremely simple, being merely a slit upon a slight eminence. There are two sternal plates beneath the “head,” and one beneath each free thoracic segment. The genital operculum is complicated, and is situated beneath the second abdominal segment.
Fig. [216].—Koenenia mirabilis, much enlarged. (After Hansen.)
Since 1885 several other species have been discovered in various parts of the world. Two American forms possess three pairs of lung-sacs on segments 4, 5, and 6 of the abdomen. Rucker[[324]] has suggested for them the generic name of Prokoenenia, including P. wheeleri, Rucker, from Texas, and P. chilensis, Hansen, from Chili. The others, styled by that author Eukoenenia, have no lung-sacs. There are about ten species, mostly from the Mediterranean region, but E. augusta, Hansen, is found in Siam, E. florenciae, Rucker, in Texas, and E. grassii, Hansen, in Paraguay.
Order V. Solifugae (Solpugae).
Tracheate Arachnids, with the last three segments of the cephalothorax free and the abdomen segmented. The chelicerae are largely developed and chelate, and the pedipalpi are leg-like, possessing terminal sense-organs.
The Solifugae are, in some respects, the most primitive of the tracheate Arachnida. Their general appearance is very spider-like, and by the old writers they are uniformly alluded to as spiders. The segmented body and the absence of spinning organs, however, make them readily distinguishable on careful inspection. They are for the most part nocturnal creatures, though some seem to rove about by day, and are even called “Sun-spiders” by the Spaniards. The night-loving species are attracted by light. They are, as a rule, exceedingly hairy. Some are extremely active, while the short-legged forms (e.g. Rhagodes, see p. [429]) move slowly. They are capable of producing a hissing sound by the rubbing together of their chelicerae. Only the last three pairs of legs are true ambulatory organs, the first being carried aloft like the pedipalps, and used for feeling and manipulating the prey.
There has been much controversy as to the poisonous properties with which these creatures have been very widely credited by both ancient and modern writers. The people of Baku on the Caspian consider them especially poisonous after their winter sleep. The Russians of that region much dread the “Falangas,” as they call them, and keep a Falanga preserved in oil as an antidote to the bite. The Somalis, on the other hand, have no fear of them, and, though familiar with these animals, have not thought them worthy of the dignity of a name.
Several investigators have allowed themselves to be bitten without any special result. Some zoologists have found and described what they have taken to be poison-glands, but these appear to be the coxal glands, which have an excretory function. Bernard[[325]] suggests that, if the bite be poisonous, the virus may exude from the numerous setal pores which are found on the extremities of the chelicerae. The cutting powers of the immensely-developed chelicerae are usually sufficient to ensure fatal results on small animals without the agency of poison. Distant,[[326]] indeed, thinks they cannot be poisonous, for when birds attack them they flee before their assailants.
The Solifugae require a tolerably warm climate. In Europe they are only found in Spain, Greece, and Southern Russia. They abound throughout Africa, and are found in South-Western Asia, the southern United States, and the north of South America. They appear to be absent from Australia, nor have any been found in Madagascar. Their usual food appears to be insects, though they devour lizards with avidity. Some interesting observations on their habits are recorded by Captain Hutton,[[327]] who kept specimens in captivity in India. An imprisoned female made a burrow in the earth with which her cage was provided, and laid fifty eggs, which hatched in a fortnight, but the young remained motionless for three weeks longer, when they underwent their first moult, and became active.
A sparrow and musk rats were at different times placed in the cage, and were speedily killed, but not eaten. Two specimens placed in the same cage tried to avoid each other, but, on coming into contact, fought desperately, the one ultimately devouring the other. It was noteworthy that the one which was first fairly seized immediately resigned itself to its fate without a struggle. As is the case with some spiders, the female is said occasionally to kill and devour the male. A Mashonaland species, Solpuga sericea, feeds on termites,[[328]] while a South Californian Galeodes kills bees,[[329]] entering the hives in search of them. They are fairly good climbers. In Egypt Galeodes arabs climbs on to tables to catch flies, and some species have been observed to climb trees.
Fig. [217].—Rhagodes sp., ventral view. Nat. size. a, Anus; ch, chelicerae; g.o, genital operculum; n, racket organs; p, pedipalp; 1, 2, 3, 4, ambulatory legs. (After Bernard.)
That their pedipalps, in addition to their sensory function (see p. [426]), possess a sucking apparatus, is clear from an observation of Lönnberg,[[330]] who kept specimens of Galeodes araneoides imprisoned in rectangular glass boxes, up the perpendicular sides of which they were able to climb for some distance by their palps, but, being able to obtain no hold by their legs, they soon tired.
External Anatomy.—The body of Galeodes consists of a cephalothorax and an abdomen, both portions being distinctly segmented. The cephalothorax consists of six segments, the first thoracic segment being fused with the two cephalic segments to form a sort of head, while the last three thoracic segments are free, and there is almost as much freedom of movement between the last two thoracic segments as between the thorax and the abdomen. The “cephalic lobes,” which give the appearance of a head, have been shown by Bernard[[331]] to be due to the enormous development of the chelicerae, by the muscles of which they are entirely occupied.
The floor of the cephalothorax is for the most part formed by the coxae of the appendages, and the sternum is hardly recognisable in many species. In Solpuga, however (see p. [429]), it exists in the form of a long narrow plate of three segments, ending anteriorly in a lancet-shaped labium.
A pair of large simple eyes are borne on a prominence in the middle of the anterior portion of the cephalothorax, and there are often one or two pairs of vestigial lateral eyes.
The first pair of tracheal stigmata are to be found behind the coxae of the second legs.
The mouth-parts take the form of a characteristic beak, consisting of a labrum and a labium entirely fused along their sides. The mouth is at the extremity of the beak, and is furnished with a straining apparatus of complicated hairs.
The abdomen possesses ten free segments, marked off by dorsal and ventral plates, with a wide membranous lateral interval. The ventral plates are paired, the first pair forming the genital opercula, while behind the second and third are two pairs of stigmata. Some species have a single median stigma on the fourth segment, but this is in some cases permanently closed, and in the genus Rhagodes entirely absent, so that it would seem to be a disappearing structure.
The appendages are the six pairs common to all Arachnids—chelicerae, pedipalpi, and four pairs of legs. The chelicerae, which are enormously developed, are two-jointed and chelate, the distal joint being articulated beneath the produced basal joint. In the male there is nearly always present, on the basal joint, a remarkable structure of modified hairs called the “flagellum,” and believed to be sensory. It differs in the different genera, and is only absent in the Eremobatinae (see p. [429]). The pedipalpi are strong, six-jointed, leg-like appendages, without terminal claw. They end in a knob-like joint, sometimes movable, sometimes fixed, which contains a very remarkable eversible sense-organ, which is probably olfactory. It is concealed by a lid-like structure, and when protruded is seen to be furnished, on its under surface, with a pile of velvet-like sensory hairs.
The legs differ in the number of their joints, as the third and fourth pairs have the femora divided, and the tarsus jointed. The first pair has only a very small terminal claw, but two well-developed claws are borne by the tarsi of the other legs. Each of the last legs bears, on its under surface, five “racket organs,” believed to be sensory.
Internal Structure.—The alimentary canal possesses a sucking chamber within the beak, after which it narrows to pass through the nerve-mass, and after an S-shaped fold, joins the mid-gut. This gives off four pairs of thin diverticula towards the legs, the last two entering the coxae of the third and fourth pairs.
At the constriction between the cephalothorax and the abdomen there is no true pedicle, but there is a transverse septum or “diaphragm,” through which the blood-vessel, tracheal nerves, and alimentary canal pass. The gut narrows here, and, on entering the abdomen, proceeds straight to a stercoral pocket at the hind end of the animal, but gives off, at the commencement, two long lateral diverticula, which run backwards parallel with the main trunk. These are furnished with innumerable secondary tube-like diverticula, which proceed in all directions and fill every available portion of the abdomen. The caeca, which are so characteristic of the Arachnidan mid-gut, here reach their extreme development. A pair of Malpighian tubules enter the main trunk in the fourth abdominal segment.
Other excretory organs are the coxal glands, which form many coils behind the nerve-mass, and open between the coxae of the third and fourth legs. They have been taken for poison-glands.
There is a small endosternite in the hinder portion of the cephalothorax under the alimentary canal.
The vascular system is not completely understood. The heart is a very long, narrow, dorsal tube, extending almost the entire length of the animal, and possessing eight pairs of ostia, two in the cephalothorax and six in the abdomen. It gives off an anterior and a posterior vessel, the latter apparently a vein, as it is guarded at its entrance by a valve. The blood seems to be delivered by the anterior artery on to the nerve-mass, and, after percolating the muscles and viscera, to divide into two streams—the one returning to the heart by the thoracic ostia, the other passing through the diaphragm and bathing the abdominal organs, finally to reach the heart either by the abdominal ostia or by the posterior vein.
Fig. [218].—Nervous system of Galeodes. abd.g, Abdominal ganglion; ch, cheliceral nerve; ch.f, chitinous fold; ch.r, chitinous rod; g.n, generative nerve; l, labial nerve; st, position of stigma. (After Bernard.)
The nervous system, notwithstanding the fact that the three last thoracic segments are free, is chiefly concentrated into a mass surrounding the oesophagus. Nerves are given off in front to the eyes, the labrum, and the chelicerae, while double nerves radiate to the pedipalps and to the legs. From behind the nerve-mass three nerves emerge, and pass through the diaphragm to enter the abdomen. The median nerve swells into an “abdominal ganglion” just behind the diaphragm, and is then distributed to the diverticula of the alimentary canal. The lateral nerves innervate the generative organs.
The respiratory system consists of a connected network of tracheae communicating with the exterior by the stigmata, whose position has already been described. There are two main lateral trunks extending nearly the whole length of the body, and giving off numerous ramifications, the most important of which are in the cephalothorax, and supply the muscles of the chelicerae and of the other appendages.
The generative glands do not essentially differ from the usual Arachnid type, though the paired ovaries do not fuse to form a ring. There are no external organs, and the sexes can only be distinguished by secondary characteristics, such as the “flagellum” already mentioned.
Classification.—There are about a hundred and seventy species of Solifugae inhabiting the warm regions of the earth. No member of the group is found in England, or in any except the most southern portions of Europe.
Kraepelin[[332]] has divided the group into three families—Galeodidae, Solpugidae, and Hexisopodidae.
Fam. 1. Galeodidae.—The Galeodidae have a lancet-shaped flagellum, directed backwards. There is a characteristic five-toothed plate or comb covering the abdominal stigmata. The tarsus of the fourth leg is three-jointed, and the terminal claws are hairy.
Fig. [219].—Chelicera and flagellum of Galeodes. (After Kraepelin.)
There are two genera, Galeodes, with about twelve species, and Paragaleodes, with six species, scattered over the hot regions of the Old World.
Fam. 2. Solpugidae.—The Solpugidae comprise twenty-four genera, distributed under five sub-families. The toothed stigmatic plate is absent, and the tarsal claws are smooth. The ocular eminence is furnished with irregular hairs. The “flagellum” is very variable.
Fig. [220].—Chelicerae and flagella of A, Rhagodes; B, Solpuga; and C, Daesia. (After Kraepelin.)
(i.) The Rhagodinae include the two genera, Rhagodes (Rhax) and Dinorhax. The first has twenty-two species, which inhabit Africa and Asia. The single species of Dinorhax belongs to East Asia. These creatures are short-legged and sluggish.
(ii.) The Solpuginae contain two genera—Solpuga with about fifty species, and Zeriana with three. They are all inhabitants of Africa, and some occur on the African shore of the Mediterranean.
(iii.) The Daesiinae number about forty species, divided among several genera, among which the principal are Daesia, Gluvia, and Gnosippus. They are found in tropical regions of both the Old and the New World.
(iv.) The Eremobatinae are North American forms, the single genus Eremobates numbering about twenty species. The flagellum is here entirely absent.
Fig. [221].—Chelicera and flagellum of Hexisopus. (After Kraepelin.)
(v.) The Karshiinae include the five genera Ceroma, Gylippus, Barrus, Eusimonia, and Karshia. They are universally distributed.
Fam. 3. Hexisopodidae.—This family is formed for the reception of a single aberrant African genus, Hexisopus, of which five species have been described.
There are no claws on the tarsus of the fourth leg, which is beset with short spine-like hairs, and in other respects the genus is peculiar.
Order VI. Chernetidea.
(CHERNETES, PSEUDOSCORPIONES.)
Tracheate Arachnids, with the abdomen united to the cephalothorax by its whole breadth. Eyeless, or with two or four simple eyes placed laterally. Abdomen segmented, with four stigmata. Chelicerae chelate, bearing the openings of the spinning organs. Pedipalpi large, six-jointed, and chelate. Sternum absent or rudimentary.
The Chernetidea or “False-scorpions” constitute the most compact and natural order of the Arachnida. There are no extreme variations within the group as at present known, while all its members differ so markedly from those of other Arachnidan orders that their true affinities are by no means easy to determine.
The superficial resemblance to Scorpions which has won these animals their popular name is almost entirely due to the comparative size and shape of their pedipalpi, but it is probable that they are structurally much more closely allied to the Solifugae.
Chernetidea are not creatures which obtrude themselves on the general notice, and it is highly probable that many readers have never seen a living specimen. This is largely due to their minute size. Garypus littoralis, a Corsican species, nearly a quarter of an inch in length of body, is a veritable giant of the tribe, while no British species boasts a length of more than one-sixth of an inch.
Moreover, their habits are retiring. They are to be sought for under stones, under the bark of trees, and among moss and débris. One species, probably cosmopolitan, certainly lives habitually in houses, and is occasionally noticed and recognised as the “book-scorpion,” and one or two other species sometimes make themselves conspicuous by the remarkable habit of seizing hold of the legs of flies and being carried about with them in their flight. With these exceptions, the Chernetidea are not likely to be seen unless specially sought for, or unless casually met with in the search for small beetles or other creatures of similar habitat. Nevertheless they are very widely distributed, and though more numerous in hot countries, are yet to be found in quite cold regions.
Though comparatively little attention has been paid to them in this country, twenty British species have been recorded, and the known European species number about seventy.
As might be expected from their small size and retiring habits, little is known of their mode of life. They are carnivorous, feeding apparently upon any young insects which are too feeble to withstand their attacks. The writer has on two or three occasions observed them preying upon Homopterous larvae. As a rule they are sober-coloured, their livery consisting of various shades of yellow and brown. Some species walk slowly, with their relatively enormous pedipalps extended in front and gently waving, but all can run swiftly backwards and sideways, and in some forms the motion is almost exclusively retrograde and very rapid. A certain power of leaping is said to be practised by some of the more active species. The Chernetidea possess spinning organs, opening on the movable digit of the chelicera. They do not, however, spin snares like the Spiders, nor do they anchor themselves by lines, the sole use of the spinning apparatus being, apparently, to form a silken retreat at the time of egg-laying or of hibernation.
External Structure.—The Chernetid body consists of a cephalothorax, and an abdomen composed of twelve segments. The segmentation of the abdomen is emphasised by the presence of chitinous plates dorsally and ventrally, but the last two dorsal plates and the last four ventral plates are fused, so that ordinarily only eleven segments can be counted above and nine below.
The cephalothorax presents no trace of segmentation in the Obisiinae (see p. [437]), but in the other groups it is marked dorsally with one or two transverse striae. The eyes, when present, are either two or four in number, and are placed near the lateral borders of the carapace towards its anterior end. They are whitish and only very slightly convex, and are never situated on prominences. Except in Garypus there is no trace of a sternum, the coxae of the legs and pedipalps forming the ventral floor of the cephalothorax.
In the Obisiinae a little triangular projection in front of the cephalothorax is regarded by Simon[[333]] as an epistome. It is absent in the other sub-orders.
The abdomen is armed, dorsally and ventrally, with a series of chitinous plates with membranous intervals. The dorsal plates are eleven in number (except in Chiridium, which has only ten), and are frequently bisected by a median dorsal membranous line. There are nine ventral plates. There is a membranous interval down each side between the dorsal and ventral series of plates.
Fig. [222].—A, Chernes sp., diagrammatic ventral view, × about 12. a, Anus; ch, chelicera; g, generative opening; p, pedipalp; 1, 2, 3, 4, legs. (The stigmata are at the postero-lateral margins of the 1st and 2nd abdominal segments.) B, Tarsus, with claws and sucking disc.
The chitinous membrane between the plates is very extensible, rendering measurements of the body in these animals of little value. In a female full of eggs the dorsal plates may be separated by a considerable interval, while after egg-laying they may actually overlap. The four stigmata are not situated on the plates, but ventro-laterally, at the level of the hinder borders of the first and second abdominal plates.
The first ventral abdominal plate bears the genital orifice. In the same plate there are two other orifices, an anterior and a posterior, which belong to the “abdominal glands.” They were taken by some authors for the spinning organs, but their function is probably to supply material for the capsule by which the eggs are suspended from the body of the mother (see p. [434]).
The Chernetidea possess chelicerae, pedipalpi, and four pairs of ambulatory legs, all articulated to the cephalothorax.
Fig. [223].—Chelicera of Garypus. f, Flagellum; g, galea; s, serrula. (After Simon.)
The chelicerae are two-jointed, the upper portion of the first joint being produced forward into a claw, curving downward. The second joint is articulated beneath the first, and curves upward to a point, the appendage being thus chelate. This second joint, or movable digit, bears, near its extremity, the opening of the spinning organ, and is furnished, at all events in the Garypinae and Cheliferinae (see p. [437]), with a pectinate projection, the “galea,” arising at its base, and extending beyond the joint in front. In the Obisiinae it is only represented by a slight prominence.
Two other organs characterise the chelicerae of all the Chernetidea; these are the “serrula” and the “flagellum.” Their minute size and transparency make them very difficult of observation, and for a long time they escaped notice. The serrula is a comb-like structure attached to the inner side of the distal joint. The flagellum is attached to the outer side of the basal joint, and recalls the antenna of a Lamellicorn beetle, or the “pectines” of scorpions, a resemblance which gave rise to the supposition that they are olfactory organs. It is more likely, however, that they are of use in manipulating the silk.
The pedipalpi are six-jointed and are very large, giving these animals a superficial resemblance to scorpions. According to Simon,[[334]] the patella is absent, and the joints are coxa, trochanter, femur, tibia, tarsus, with an apophysis forming the fixed digit of the chela, while the sixth joint is the movable digit, and is articulated behind the tarsus. These joints, especially the tarsus, are often much thickened, but however strongly developed, they are always narrow and pediculate at the base. The coxae of the pedipalps are closely approximated, and are enlarged and flattened. They probably assist in mastication, but there is no true maxillary plate articulated to the coxa as in some Arachnid groups.
The legs are usually short and feeble, and the number of their articulations varies from five to eight, so that it is not easy to be certain of the homologies of the individual joints to those of other Arachnids. The coxae are large, and form the floor of the cephalothorax. They are succeeded by a short trochanter, which may be followed by another short joint, the “trochantin.” Then come the femur and tibia, elongated joints without any interposed patella, and finally the tarsus of one or two joints, terminated by two smooth curved claws, beneath which is situated a trumpet-shaped membranous sucker.
Internal Structure.[[335]]—The internal structure of the Chernetidea, as far as their small size has permitted it to be made out, bears a considerable resemblance to that of the Phalangidea. The alimentary canal dilates into a small sucking pharynx before passing through the nerve-mass into the large many-lobed stomach, but the narrow intestine which terminates the canal is convoluted or looped, and possesses a feebly-developed stercoral pocket.
Above the stomach are situated the spinning glands, the products of which pass, by seven or more tubules, to the orifice already mentioned on the distal joint of the chelicerae. The abdominal or cement-glands are in the anterior ventral portion of the abdomen. No Malpighian tubes have been found.
The tracheae from the anterior stigmata are directed forward; those from the posterior stigmata backward. Bernard[[336]] has found rudimentary stigmata on the remaining abdominal segments.
The heart is the usual dorsal tube, situated rather far forward, and probably possessing only one pair of ostia. The nerve-cord is a double series of ventral masses, united by transverse commissures. These undergo great concentration in the last stages of development, but in the newly-hatched Chernetid a cerebral mass and five pairs of post-oesophageal ganglia can be distinguished.
There are two peculiar eversible “ram’s-horn organs,” opening near the genital opening. They are said to be present only in the male, and have been taken for the male organs, though other writers consider them to be tracheal in function.
Development.—Some points of peculiar interest are presented by the embryology of these animals, the most striking facts being, first, that the whole of the egg is, in some cases at all events, involved in the segmentation; and, secondly, that there is a true metamorphosis, though the larva is not free-living, but remains enclosed with others in a sac attached to the mother.
At the beginning of winter the female immures herself in a silken retreat, her body distended with eggs and accumulated nourishment. About February the egg-laying commences, thirty eggs, perhaps, being extruded. They are not, however, separated from the mother, but remain enclosed in a sac attached to the genital aperture, and able, therefore, to receive the nutritive fluids which she continues to supply throughout the whole period of development.
The eggs, which line the periphery of the sac, develop into embryos which presently become larvae, that is to say, instead of further development at the expense of yolk-cells contained within themselves, they develop a temporary stomach and a large sucking organ, and become for a time independent sucking animals, imbibing the fluids in the common sac, and arranged around its circumference with their mouths directed towards the centre. Subsequently a second embryonic stage is entered upon, the sucking organ being discarded, and the albuminous matter which the larva has imbibed being treated anew like the original yolk of the egg.
It is an interesting fact that in this second embryonic stage a well-marked “tail” or post-abdomen is formed, and the ganglionic nerve-masses increase in number, a cerebral mass being followed by eight pairs of ganglia in the body and eight in the tail. Subsequently a great concentration takes place till, besides the cerebral mass, only five closely-applied pairs of ganglia remain, corresponding to the pedipalpi and the four pairs of legs. Moreover, the first pair advances, so as to lie on the sides of, and not behind, the oesophagus.
Fig. [224].—Three stages in the development of Chelifer. A, Segmenting ovum; B, embryo, with post-abdomen, maximum number of ganglia, and developing sucking apparatus; C, larva. (After Barrois.)
There are two ecdyses or moults during development, a partial moult, concerning only the ventral surface of the “pro-embryo” as it assumes the larval form, and a complete moult at the final stage, before emergence from the incubating sac.
At the end of winter the mother cuts a hole in the silken web, and the young brood issues forth.[[337]]
Classification.—The order Chernetidea consists of a single family, Cheliferidae. Nine genera are recognised by most authors, but their grouping has been the subject of a good deal of difference of opinion, largely dependent on the different systemic value allowed by various arachnologists to the absence or presence of eyes, and to their number when present. Simon takes the extreme view that the eyes are only of specific value, and he is thus led to suppress two ordinarily accepted genera, Chernes and Roncus, which are separated chiefly by eye-characters from Chelifer and Obisium respectively. He relies rather on such characters as the presence or absence of galea, epistome, and trochantin, and establishes three sub-families as follows:—
(i.) Cheliferinae.—Galea. No epistome. Trochantin on all legs. Eyes two or none. Sole genus, Chelifer (Chelifer + Chernes).
(ii.) Garypinae.—Galea. No epistome. Trochantin on legs 3 and 4 only. Eyes four or none. Genera Chiridium, Olpium, and Garypus.
(iii.) Obisiinae.—No galea. An epistome. No trochantin. Genera Chthonius and Obisium (which includes Roncus).
Whatever be the value of the eyes in the classification of this group—and Simon adduces strong arguments for his view—there can be no doubt of their convenience in practical identification. Moreover, as Pickard-Cambridge[[338]] points out, a grouping of the genera according to the eyes results, as regards British species, in pretty much the same linear arrangement as Simon’s classification, and it may therefore be convenient to mention that, of the six genera represented in this country, Chthonius and Obisium are four-eyed, Roncus and Chelifer two-eyed, while Chernes and Chiridium are eyeless.
Sub-Fam. 1. Cheliferinae.—These Chernetidea have the cephalothorax slightly narrowed in front, and generally marked dorsally with two transverse striae, while the abdominal plates are bisected by a dorsal longitudinal line. With the exception of Chelifer cancroides, which is always found in houses, all the species are to be sought under bark, though occasionally they are discovered under stones.
The two genera of this sub-family are Chelifer and Chernes, the species of Chelifer being two-eyed, and those of Chernes blind.
As already stated, Simon does not consider the possession of the two—often very feebly developed—eyes of generic importance, and admits only the genus Chelifer.
Five species of Chelifer (including Ch. cancroides) and five of Chernes have been recorded in England.
Fig. [225].—Chelifer cyrneus, enlarged. (After Simon.)
Fig. [226].—Chiridium museorum, enlarged. (After Simon.)
Sub-Fam. 2. Garypinae.—The Garypinae have the cephalothorax greatly contracted in front and often projecting considerably.
Fig. [227].—Olpium pallipes, enlarged. (After Simon.)
There are three genera, Chiridium, Olpium, and Garypus. Chiridium is eyeless, and appears to have only ten segments in the abdomen, the tergal plates of which are bisected. C. museorum is found in England, and is the only Chernetid except Chelifer cancroides which habitually lives in houses. C. ferum is found under bark in the south of France.
Neither Olpium nor Garypus, which both possess four eyes and eleven abdominal segments, have as yet been found in this country. Garypus, like Chiridium, has the dorsal abdominal plates bisected. There is a transverse stria on the cephalothorax, and the eyes are far from the anterior border. In Olpium the dorsal plates are undivided and the eyes more anterior.
Sub-Fam. 3. Obisiinae.—The cephalothorax of the Obisiinae does not narrow—and is, indeed, sometimes broadest—anteriorly. The chelicerae are notably large, and the dorsal abdominal plates undivided. They are the most active of the Chernetidea, ordinarily running backwards or sideways with their pedipalpi closely folded up against the body. Four genera usually admitted fall within this group:—Obisium, Roncus, Blothrus, and Chthonius.
Obisium has four eyes, parallel-sided cephalothorax, and curved chelae on the palps. Roncus is like Obisium except in having only two eyes, and is therefore disallowed by Simon, who also considers Blothrus to comprise merely eyeless species of Obisium. In Chthonius the cephalothorax is broadest in front, and the digits of the chelae are straight.
The Obisiinae are found in moss and débris, and under stones. Three species of Obisium, two of Roncus, and four of Chthonius are recorded in England.
The subjoined list of British species of Chernetidea is taken from the monograph of the Rev. O. P. Cambridge, cited above:—
Group I.—Four eyes.
Chthonius orthodactylus, Leach.
„ rayi, L. Koch.
„ tetrachelatus, Preyssler.
„ tenuis, L. Koch.
Obisium muscorum, Leach.
„ sylvaticum, C. L. Koch.
„ maritimum, Leach.
Group II.—Two eyes.
Roncus cambridgii, L. Koch.
„ lubricus, L. Koch.
Chelifer hermannii, Leach.
„ cancroides, Linn.
„ meridianus, L. Koch.
„ subruber, Simon.
„ latreillii, Leach.
Group III.—No eyes.
Chernes nodosus, Schr.
„ insuetus, Camb.
„ cimicoides, Fabr.
„ dubius, Camb.
„ phaleratus, Simon.
Chiridium museorum, Leach.
CHAPTER XVII
ARACHNIDA EMBOLOBRANCHIATA (CONTINUED)—PODOGONA—PHALANGIDEA = OPILIONES—HABITS—STRUCTURE—CLASSIFICATION
Order VII. Podogona (Ricinulei).
Tracheate Arachnids with two-jointed chelate chelicerae and prehensile pedipalpi. The tarsus of the third leg of the male bears a copulatory organ.
Fig. [228].—Cryptocellus simonis, × 4. (After Hansen and Sörensen.)
In 1838 Guérin-Méneville[[339]] described an Arachnid from West Africa which he named Cryptostemma westermannii. At rare intervals occasional specimens of allied forms have been taken in the same region until six species of Cryptostemma have been established. In South America, also, two unique examples of very similar creatures are the only known representatives of the two species of the allied genus Cryptocellus. All the examples hitherto found are of fair size (between ⅕ inch and ½ inch in length), and bear some general, though superficial, resemblance to the Trogulidae, which has led to their being placed among the Phalangidea by almost all the Arachnologists who have noticed them. Their claim to this systematic position, however, is extremely doubtful, and Hansen and Sörensen, who have had the opportunity of studying the group much more minutely than previous writers, are of the opinion that they ought to constitute a separate order of Arachnids, more nearly allied to the Pedipalpi than to the Phalangidea. In this place it is only possible to indicate some of their peculiar characteristics. Their integuments are particularly hard and coriaceous. The cephalothorax is united to the abdomen by a rather broad pedicle, but there is also a remarkable coupling apparatus which makes the constriction between cephalothorax and abdomen appear very slight. There is a movable anterior projection of the cephalothorax, the “cucullus.” The two-jointed chelicerae terminate in minute chelae, as also do the five-jointed pedipalps. There are no spiracles on the abdomen, but two are situated on the thorax above the coxae of the third pair of legs. Perhaps the most remarkable fact is that, as in the Araneae, a modified limb is used by the male for the fertilisation of the female; but in this case it is not the tarsus of the pedipalp, but of the third leg of the male, which is specially developed as an intromittent organ.
Ordinal rank is not universally accorded to the group, but whatever its true position, the known forms fall under a single family Cryptostemmatidae, including the two genera Cryptostemma and Cryptocellus.
Order VIII. Phalangidea (Opiliones).
Tracheate Arachnids, with abdomen united to the cephalothorax by its whole breadth. They are oviparous, and undergo no metamorphosis. Abdomen always segmented. A pair of odoriferous glands opening on the thorax. Two simple eyes; three-jointed chelate chelicerae; pedipalpi not chelate. Spinning organs absent.
“Harvesters,” “Harvestmen,” or “Harvest-spiders,” as these animals are popularly called, need never be confounded with true Spiders if the absence of a constriction between the cephalothorax and abdomen be noted. They are more difficult to distinguish from Mites, members of which group have sometimes been described as Phalangids. The Phalangid is, however, generally recognisable by its segmented abdomen, and as a further point of distinction, it may be noted that, whereas the anal orifice is always transverse or circular in Phalangids, it is uniformly longitudinal in the Acarines.
Fig. [229].—Oligolophus spinosus. (After Pickard-Cambridge.)
Members of this group vary considerably in habit. The best known forms are exceedingly active, and trust to their speed in endeavouring to escape from danger, at the same time emitting an odorous fluid from two apertures situated just above the coxae of the first pair of legs. These active Harvestmen are only found in the mature state at certain seasons of the year, and are believed, therefore, to live only for a single season. Slow-moving forms, like the Nemastomatidae and the Trogulidae, which live amidst grass and herbage, have a much longer duration of life. In danger they remain perfectly still, and trust to their earthy appearance to escape observation.
They are stated to be extremely thirsty animals, and have been observed drinking from the dewdrops on herbage. It is probably on this account that they are sometimes seen attacking juicy vegetable matter, for without doubt they are essentially carnivorous. The larvae of insects, young spiders, mites, and myriapods are their customary food. It is not requisite that the prey should be alive, but they will not touch anything mouldy.
Notwithstanding their apparently weak mouth-parts, they do not merely suck the juices of their victims, but masticate and swallow solid particles. Cannibalism is frequently observed among them.
The males fight fiercely with one another at the breeding time. The females, with their long extrusible ovipositors, place groups of twenty to forty eggs in small holes in the ground or under stones or bark, unprotected by any form of cocoon. The eggs hatch into fully-formed Phalangids, which are at first white, but attain their coloration after the first moult. They subsequently moult from five to nine times.
The distribution of this group is world-wide, and some of the exotic species are very remarkable in form. Only twenty-four species have as yet been recorded in this country.
External Structure.—In the Phalangidea there is no constriction between the cephalothorax and the abdomen, and in the Ischyropsalidae alone is the distinction between them readily observable. This is due to the partial or complete fusion of the first five segments of the abdomen with the carapace or cephalothoracic shield in most species, these segments being indicated, if at all, merely by faint striae or successive transverse rows of spines or tubercles. In the forms possessing hard integuments (Gonyleptidae, Nemastomatidae, Trogulidae) this fusion results in a dorsal “scutum,” the component parts of which cannot easily be distinguished.
The cephalothorax is often surmounted by a turret—usually grooved dorsally, and beset on its edges with a spiny armature—on the sides of which are the two simple eyes. The position and shape of this turret and the arrangement of its spines are of importance in the classification of the group.
Fig. [230].—Hood of Metopoctea. (After Simon.)
In the Trogulidae the base of the turret gives rise to a remarkable, forwardly-directed, bifurcate structure, furnished with numerous strong tubular bristles. This is called the “hood,” and its hollowed-out under surface forms a chamber, the “camerostome,” in which lie the basal joints of the pedipalpi.
In most European Phalangids the under surface of the cephalothorax is almost entirely concealed by the forwardly-projecting portion of the abdomen bearing the generative opening, and by the gnathobases, not only of the pedipalpi, but of the first and sometimes of the second legs. As in Spiders, however, there is always present a “sternum” and generally a “labium.” The sternum is long and narrow in the Mecostethi, and Cyphophthalmi, but in the Plagiostethi, which include most of the forms found in temperate regions, it is very short and transverse, and is hidden by the abdominal prolongation before mentioned.
Fig. [231].—Mouth-parts of Phalangium. A, B, C, Gnathobases of pedipalp and first and second legs; ch, chelicera; ep, epistome; lab, labium; m, mouth; ped, pedipalp; pre.ep, pre-epistome; st, sternum, shown by the removal of the anterior part of the genital process, which extends to the dotted line; 1, 2, 3, 4, legs.
The anterior wall of the mouth is formed by a beak-like plate, the “epistome,” the basal portion of which is covered externally by a second plate, for which Simon[[340]] proposes the name “pre-epistome.” In some Phalangids there are three little chitinous plates, one median and two lateral, on the clypeus, between the anterior border of the carapace and the insertion of the chelicerae. They are best seen in Nemastoma.
The abdomen always presents evidences of segmentation, though there is a difference of opinion as to the number of segments of which it is composed. This is due to the already mentioned partial or complete fusion of the anterior segments with the cephalothorax. From the admirable researches of Hansen and Sörensen[[341]] it seems likely that the normal number of abdominal segments is ten. Ventrally, the abdomen is produced forward into a “sternal process” which is capped by a genital plate, hardly distinguishable in the Phalangidae, but readily visible in the other families, which surrounds and masks the unpaired genital orifice. Two stigmata or breathing pores are situated on the sides of the first ventral plate, which these authors consider to be composed of two fused sternites.
As in other Arachnids there are six pairs of appendages articulated to the cephalothorax. They are the chelicerae, the pedipalpi, and the four pairs of ambulatory legs.
The chelicerae are three-jointed and chelate, the second joint having its inner portion produced into an apophysis to which the final joint is apposed. In certain forms (Gonyleptidae, Ischyropsalis) the chelicerae are remarkably long, and may considerably exceed the total length of the trunk.
The pedipalpi are six-jointed, possessing coxa, trochanter, femur, patella, tibia, and tarsus. They are leg-like and are never chelate, but in some forms terminate in a single movable claw. The coxal joints are provided with maxillary plates.
The legs are normally seven-jointed, as in Spiders, the penultimate joint being the metatarsus. The tarsus is always multi-articulate, the number of its joints being variable. It bears terminally one or two simple claws. “False articulations” (where the parts are not inserted one into the other, but are only marked off by a membranous ring) are of frequent occurrence in the legs of these creatures. The first legs, like the pedipalps, bear maxillary plates, as do also the second in most Phalangids. The maxillae of the second legs are, however, entirely absent in Nemastoma, and rudimentary in the Gonyleptidae and the Ischyropsalidae. The coxae of the legs are all largely developed, but are not capable of free motion, being soldered to, and practically forming part of, the cephalothoracic floor. In some forms they are only separated from one another by slight grooves. The extreme length of the legs, and their hard and brittle nature, are characteristic features of the Phalangids, though in some species (Trogulidae) they are comparatively short. The first pair of legs are always the shortest, and the second the longest.
The sexual organs of Phalangids are ordinarily concealed, and the sexes can only be distinguished by certain very variable secondary characters, the males being usually smaller of body and longer of leg than the females, besides being more distinctly coloured and being armed with more numerous and longer spines. Sometimes the male chelicerae are highly characteristic.
Phalangids are usually destitute of spinning organs, but such have been discovered, in a rudimentary state, in the Cyphophthalmi, which are said to spin slight webs.
Internal Structure.—In Phalangium the mouth leads upwards into a membranous pharynx, wider than that of Spiders, but narrowing into an oesophagus which passes between the cerebral and thoracic ganglionic nerve-masses. It then turns backwards over the thoracic ganglion, being slightly dilated at that point. Immediately afterwards it dilates into a flask-like gastric sac which occupies almost the whole width of the abdomen, and proceeds straight to the anus. Viewed from above, the shape of this sac is entirely concealed by the large number of caeca (thirty) to which it gives rise dorsally and laterally. The two largest of these caeca extend, parallel to each other, over the whole of the abdominal portion of the gastric sac, and are flanked by four lateral pairs of smaller caeca, while there is a cluster of small caeca covering the anterior and narrower portion of the flask-like stomach.
The large hepatic mass so conspicuous on opening dorsally the abdomen of a Spider is here entirely absent, but its functions are believed to be performed by certain wrinkled, tubular, longitudinally parallel bodies, about seven in number, closely applied to the under surface of the flask.
The masticating portions of the maxillae of the pedipalpi and the first pair of legs are hollow distensible sacs, often seen in a swollen condition in specimens kept in spirits. They are furnished, on the inner surface, with a horny ridge.
Owing to the fixity of the coxae of the legs, their maxillary plates are incapable of much lateral motion, but are rubbed against each other vertically.
Beyond the fact that the heart is a dorsal tube lying along the anterior two-thirds of the alimentary canal, and divided by constrictions into three well-marked and equal portions, little is known of the blood-system of these animals. It is probably essentially like that of Spiders, but the presence of a pericardial sac has not yet been established, nor has the course of the blood-vessels been described in detail.
As in other Arachnids, the principal ganglionic nerve-masses closely embrace the oesophagus. Immediately anterior to it, forming a conical mass with its base on the oesophagus, is the cerebral ganglion, while just behind it is the transverse portion of the large thoracic nerve-centre. In Phalangium opilio, according to Tulk,[[342]] a median nerve is given off from the apex of the cerebral mass (the paired nature of which is apparent) and bifurcates to the two eyes. Two lateral nerves proceed to certain organs near the origin of the second pair of legs, which were thought by the old writers to be lateral eyes, but which are now known to be glands for the manufacture of the odorous fluid which these animals can exude.
Fig. [232].—Nervous and respiratory systems of a Phalangid. Nerves black, tracheae white. c.g, Cerebral ganglion; g′, g″, g‴, ganglia supplying viscera; m.n, median abdominal nerve; oe, passage for oesophagus; st, stigma; th.g, thoracic ganglion; tr, main trunk of tracheae.
The thoracic ganglion expands, on either side of the oesophagus, into a mass which extends nearly as far forward as the apex of the cerebral ganglion. These lateral masses give off nerves to the appendages. From the back of the transverse portion proceed three nerves. The median nerve passes above the generative organs, and soon branches into two nerves which presently swell out to form ganglia of considerable size, beyond which they soon join again and give off an anastomosing network of nerve-fibres. The lateral nerves immediately branch. The outer branch dilates into a ganglion which supplies the external part of the generative organ. The inner branch, which is longer, also forms a ganglion the nerves from which are chiefly distributed to the under surface of the alimentary canal.
The respiratory organs consist of two large tracheal tubes with numerous branches, having their external openings or “stigmata” near the base of the fourth pair of legs. The two main tubes are directed forwards, and are mainly concerned with supplying the largely developed muscles of the legs. The distribution of branches to the abdomen is comparatively feeble. The particular arrangement of tubes in P. opilio, according to Tulk, may be seen in the accompanying figure. There are a pair of coxal glands, of excretory function, opening in the neighbourhood of the coxae of the third pair of legs.
The Phalangidea are remarkable among Arachnids in the possession of large protrusible external organs of generation. The ovipositor of the female may be as long as the whole body of the animal, and the intromittent organ of the male is of almost equal length. The pedipalpi take no part in the fertilisation of the female, which is accomplished directly.
The protrusible organs are concealed under the forwardly-projecting anterior segment of the abdomen beneath, the genital orifice being thus in many cases quite near the head region. The internal sexual organs are not very complex. The ovary re-enters upon itself, forming a ring, and from the point of re-entry a tube proceeds towards the centre of the ring, dilating to form an ovisac. It then narrows, turns forward, dilates once more into a second ovisac, from which the oviduct proceeds to the base of the ovipositor. This is a flattened organ, grooved on its upper surface and bifid at its extremity. The testis of the male is a single sac-like gland, from either end of which proceeds a vas deferens, which, after several convolutions, unite into a sperm-sac which opens at the base of the penis.
Partial hermaphroditism is a very frequent phenomenon among the Phalangids, the testis often producing ova as well as spermatozoa.
Though the males fight fiercely at the breeding time, the animals for the most part live peacefully together. Henking[[343]] found that the eggs of Liobunum, which were about half a millimetre in diameter, were laid during October and hatched out in the following April.
Classification.—The Order Phalangidea is divided into three Sub-orders: 1, Cyphophthalmi; 2, Mecostethi; 3, Plagiostethi.
Sub-Order 1. Cyphophthalmi
Phalangids with dorsal and ventral scutum, only the last abdominal segment remaining free. Eyes two or absent. Maxillary lobe on coxae of first pair of legs rudimentary. Sternum long and narrow. Anterior segment of abdomen not projecting ventrally beyond the coxae of the fourth pair. Odoriferous glands open on prominences.
In 1875 Stecker published a description of a remarkable creature which he said he had found in Bohemia, and which he named Gibocellum sudeticum. Among other points it possessed four eyes and four spinning mammillae, and it differed so much from other Cyphophthalmi as to necessitate the foundation of a family, Gibocellidae, for its reception. No one else appears to have seen the animal, or any of Stecker’s preparations of it, and Hansen and Sörensen[[344]] adduce grave reasons for believing that it never existed at all. If this species is to be disallowed, the Cyphophthalmi all fall into a single family.
Fig. [233].—Parasiro corsicus, enlarged. (After Simon.)
Fam. Sironidae.—These somewhat Mite-like Phalangids are rarely met with, partly, no doubt, because of their retiring habits and small size, the known forms ranging from 6 mm. to less than 2 mm. in length. Of the seven genera which have been established, Stylocellus numbers eight species from Borneo and Sumatra, and Pettalus two species from Ceylon. Ogovia, Miopsalis, and Purcellia have one species each, from South Africa, Further India, and the Cape, respectively. The only European forms are the two species of Siro (France and Austria), and Parasiro corsicus. No species has yet been found in England.
Sub-Order 2. Mecostethi.[[345]]
(LANIATORES).
Sternum long and narrow. Dorsal scutum leaving at least the last three segments free. Openings of odoriferous glands not on prominences. The fourth pair of legs usually long and powerful. One terminal claw on each of the first two pairs of legs; two on the last two pairs.
The Mecostethi are essentially tropical forms, though a few representatives are found in the caves of Southern Europe. One family (Phalangodidae) has its headquarters in the hot regions of the Old World, while the other two (Cosmetidae, Gonyleptidae) are confined to Central and South America.
Fam. 1. Phalangodidae.—Body piriform or triangular, broadest behind. Last ventral segment of abdomen much the largest. Very narrow sternum. Eye-turret near anterior border of cephalothorax. Chelicerae narrow at base. Pedipalpi long and strong. Maxillary plates on first pair of legs rudimentary. No stigmata visible.
The only European forms of this family belong to the genus Phalangodes. They all avoid the light, and are usually found in caves. Simon[[346]] records six species found in France. A North American species, P. armata, is entirely destitute of eyes.
Fig. [234].—Phalangodes terricola, enlarged. (After Simon.)
The family has representatives in Australia and in tropical Africa and Asia. Mermerus, Epidanus, Maracaudus, and Sitalces are some of the exotic genera.
The other two families of this Sub-order—Fam. 2, Cosmetidae; Fam. 3, Gonyleptidae—include a large number of species, some of considerable size (up to an inch in length of body), found in Central and South America.
Sub-Order 3. Plagiostethi.[[347]]
(PALPATORES.)
First abdominal segment produced forward ventrally to the level of the first pair of legs, bringing the mouth and the genital opening very near together. Sternum consequently much reduced. Pedipalpi thin, with terminal claw absent or rudimentary. Terminal claws of the legs single.
The Plagiostethi include most of the Harvestmen of temperate regions, the most familiar examples of these creatures belonging to the large family Phalangidae, and being much more in evidence than the slow-moving and ground-living forms included in the other families.
Fam. 1. Phalangiidae.—Eye-turret always far removed from anterior border of cephalothorax. Second pair of legs with well-marked maxillary lobes. Legs similar, without the false joint called “trochantin.” Multiarticulate tarsi. Simple pedipalpi, with tarsus much longer than tibia, and possessing terminal claw. Some have soft, some coriaceous integuments.
The Phalangidae fall naturally into two groups or sub-families, named by Simon Sclerosomatinae and Phalangiinae. The first group consists of more or less coriaceous forms living among moss and herbage. They are not very numerous, there being only about twelve known European species divided among the three genera, Sclerosoma, Mastobunus, and Astrobunus.
Fig. [235].—Sclerosoma quadridentatum. (After Pickard-Cambridge.)
Two species of Sclerosoma are found in England, S. quadridentatum occurring not uncommonly among moss or under stones in various parts of the country. Its back is studded with wart-like tubercles, which give it a characteristic appearance.
The Phalangiinae are soft-bodied Harvestmen, always with long legs, which in the genus Liobunum attain an inordinate length. There are nine European genera, Liobunum, Prosalpia, Gyas, Oligolophus, Acantholophus, Phalangium, Dasylobus, Platybunus, and Megabunus, comprising in all about fifty species. Five of these genera are represented in England.
The familiar Phalangids, with small, almost spherical bodies and ridiculously long legs, belong to the genus Liobunum, L. rotundum being the common species. It is mature in autumn, when it may be seen scampering at a great pace among the herbage. It very readily parts with its limbs, and Pickard-Cambridge[[348]] relates that he once “saw one running with very fair speed and facility, having lost all but two legs, an anterior one on one side and a posterior one on the other.”
The Harvestmen so frequently seen on walls belong, as a rule, to the genus Phalangium. The best known example is Phalangium opilio (the P. cornutum of Linnaeus), the male of which possesses a remarkable development of the chelicerae.
The genus Oligolophus is well represented in this country, nine species having been recorded. They do not differ greatly from Phalangium, but have, as a rule, more massive bodies, and rather stout, though tolerably long legs. The largest English Harvestman, not rare under stones at Cambridge, is O. spinosus, whose body measures half an inch in length. O. agrestis is perhaps the commonest British Phalangid, and is abundant in woods and among herbage, and on low trees.
Fig. [236].—Oligolophus spinosus. (After Pickard-Cambridge.)
Platybunus has two, and Megabunus one British representative. They are of small size, and are to be sought for among heather or dead leaves in spring or early summer.
Fam. 2. Ischyropsalidae.—Coriaceous Phalangids, with eye-turret far removed from anterior border of cephalothorax. Maxillary lobes of second pair of legs rudimentary, in the form of tubercles. Legs similar, without “trochantin.” Multiarticulate tarsi. Tarsus of pedipalp without claw, and shorter than metatarsus. Pedipalps long and horizontal.
This family includes a small number of large or moderate-sized Phalangids, which are found occasionally in thick moss, or in caves, in mountainous regions of the south of Europe, and belong to the genera Ischyropsalis and Sabacon. There is a North American genus, Taracus.
Fam. 3. Nemastomatidae.—Coriaceous Phalangids, with cephalothorax fused with the first five segments of the abdomen, forming a scutum. Eye-turret near anterior border. No maxillary lobe on second coxae. Similar legs, without “trochantin.” Multiarticulate tarsi. Tarsus of pedipalp without claw, and shorter than metatarsus.
Fig. [237].—Nemastoma lugubre.
There is but one genus, Nemastoma, in this family, and the members of it are, as a rule, rather small and dark Phalangids, which live under stones or in moss or débris, and are found in the mature state at all seasons of the year. There are about twenty European species, but only two of these, N. lugubre and N. chrysomelas, have as yet been found in Britain. N. lugubre is a very common animal, and though it does not obtrude itself upon public notice, its little black body with two pearly white spots must be a familiar object to all insect collectors who have occasion to search under stones or among moss in damp places. Its legs are short and stout, but those of N. chrysomelas, which is a brighter coloured Harvestman with spots of dull gold colour, are long and slender.
Fam. 4. Trogulidae.—Coriaceous and very hard integument. Anterior part of cephalothorax produced into a bifurcate “hood.” Often a “trochantin.”
Fig. [238].—Trogulus aquaticus. a, Hood. (After Simon.)
The Trogulidae are very slow-moving Phalangids of moderate or large size (a sixth to half an inch in body), found under stones or in damp moss and débris. They are Mite-like in general appearance, and may readily be distinguished from all other Harvestmen by the presence of the “hood” (Fig. [230], p. 442), the hollowed-out under surface of which forms a chamber, called by Simon the “camerostome,” in which lie the basal portions of the pedipalps.
Only a single immature specimen has been found in England, belonging probably to the species Trogulus tricarinatus. It was found in Dorsetshire. Some members of the family are not uncommon in various regions of the Continent. There are four genera, Dicranolasma, Anelasmocephalus, Calathocratus, and Trogulus. Two other genera, Amopaum and Metopoctea, have been established, but the former is probably the young of Dicranolasma and the latter of Trogulus.
According to the monograph on the British Phalangidea by the Rev. O. Pickard-Cambridge, cited above, the following species have been recorded in this country. They all fall under the sub-order Plagiostethi:—
BRITISH PHALANGIDEA.
Phalangiidae.
Sclerosoma quadridentatum, Cuvier.
„ romanum, L. Koch.
Liobunum rotundum, Latr.
„ blackwallii, Meade.
Phalangium opilio, Linn.
„ parietinum, De Geer.
„ saxatile, C. L. Koch.
„ minutum, Meade.
Platybunus corniger, Meade.
„ triangularis, Herbst.
Megabunus insignis, Meade.
Oligolophus morio, Fabr.
„ alpinus, Herbst.
„ cinerascens, C. L. Koch.
„ agrestis, Meade.
„ tridens, C. L. Koch.
„ palpinalis, Herbst.
„ ephippiatus, C. L. Koch.
„ spinosus, Bosc.
Nemastomatidae.
Nemastoma lugubre, Müller.
„ chrysomelas, Hermann.
Trogulidae.
Anelasmocephalus cambridgii, Westwood.
Trogulus tricarinatus, Linn.
CHAPTER XVIII
ARACHNIDA EMBOLOBRANCHIATA (CONTINUED)—ACARINA—HARVEST-BUGS—PARASITIC MITES—TICKS—SPINNING MITES—STRUCTURE—METAMORPHOSIS—CLASSIFICATION
Order IX. Acarina (Acari, Acaridea).
Arachnids with unsegmented,[[349]] non-pediculated abdomen. Respiration by tracheae, or by the general surface of the body. Month parts suctorial, but frequently capable of biting or piercing. Metamorphosis always observable.
The Acarina or Mites are remarkable not so much for the number of their species, which is very considerable, as for the vast multitude of individuals of the Order, which is far in excess of that of any other Arachnid group. This fact is correlated with their minute size. Very few Mites exceed half an inch in length, while very many are microscopic creatures, often measuring less than the hundredth of an inch. Taken all round, a millimetre may be considered a large size for a Mite.
There is much variety of habit within the Order. All Mites live principally on fluid nutriment, but it may be obtained from living animals or plants or from decaying organic matter. Some are entirely parasitic upon plants or animals; others attach themselves to animals in their larval stage, but are free when adults; while others, again, live an entirely independent and predaceous life.
The greater number of the Mites are too small to strike the eye. Some of them have, however, contrived to attract attention, in no very agreeable manner. Every one knows the Mite popularly called the “Harvest-bug,” but to this day there is some uncertainty as to its identity. It was described as a separate species under the name of Leptus autumnalis, and Mégnin was the first to show that it was the larval form of one of the Trombidiidae (see p. [472]). Most authors have considered it the larva of Trombidium holosericeum, but Murray referred it to the genus Tetranychus. The difficulty is that the minute creature cannot be removed from its victim without such injury as to prevent it from being bred out and the mature form determined. Brucker[[350]] has recently compared a large number of “Harvest-bugs” taken from human beings with the figures and descriptions of the larvae of certain Trombidiidae given by Henking and Berlese, and he determined them as the larvae of T. gynopterorum. Quite possibly, however, more than one genus is concerned in the production of this pest.
That certain skin-diseases are due to Mites (Demodicidae, Sarcoptidae) is a fact which is widely known. The fruit-grower, too, has to take cognisance of the Order, for his trees may suffer from “Red-spider” (Tetranychus telarius), and his black-currant bushes fail under the attack of the “Gall-mite” (Eriophyes or Phytoptus ribis). The curious swellings or galls which disfigure the leaves of many trees are sometimes of insect origin, but they are often due to Mites.
Domestic pets suffer greatly from Acarine parasites. A large number of species confine their attention exclusively to the feathers of birds (Analges, etc., see p. [466]). One curious genus, Syringophilus, is parasitic within the feathers, feeding upon the pith of the quill. Heller of Kiel discovered them in 1879, but the researches of Trouessart first showed their frequent presence and very wide distribution. He found that they entered by the superior umbilicus of the feather, and disappeared by the inferior umbilicus when the feathers moulted or the infested bird died.
It is probable that the comparatively large Mites of the group Ixodoidea (see p. [468]), commonly called “Ticks,” are the most widely known of the order. They attack wild and domestic animals and man, and are nearly always acquired from vegetables, such as brush or herbage. It would seem likely that many of these creatures can never have the chance of attaching themselves to animals, and it has been suggested that animal juices are a luxury but not a necessity to them, and that they can live, if need be, on vegetable sap, but further investigations have quite dispelled this view.
The suspected connection between the North American Tick, Boophilus annulatus, and the cattle disease known as Texas fever or “red water,” since clearly proved by the researches of Smith and Kilborne, led to the careful investigation of the life-history of that creature, and this was undertaken by Curtice.[[351]]
The female Ticks laid eggs a few days after dropping off the cattle, egg-laying lasting a week or more. The eggs took from three to five weeks to hatch, and the larvae attached themselves to cattle, on which they remained a fortnight, becoming mature and fertilised before they again sought the ground. The whole cycle occupied a time varying from six to ten weeks, a period apparently much exceeded by some members of the family.
Lounsbury[[352]] has recently made out the life-history of the South African “Bont” tick, Amblyomma hebraeum.
The eggs are deposited in the soil, ten to twenty thousand eggs in all being laid by one female. The larvae climb neighbouring plants and seize passing animals. After the third day of attachment they begin to distend, and they generally fall off, fully distended, on the sixth day, immediately seeking a place of concealment, where they become torpid. Under natural conditions the nymph does not emerge for at least eleven weeks, and then it behaves in the same way as the larva, again attaching itself to an animal for six days. A new time of torpidity and concealment ensues, again of at least eleven weeks’ duration, when the final moult takes place and the mature tick emerges. The males at once attach themselves to animals, but the females hesitate to fix themselves, except close by a male. For four days after fixation the male appears to exercise no attraction for the female, but after that period he shows great excitement at her approach. She, however, does all the courting, the male remaining fixed in the skin of the host. After pairing, the female distends greatly, attaining her maximum size (nearly one inch in length) in about a week, when she lets go and descends to the earth to lay eggs. If unmated, she detaches herself within a week, and seeks another host. Oviposition lasts from three to nine weeks, and the development of the egg from eleven weeks to six months. At least a year is occupied in the whole cycle. These ticks, and many others, communicate disease[[353]] by inoculation, conveying it from one animal to another.
No poison-glands have been demonstrated in the Acari, the function of the salivary glands of the Ticks being probably to prevent the coagulation of the blood of their victims.
It is an important point in the mode of life of the Ticks that they can live for a long time without food. Mégnin[[354]] states that he kept an Argas alive for four years, entirely without nutriment.
In the Tetranychinae (see p. [472]), glands apparently homologous with the salivary glands of the Ticks have taken on the function of spinning organs. According to Donnadieu,[[355]] these glands, which resemble bunches of grapes, and are possessed by both sexes, open into the buccal cavity at the base of the chelicerae. The gummy fluid exudes from the mouth, and is combed into threads by the pedipalps. The legs of these mites are furnished terminally with curious hairs ending in a round knob, which are supposed to have some relation to their spinning habits.
The males are the busiest spinners, the time of the females being largely occupied in laying eggs among the excessively fine threads of silk with which the Mites cover the under surface of leaves. In the Eriophyidae (see p. [464]) corresponding glands are thought to furnish an irritant fluid which causes abnormal growths or galls upon vegetable tissues.
External Structure.—It is often stated, but erroneously, that there is no distinction between cephalothorax and abdomen in the Mites. Certainly no such division can be made out in the Hydrachnidae (see p. [472]) or in some other forms, but in the majority of Acari the cephalothorax is clearly marked off by a transverse groove or suture. In some cases the anterior portion of the cephalothorax is movably articulated with the rest, and forms a sort of false head called a “capitulum.” In most Mites the chitinous integument is soft and non-resistant, but it is otherwise with the Oribatidae or “Beetle-mites” (see p. 467), which are nearly all covered by an extremely hard and coriaceous armature.
Eyes are sometimes absent, sometimes present in varying numbers. They seem here to be of remarkably little systematic importance, as otherwise closely allied species may be either eyed or eyeless.
Normally Mites possess the usual Arachnid appendages, chelicerae, pedipalpi, and four pairs of ambulatory legs. The anterior appendages are, however, subject to a very great degree of modification, while in one Family, the Eriophyidae (Phytoptidae), the legs are apparently reduced to two pairs.
The chelicerae are sometimes chelate, in which case they are two-jointed, the distal joint or movable finger being always articulated below the immovable finger. Sometimes they terminate in a single claw or blade, the movable joint being obsolete. In the Ticks they exist as two long styles or piercing weapons, serrate on the outer edge.
The pedipalpi vary very much in structure, according to the habits of the particular form to which they belong. In the Sarcoptidae (see p. [466]) they are hardly recognisable owing to the extent to which they have coalesced with the maxillary plate. In many of the free-living forms they are leg-like feeling organs, but in others they are raptorial, being not precisely chelate, but terminating in a “finger-and-thumb” arrangement which is of use in holding prey. The extreme development of the raptorial palp is found in Cheyletus (see p. [473]), in which the whole appendage is remarkably thick and strong, and the “finger” is a powerful chitinous claw, while the “thumb” is replaced by movable pectinated spines of chitin. The Water-mites have a palpus adapted for anchoring themselves to water-weeds, the last joint being articulated terminally with the penultimate joint, and bending down upon it. Finally, in the “Snouted mites” (Bdellidae, see p. [471]) the palpi are tactile or antenniform, often strongly recalling the antennae of weevils.
The maxillary plates which arise from the basal joints of the pedipalps are always more or less fused, in the Mites, to form a single median transverse plate, constituting the lower lip or “labium” of some authors. In some of the Oribatidae the fusion of the maxillae is only complete at the base, and the free points are still of some use as masticating organs. In those free living Mites which have undergone no great modification of the mouth-parts two other portions can be distinguished, the upper lip or “epipharynx,” and the “lingua,” which forms the floor of the mouth, and is for the most part concealed by the maxillary plate.
The legs are usually six- or seven-jointed, and are subject to great variation, especially as regards the tarsus or terminal joint. This may bear claws (1–3) or sucking disks, or a combination of the two, or may simply take the form of a long bristle or hair.
The Cheese-mite has a claw surrounded by a sucker—like Captain Cuttle’s hook within his sleeve. The claws of those species which are parasitic on the hairs of animals are sometimes most remarkably modified.
Fig. [239].—Diagram of the viscera of an Oribatid Mite, greatly enlarged. C, C, Lateral caeca of stomach; g, cerebral ganglion; od, od, oviducts; oe, oesophagus; pr.g, pro-ventricular gland; ps, pseudo-stigmatic organ; st, stomach; tr, tr, tracheae. (Partly after Michael.)
Internal Structure.—The minute size of most Mites has rendered research upon their internal structure a matter of great difficulty, and there are still many obscurities to be removed. Those forms which have been subjected to examination present a tolerable uniformity in the structure of the principal organs, but the brief description here given will not, of course, apply to aberrant groups like the Vermiformia. A marked concentration is noticeable throughout the Order, and is best exemplified by the nervous system.
The mouth leads into a sucking pharynx, which narrows to form the oesophagus. This passes through the nerve-mass in the usual Arachnid fashion, and widens to form the ventriculus or stomach. The oesophagus varies considerably in width in the various groups, being very narrow in those Mites which merely suck blood, but wider in vegetable-feeders like the Oribatidae.
The stomach is always provided with caeca, but these are not nearly so numerous as in some other Orders of Arachnida. There are always two large caeca directed backwards, and there may be others. They are most numerous in the Gamasidae (see p. 470), which sometimes possess eight, some being prolonged into the coxae of the legs, as in Spiders. At the sides of the anterior part of the stomach there are usually two glandular bodies, the pro-ventricular glands. In those Mites in which the alimentary canal is most differentiated (e.g. Oribatidae) three parts are distinguishable behind the stomach, a small intestine, a colon, and a rectum, but in most groups the small intestine is practically absent. The Malpighian tubes, very variable in length, enter at the constriction between colon and rectum.
In some of the Trombidiidae there appears to be a doubt as to the existence of a hind-gut at all. A body having the appearance of the hind-gut, and occupying its usual position, is found to contain, not faecal matter, but a white excretory substance, and all efforts to discover any passage into it from the stomach have been unsuccessful. Both Croneberg[[356]] and Henking[[357]] came to the conclusion that the stomach ended blindly, and that the apparent hind-gut was an excretory organ. Michael,[[358]] in his research upon a Water-mite, Thyas petrophilus, met with precisely the same difficulty, and was led to the belief that what was originally hind-gut had become principally or entirely an excretory organ.
The nervous system chiefly consists of a central ganglionic mass, usually transversely oval, and presenting little or no indication of the parts which have coalesced in its formation. Nerves proceed from it in a radiate manner, but no double nerve-cord passes towards the posterior end of the body. As above stated, it is perforated by the oesophagus.
The vascular system is little understood. In 1876 Kramer[[359]] wrote that he was able to perceive an actively pulsating heart in the posterior third of the abdomen in specimens of Gamasus which had recently moulted, and were therefore moderately transparent. No other investigator has been equally fortunate, though many capable workers have sought diligently for any trace of a dorsal vessel in various Acarine groups.
It would appear that the blood-flow in most Mites is lacunar and indefinite, aided incidentally by the movements of the muscles, and perhaps by a certain rhythmic motion of the alimentary canal, which has been observed to be most marked during the more quiescent stages of the life-history.
The internal reproductive organs have the ringed arrangement generally observed in the Arachnida. The two testes, which are sometimes bi-lobed, are connected by a median structure which may serve as a vesicula seminalis, and there are two vasa deferentia which proceed to the intromittent organ, which is sometimes situated quite in the anterior part of the ventral surface, but at others towards its centre. The male Mite is often provided with a pair of suckers towards the posterior end of the abdomen, and sometimes accessory clasping organs are present.
In some Mites there is no intromittent organ, and Michael[[360]] has described some remarkable cases in which the chelicerae are used in the fertilisation of the female, a spermatophore, or bag containing spermatozoa, being removed by them from the male opening and deposited in that of the female. The most remarkable instance is that of Gamasus terribilis, the movable joint of whose chelicera is perforated by a foramen through which the spermatophore is, so to speak, blown and carried as a bi-lobed bag, united by the narrow stalk which passes through the foramen, to the female aperture.
The ovaries are fused in the middle line, and connected by oviducts with the tube (vagina or uterus) which passes to the exterior. There is often an ovipositor.
Professor Gené of Turin[[361]] described, in 1844, some remarkable phenomena in connection with the reproduction of Ticks. The male Ixodes introduced his rostrum into the female aperture, two small white fusiform bodies emerging right and left from the labium at the moment of introduction. On retraction they had disappeared. When the female laid eggs, a bi-lobed vesicle was protruded from beneath the anterior border of the scutum and grasped the egg delivered to it by the ovipositor, appearing to manipulate it for some minutes. Then the vesicle was withdrawn, and the egg was left on the rostrum, and deposited by it in front of the animal. When the vesicle was punctured, and so rendered useless, the unmanipulated eggs quickly shrivelled and dried up.
Lounsbury[[362]] has recently confirmed Professor Gené’s observation as to oviposition in the case of a South African Tick, Amblyomma hebraeum.
The respiratory organs, if present, are always in the form of tracheae. These are usually long and convoluted, but not branching. The spiral structure is difficult to make out in these animals, and in the Oribatidae at least, instead of the external sheath being fortified with a spiral filament of chitin, there is a very delicate enveloping membrane with an apparently unbroken chitinous lining, which can, however, by suitable treatment, be resolved into a ribbon-like spiral band.[[363]] The position of the stigmata is very variable, and is utilised to indicate the main groups into which the Mites have been divided.
The Oribatidae possess two curious cephalothoracic organs which were for a long time considered respiratory. These are in the form of two bodies, like modified hairs, which protrude from sockets on the dorsal surface of the cephalothoracic shield. Michael[[364]] has shown that these have no connection with the tracheae, and he regards them as sensory organs—possibly olfactory. They are generally referred to as the “pseudo-stigmatic” organs.
In the Oribatidae, at all events, well-developed coxal glands are present. In many Mites, especially the Ixodoidea or Ticks, the salivary glands are large and conspicuous.
Metamorphosis.—All Mites undergo a metamorphosis, varying in completeness in the different groups. Altogether six stages can be recognised, though they are seldom or never all exhibited in the development of a single species. These are ovum, deutovum, larva, nymph, hypopial stage, and imago.
The Ovum.—All Mites lay eggs. It is frequently stated that the Oribatidae are viviparous exceptions, but though some of them are perhaps ovoviviparous, most deposit eggs like the rest of the Order. A phenomenon which has probably helped to foster this erroneous view is the occasional emergence from the dead body of the mother of fully-formed larvae. Towards winter it is not unusual for the mother to die at a time when her abdomen contains a few ripe eggs, and these are able to complete their development internally.
The Deutovum.[[365]]—In a few cases (Atax, Damaeus) a stage has been observed in which the outer envelope of the egg becomes brown and hard, and splits longitudinally, so as to allow the thin inner membrane to become visible through the fissure. More room is thus obtained for the developing larva, which is, moreover, protected, over most of its surface, by a hard shell. The deutovum stage may occur either within the body of the mother, or after the egg has been laid.
The Larva.—Omitting, for the moment, the very aberrant Vermiformia (see p. [464]), it is the almost universal rule for the egg to hatch out as a hexapod larva. The larvae of the genus Pteroptus are said to be eight-legged. Winkler has stated that the early embryo of Gamasus possesses eight legs, of which the last pair subsequently atrophy, but this observation requires confirmation.
The Nymph.—The nymph-stage commences on the acquisition of eight legs, and lasts until the final ecdysis which produces the imago. This is the most important period of Acarine life, and is divided into a prolonged active period, during which the animal feeds and grows, and an inert period, sometimes prolonged, but at others very short, and differing little from the quiescence observable at an ordinary moult, during which the imago is elaborated. In many species the nymph is strikingly different from the imago; in others there is a close resemblance between them. It would appear, from the cases which have been most thoroughly investigated, that the imago is not developed, part for part, from the nymph, but that there is an “histolysis” and “histogenesis” similar to that which occurs among certain insects (see vol. v. p. 165). There may be more than one nymphal stage.
The hypopial stage occurs in the Tyroglyphinae, the “Cheese-mite” sub-family. Here some of the young nymphs assume an entirely different form, so different that it was for a long time considered to constitute a separate genus, and was named Hypopus. The animal acquires a hard dorsal covering. The mouth-parts are in the form of a flat blade with two terminal bristles, but with no discernible orifice. The legs are single-clawed, and all more or less directed forward, and they are articulated near the middle line of the ventral surface. Suckers are always present under the hind part of the abdomen.
It appears that these remarkably modified nymphs are entrusted with the wider distribution of the species, and that they are analogous to the winged individuals which occur in the parthenogenetic generations of the Aphidae. The ordinary Tyroglyphus is soft-bodied, and requires a moist environment, and exposure to the sun or prolonged passage through the air would be fatal to it. The hypopial form is much more independent of external conditions, and its habit is to attach itself by its suckers to various insects, and by this means to seek a new locality, when it resumes the ordinary nymph-form and proceeds with its development.
Classification.—There is no generally accepted classification of the Acarina, though several eminent Arachnologists have attempted of late years to reduce the group to order. Widely different views are held concerning the affinities of certain groups, and there is no agreement as to the value to be accorded to the characters which all recognise. Thus Canestrini[[366]] allows thirty-four families, while according to Trouessart[[367]] there are only ten.
Trouessart’s scheme of classification is in the main followed in the present chapter.
Sub-Order 1. Vermiformia.
This Sub-order includes the lowest and most aberrant forms of the Mites. They are entirely parasitic, and of very small size. The abdomen is much elongated, and is transversely striated. There are two families, Eriophyidae[[368]] (Phytoptidae) and Demodicidae.
Fam. 1. Eriophyidae (Phytoptidae).—These are the so-called Gall-mites. The curious excrescences and abnormal growths which occur on the leaves and buds of plants are familiar to every one. Various creatures are responsible for these deformities, many being the work of insects, especially the Cynipidae among the Hymenoptera, and the Cecidomyiidae among the Diptera. Others, again, are due to Eriophyid Mites.
Though the galls originated by Mites are often outwardly extremely similar to those of insect origin, they can be at once distinguished on close examination. Mite-galls contain a single chamber, communicating with the exterior by a pore, usually guarded with hairs, and the Mites live gregariously within it, apparently feeding upon the hairs which grow abundantly on its inner surface. In Insect-galls each insect larva lives in a separate closed chamber.
Fig. [240].—Vermiform Mites, highly magnified. A, Demodex folliculorum; B, Eriophyes (Phyptoptus) ribis.
The Eriophyidae are unique among Mites in possessing only two pairs of legs, situated quite at the anterior part of the body. The mouth-parts are very simple.
There are three genera, Eriophyes (Phyptoptus) with about one hundred and fifty known species, Monochetus with a single species, and Phyllocoptes with about fifty species.
Among the best known examples are Eriophyes tiliae, which produces the “nail-galls” on lime-leaves, and E. ribis, the “black-currant Gall-mite,” which feeds between the folded leaves of the leaf-buds, and gives rise to swelling and distortion.
Fam. 2. Demodicidae.—The single genus Demodex which constitutes this family consists of a few species of microscopic Mites which inhabit the hair-follicles of mammals, and are the cause of what is known as “follicular mange,” some other forms of mange being due to members of the succeeding family. Demodex possesses eight short, three-jointed legs, each terminated by two claws. The abdomen is much produced, and is transversely striated. About ten species have been described, but of these five are probably varieties of D. folliculorum (Fig. [240], A), which infests Man.
Sub-Order 2. Astigmata.
The Astigmata are Mites of more or less globular form, with chelate chelicerae and five-jointed legs. All members of the group are eyeless. Their habits are very various, some feeding on vegetable matter and others on carrion, while a large number are parasitic on animals. Tracheae are absent. There is only one family.
Fig. [241].—A, Leg of a fowl infested with “leg-scab”; B, female of Sarcoptes mutans, greatly magnified. (After Neumann.)
Fam. 1. Sarcoptidae.—No tracheae or stigmata. Apical rostrum. Oviparous or ovoviviparous. The seventy genera and 530 odd species of this family are divided into a number of sub-families, of which the principal are the Sarcoptinae, the Analgesinae, and the Tyroglyphinae.
(i.) The Sarcoptinae are the so-called “Itch-mites.” They are minute animals, with bodies transversely wrinkled and legs terminating in suckers or bristles. The genus Sarcoptes, which includes about fifteen species, lives in tunnels which it burrows in the skin of mammals.
(ii.) The Analgesinae are the “Birds’-feather Mites.” The principal genera are Pterolichus (120 species), Pteronyssus (33 species), Analges (23 species), Megninia (42 species), and Alloptes (33 species).
(iii.) The Tyroglyphinae[[369]] have received the popular name of “Cheese-mites,” from the best known example of the group. They are smooth-bodied, soft-skinned white Mites, with legs usually terminating in a single claw, sometimes accompanied by a sucker. They are for the most part carrion-feeders, living upon decaying animal or vegetable matter, but a few are parasitic on mammals, insects, and worms.
There are sixteen genera, including about fifty species. Tyroglyphus siro and T. longior are common Cheese-mites. Other species live in decaying vegetables and food-stuffs. Some of the genus Glycyphagus (G. palmifer, G. plumiger) are very remarkable for the palmate or plumose hairs which decorate their bodies. The remarkable hypopial stage in the development of Tyroglyphus has been mentioned on p. 463. The Tyroglyphinae are the lowest of the free-living Acarine forms.
Sub-Order 3. Metastigmata.
The four families which constitute this sub-order comprise a large number of Mites in which the tracheae open near the articulation of the legs, and consequently in a somewhat posterior situation. The families are Oribatidae, Argasidae, Ixodidae, and Gamasidae.
Fam. 1. Oribatidae.—The Oribatidae or “Beetle-mites” are free-living Acari, with tracheae of which the stigmata are concealed by the articulation of the legs. The cephalothorax is distinctly marked off from the abdomen, and bears dorsally two “pseudo-stigmatic” organs. The rostrum is inserted below the cephalothorax. These Mites gain their popular name from the beetle-like hardness of their integuments. They are oviparous or ovoviviparous. Eyes are always absent.
Fig. [242].—Oribatid Mites. A, Cepheus ocellatus, × 24; B, ventral view of Hoploderma magnum, closed, × 20. (After Michael.)
These are small creatures, seldom attaining the twentieth of an inch in length. They are vegetable-feeders (except, perhaps, Pelops), and are to be found in dead wood or vegetable débris, under bark, or among moss and lichen. In winter they often take refuge under stones. It is impossible at present to estimate the number of existing species, for only a few localities have been systematically worked for them, and their small size has prevented their inclusion, in any numbers, in the collections of scientific expeditions. Our knowledge of the group is likely, however, to be largely extended, for it has been found that they reach England alive and in good condition from the most remote regions if moss or other material in which they live is collected when not too dry, and hermetically sealed up in tin cases.
About twenty genera and more than 220 species are at present known. Pelops has much elongated chelicerae, with very small chelae at the end. There are ten species, found in moss and on bushes. Oribata numbers about fifty species, found in moss and on trees. Notaspis, in which the last three legs are inserted at the margin of the body, has about thirty species, found among moss and dead leaves. Nothrus is a short-legged genus with flat or concave dorsal plate, often produced into very remarkable spiny processes. There are twenty-two species found under bark and among moss and lichen. Hoploderma (Hoplophora) is remarkable for its power of shutting down its rostrum and withdrawing its legs in a manner which leaves it as unassailable as a tortoise or an armadillo.
Though the Oribatidae are all eyeless, they are distinctly sensitive to light, not wandering aimlessly till they reach a shadow, but apparently making straight for a dark spot when subjected to strong illumination. Some species have a curious habit of collecting dirt and débris on their backs, so as entirely to obscure the often very remarkable disposition of the spines and processes with which they are furnished.
Fig. [243].—Capitulum of Boophilus australis; ventral view. p1, p2, p3, p4, The four articles of the palp; m, the mandible or chelicera; d, its digit; n, the hypostome.
The next two families include the animals commonly known as Ticks, the largest and most familiar of the Mite tribe. Of recent years they have attracted much attention as the conveyers, to man and domestic animals, of certain diseases due to blood-parasites (see p. [457], n.), and our knowledge of their structure and habits has greatly increased in consequence. Hitherto they have generally been considered to constitute a single family, the Ixodidae, but a section of them so differ from the rest as to require their removal to another family, the Argasidae, so that it is necessary to employ a super-family name—Ixodoidea—to embrace the whole group.
Ticks are parasitic on mammals, birds, and reptiles, some shewing a marked partiality for a particular host, others being much more catholic in their tastes. Both sexes in the Argasidae, but the females only of the Ixodidae, are capable of great distension, but when unfed they are all somewhat flat animals with laterally extended legs and rather crab-like movement.
All Ticks possess a small, movable “false head” or capitulum bearing mouth-parts which are exceedingly characteristic of the group. The chelicerae are cutting instruments with their distal ends serrated outwardly, and there is always present a hypostome beset with recurved teeth which serve to maintain a firm hold on the tissues into which it is thrust. On either side of the chelicerae are the four-jointed palps, leg-like in the Argasidae, but more rigid and rod-like in the Ixodidae, where their inner margin is often hollowed so as to enclose the chelicerae and hypostome when the palps are apposed. There is a conspicuous pair of spiracles near the coxae of the fourth pair of legs.
Fig. [244].—Ornithodoros talaje, under surface, × 5. (After Canestrini.)
Fam. 2. Argasidae.—The Argasidae are leathery Ticks without a shield or scutum, and with free, leg-like palps. The capitulum is never more than partially visible when the adult animal is viewed dorsally. Their hosts are always warm-blooded animals. Two genera are usually recognised, Argas and Ornithodoros, though recent discoveries of new forms have tended towards their fusion. Argas reflexus and A. persicus have been proved to convey a Spirochaete disease to fowls, and the latter, under the name of the “Mianeh Bug” has long possessed an evil reputation for the “poisonous” effect of its bite on human beings. In Mexico the “Turicata” (Ornithodoros turicata) and the “Garapata” (O. megnini) are greatly dreaded, while human “tick fever” on the Congo has been traced to the instrumentality of O. moubata.
Fig. [245].—Female Sheep-tick, Ixodes ricinus.
Fam. 3. Ixodidae.—These are the more familiar Ticks, possessing a scutum or shield, which covers the whole back of the male, which is capable, therefore, of little distension, whereas it forms only a small patch on the front part of the body of the distended female. There are ten genera, Ixodes, Haemaphysalis, Dermacentor, Rhipicentor, Rhipicephalus, Boophilus, Margaropus, Hyalomma, Amblyomma, and Aponomma.
Ixodes ricinus is the common English sheep-tick. Species of Boophilus are parasitic on cattle the world over, and B. annulatus is the transmitter of Texas fever. Rhipicephalus and Amblyomma are large genera which include several species of economic importance. For example, R. sanguineus conveys canine piroplasmosis, and A. hebraeum causes “heart-water” in South African cattle. The genus Aponomma confines its attention to reptiles, and some of its species are exceedingly ornate.
Neglecting Margaropus and Rhipicentor, which include only a very few aberrant forms, the following entirely artificial key will serve to differentiate the genera of the Ixodidae:—
| 1. | A pair of eyes on the lateral borders of the scutum | 2 |
| No eyes | 6 | |
| 2. | Capitulum long, much longer than broad | 3 |
| Capitulum short | 4 | |
| 3. | Unicolorous, ♂ with chitinous plates near anus | Hyalomma |
| Generally ornate, ♂ without anal plates | Amblyomma | |
| 4. | Generally ornate, ♂ without anal plates, but with enlarged 4th coxae | Dermacentor |
| Unicolorous, ♂ with anal plates and normal coxae | 5 | |
| 5. | Palpi very short, spiracle circular | Boophilus |
| Palpi medium, spiracle comma-shaped | Rhipicephalus | |
| 6. | Capitulum short; 2nd article of palp projecting laterally | Haemaphysalis |
| Capitulum long | 7 | |
| 7. | Unicolorous, elongate, on birds or mammals | Ixodes |
| Generally ornate, broad-oval, on reptiles | Aponomma |
Neumann has recently revised the Ixodoidea in a series of papers published in the Mémoires de la Société zoologique de France,[[370]] but the work is not obtainable as a whole. A monograph, by Nuttall, Warburton, Cooper, and Robinson, is now in course of publication at the Cambridge University Press.[[371]]
Fam. 4. Gamasidae.—The Gamasidae are carnivorous Mites, either free-living or parasitic on animals. The chelicerae are chelate, and the palps are free. The tarsi have two claws, accompanied by a “caruncle” or sucking disc. They are mostly pale-coloured Mites, with a smooth, more or less scutate covering. The three principal sub-families are Gamasinae, Uropodinae, and Dermanyssinae.
Of the Gamasinae, Gamasus coleoptratorum is the well-known Beetle-parasite so frequently seen on Geotrupes. It is often confounded with another species of similar habits, G. crassipes.
The curious Beetle-parasites attached to their victim by a thread belong to the genus Uropoda of the Uropodinae. The connecting filament, which the Mite can sever at will, for a long time puzzled observers. It was variously construed as a silken cord of attachment, and as a sort of umbilical cord, through which the Mite drew nourishment from the Beetle. On more careful investigation it proved to be connected with the anus of the Mite, and to consist of its consolidated excrement.
The Dermanyssinae are all parasitic on warm-blooded animals, principally birds and bats. Dermanyssus avium is the common parasite infesting fowls and cage-birds.
Sub-Order 4. Heterostigmata.
Fam. Tarsonemidae.—This is the sole family of the sub-order. It comprises a number of minute vegetable-feeding Mites which have been little studied, though they are probably the cause of considerable injury to the leaves and buds of plants.
Sub-Order 5. Prostigmata.
In these Mites the stigmata are situated anteriorly, in the rostrum or the thorax. In the Water-mites the tracheae have atrophied, but these creatures are clearly Trombidiidae which have taken to an aquatic life.
Fig. [246].—Bdella lignicola, x about 50. (After Canestrini.)
Fam. 1. Bdellidae.—The Bdellidae are sometimes known as the “Snouted Mites” on account of the very prominent forwardly-directed “capitulum” or false head. They have chelate chelicerae and tactile palps, which are often “elbowed,” like the antennae of weevils. Eyes may be present or absent. They are usually of a bright red colour, and are free-living and predaceous, though in their larval stages they may often be found attached to the limbs of insects and spiders.
The minute active scarlet Mites of the genus Eupodes and its allies perhaps come within this family. Their legs are six-jointed.
The remaining families of the Prostigmata (Halacaridae, Hydrachnidae, and Trombidiidae) all have raptorial palps, and clawed or piercing chelicerae.
Fam. 2. Halacaridae.—This is a small group of marine Mites. In their usually prominent capitulum they resemble the Bdellidae. In some respects they recall the Oribatidae, having hard integuments, and their legs being articulated near the margin of the body. They do not swim, but crawl upon weeds and zoophytes, or burrow in the mud.
Fig. [247].—Atax alticola, x 16. (After Canestrini.)
Fam. 3. Hydrachnidae.—The Hydrachnidae are the Fresh-water Mites. Their legs are provided with long close-set hairs, and thus adapted for swimming. They are predaceous, and in their young stages are often parasitic on water insects. A familiar example is Atax bonzi, which lives within the shell of the fresh-water mussel.
Fam. 4. Trombidiidae.—The predaceous palps of the Trombidiidae are generally of the “finger-and-thumb” type. The tarsi are two-clawed, without caruncle. This group may be divided into six sub-families.
Fig. [248].—Tetranychus gibbosus, x 50. (After Canestrini.)
(i.) The Limnocharinae or “Mud-mites” connect the Hydrachnidae with the typical Trombidiidae. They are usually velvety and of a red colour. They do not swim, but creep. The larva of Limnocharis aquaticus is parasitic on Gerris lacustris.
(ii.) The Caeculinae bear a strong general resemblance to the Harvestmen or Phalangidae. Caeculus is so similar to the Phalangid genus Trogulus that it was considered by Dufour to belong to the same order.
(iii.) The Tetranychinae or “Spinning mites” are phytophagous, and do much injury to plants, sucking the sap from the leaves and giving them a blistered appearance. Tetranychus telarius is the “Red-spider” of popular nomenclature.
(iv.) The Cheyletinae are remarkable Mites with fleshy, semi-transparent body, and enormously developed raptorial pedipalpi, which are extremely formidable weapons of attack. They do not creep or run like most Mites but proceed by a series of short leaps. Cheyletus is the principal genus.
The curious genus Syringophilus, which is parasitic in the interior of birds’ feathers, appears to be a degenerate Cheyletine.
(v.) The Erythraeinae are minute, active Mites, usually red in colour, free-living and predaceous.
(vi.) The Trombidiinae include most of the moderate-sized, velvety red Mites which are commonly known as “Harvest-mites,” and their larvae, the so-called Harvest-bugs, frequently attack man. Trombidium holosericeum is a well-known example.
Sub-Order 6. Notostigmata.[[372]]
This sub-order has been established for the reception of the curious genus Opilioacarus.
Fam. Opilioacaridae.—Mites with segmented abdomen, leg-like palps, chelate chelicerae, and two pairs of eyes. There are four dorsal abdominal stigmata. Four species of the sole genus Opilioacarus have been recorded, O. segmentatus from Algeria, O. italicus from Italy, O. arabicus from Arabia, and O. platensis[[373]] from South America.
APPENDICES TO ARACHNIDA
I. and II.
TARDIGRADA AND PENTASTOMIDA
BY
ARTHUR E. SHIPLEY, M.A., F.R.S.
Fellow and Tutor of Christ’s College, Cambridge, and Reader in Zoology in the University
CHAPTER XIX
TARDIGRADA
OCCURRENCE—ECDYSIS—STRUCTURE—DEVELOPMENT—AFFINITIES—BIOLOGY—DESICCATION—PARASITES—SYSTEMATIC
The animals dealt with in this chapter lead obscure lives, remote from the world, and few but the specialist have any first-hand acquaintance with them. Structurally they are thought to show affinities with the Arachnida, but their connexion with this Phylum is at best a remote one.
Tardigrades are amongst the most minute multicellular animals which exist, and their small size—averaging from ⅓ to 1 mm. in length—and retiring habits render them very inconspicuous, so that as a rule they are overlooked; yet Max Schultze[[374]] asserts that without any doubt they are the most widely distributed of all segmented animals. They are found amongst moss, etc., growing in gutters, on roofs, trees or in ditches, and in such numbers that Schultze states that almost any piece of moss the size of a pea will, if closely examined, yield some members of this group, but they are very difficult to see. The genus Macrobiotus especially affects the roots of moss growing on stones and old walls. M. macronyx lives entirely in fresh water, and Lydella dujardini and Echiniscoides sigismundi are marine; all other species are practically terrestrial, though inhabiting very damp places.
In searching amongst the heather of the Scotch moors for the ova and embryos of the Nematodes which infest the alimentary canal of the grouse, I have recently adopted a method not, as far as I am aware, in use before, and one which in every case has yielded a good supply of Tardigrades otherwise so difficult to find. The method is to soak the heather in water for some hours and then thoroughly shake it, or to shake it gently in a rocking machine for some hours. The sediment is allowed to settle, and is then removed with a pipette and placed in a centrifugaliser. A few turns of the handle are sufficient to concentrate at the bottom of the test-tubes a perfectly amazing amount of cryptozoic animal life, and amongst other forms I have never failed to find Tardigrades.
Fig. [249].—Dorsal view of Echiniscus testudo, C. Sch., × 200, showing the four segments 1, 2, 3, 4. (From Doyère.)
Fig. [250].—Cast-off cuticle of Macrobiotus tetradactylus, Gr., × about 150, containing four eggs in which the boring apparatus of the embryo can be distinguished. (From R. Greeff.)
Many Tardigrades are very transparent; their cells are large, and arranged in a beautifully symmetrical manner; and since those of them that live in moss, and at times undergo desiccation, are readily thrown into a perfectly motionless state, during which they may be examined at leisure, it is not surprising that these little creatures have been a favourite object for histological research. One way to produce the above-mentioned stillness is partly to asphyxiate the animals by placing them in water which has been boiled, and covering the surface of the water with a film of oil.
Fig. [251].—Echiniscus spinulosus, C. Sch., × about 200, seen from the side. (From Doyère.)
The whole body is enclosed in a thin transparent cuticle, which must be pierced by a needle if it be desired to stain the tissues of the interior. As a rule the cuticle is of the same thickness all over the body, but in the genus Echiniscus the cuticle of the dorsal surface is arranged in thickened plates, and these plates are finely granulated. From time to time the cuticle is cast, and this is a lengthy process, so that it is not unusual to find a Tardigrade ensheathed in two cuticles, the outer of which is being rubbed off. The Macrobioti lay their eggs in their cast cuticle (Fig. [250]). The end of each of the eight legs bears forked claws of cuticular origin. The legs are not jointed except in the genus Lydella, where two divisions are apparent.
Within the cuticle is the epidermis, a single layer of cells arranged in regular longitudinal and transverse rows along the upper and under surface, where the cells are as uniformly arranged and as rectangular as bricks. The cells on the sides of the body are polygonal, and not in such definite rows. The nuclei show the same diagrammatic symmetry as the cells which contain them, and lie in the same relative position in neighbouring cells. In a few places, such as the end of each limb and around the mouth and arms, the cells of the epidermis are heaped up and form a clump or ridge. In some genera a deposit of pigment in the epidermis, which increases as the animal grows old, obscures the internal structures. It is generally brown, black, or red in colour.
Fig. [252].—Macrobiotus schultzei, Gr., × 150. (Modified from Greeff.) a, The six inner papillae of the mouth; b, the chitin-lined oesophagus; c, calcareous spicule; d, muscle which moves the spicule; e, muscular pharynx with masticating plates; f, salivary glands; g, stomach; h, ovary; i, median dorsal accessory gland; k, diverticula of rectum.
The cuticle and epidermis enclose a space in which the various internal organs lie. This space is traversed by numerous symmetrically disposed muscle-fibres, and contains a clear fluid—the blood—which everywhere bathes these organs. This fluid evaporates when desiccation takes place, and is soon replaced after rain; it forms no coagulum when reagents are added to it, and it probably differs but little from water. Floating in it are numerous corpuscles, whose number increases with age. In well-fed Tardigrades the corpuscles are packed with food-reserves, often of the same colour—green or brown—as the contents of the stomach, which soon disappear when the little creatures are starved.
The alimentary canal begins with an oral cavity, which is in many species surrounded by chitinous rings. The number of these rings and their general arrangement are of systematic importance. The oral cavity opens behind into a fine tube lined with chitin, very characteristic of the Tardigrada, which has been termed the mouth-tube. By its side, converging anteriorly, lie the two chitinous teeth, which may open ventrally into the mouth-tube, as in Macrobiotus hufelandi and Doyeria simplex, or may open directly into the oral cavity, as in Echiniscus, Milnesium, and some species of Macrobiotus. In some of the last named the tips of the teeth are hardened by a calcareous deposit. The hinder end of each stylet or tooth is supported by a second chitinous tooth-bearer,[[375]] and the movement of each is controlled by three muscles, one of which, running forwards to the mouth, helps to protrude the tooth, whilst the other two running upwards and downwards to the sheath of the pharynx, direct in what plane the tooth shall be moved.
The mouth-tube passes suddenly into the muscular sucking pharynx, which is pierced by a continuation of its chitinous tube. Roughly speaking, the pharynx is spherical; the great thickness of its walls is due to radially arranged muscles which run from the chitinous tube to a surrounding membrane. When the muscles contract, the lumen of the tube is enlarged, and food, for the most part liquid, is sucked in. Two large glands, composed of cells with conspicuous nuclei, but with ill-defined cell outlines, pour their contents into the mouth in close proximity to the exit of the teeth. The secretion of the glands—often termed salivary glands—is said in many cases to be poisonous.
The pharynx may be followed by a distinct oesophagus, or it may pass almost immediately into the stomach, which consists of a layer of six-sided cells arranged in very definite rows. In fully-fed specimens these cells project into the lumen with a well-rounded contour. Posteriorly the stomach contracts and passes into the narrow rectum, which receives anteriorly the products of the excretory canals and the reproductive organs, and thus forms a cloaca. Its transversely placed orifice lies between the last pair of legs. The food of Tardigrades is mainly the sap of mosses and other humble plants, the cell-walls of which are pierced by the teeth of the little creatures.
The organs to which an excretory function has been attributed are a pair of lateral caeca, which vary much in size according as the possessor is well or ill nourished. They recall the Malpighian tubules of such Mites as Tyroglyphus. Nothing comparable in structure to nephridia or to coxal glands has been found.
The muscles show a beautiful symmetry. There are ventral, dorsal, and lateral bundles, and others that move the limbs and teeth, but the reader must be referred to the works of Basse, Doyère,[[376]] and Plate[[377]] for the details of their arrangement. The muscle-fibres are smooth.
Fig. [253].—Brain of Macrobiotus hufelandi, C. Sch., × about 350. (From Plate.) Seen from the side. ap, Lobe of brain bearing the eye; ce, supra-oesophageal ganglion; d, tooth; Ga, first ventral ganglion; ga’, sub-oesophageal ganglion; k, thickening of the epidermis round the mouth; oc, eye-spot; oe, oesophagus; op, nerve running from the ocular lobe of the brain to the first ventral ganglion; ph, pharynx.
The nervous system consists of a brain or supra-oesophageal ganglion, whose structure was first elucidated by Plate, and a ventral chain of four ganglia. Anteriorly the brain is rounded, and gives off a nerve to the skin; posteriorly each half divides into two lobes, an inner and an outer. The latter bears the eye-spot when this is present. Just below this eye a slender nerve passes straight to the first ventral ganglion. The brain is continued round the oral cavity as a thick nerve-ring, the ventral part of which forms the sub-oesophageal ganglion, united by two longitudinal commissures to the first ventral ganglion. Thus the brain has two channels of communication between it and the ventral nerve-cord on each side, one by means of the slender nerve above mentioned, and one through the sub-oesophageal ganglion. The ventral chain is composed of four ganglia connected together by widely divaricated commissures. Each ganglion gives off three pairs of nerves, two to the ventral musculature, and one to the dorsal. The terminations of these nerves in the muscles are very clearly seen in these transparent little creatures, though there is still much dispute as to their exact nature.
The older writers considered the Tardigrada as hermaphrodites, but Plate and others have conclusively shown that they are bisexual, at any rate in the genus Macrobiotus. The males are, however, much rarer than the females. The reproductive organs of both sexes are alike. Both ovary and testis are unpaired structures opening into the intestine, and each is provided with a dorsal accessory gland placed near its orifice. In the ovary many of the eggs are not destined to be fertilised, but serve as nourishment for the more successful ova which survive.
No special circulatory or respiratory organs exist, and, as in many other simple organisms, there is no connective tissue.
Fig. [254].—Male reproductive organs of Macrobiotus hufelandi, C. Sch., × about 350. (From Plate.) a.ep, Epidermal thickening round anus; cl, cloaca; gl.d, accessory gland; gl.l, Malpighian gland; st, stomach; te, testis; x, mother-cells of spermatozoa.
The segmentation of the egg in M. macronyx is total and equal, according to the observations of von Erlanger.[[378]] A blastula, followed by a gastrula, is formed. The blastopore closes, but later the anus appears at the same spot. There are four pairs of mesodermic diverticula which give rise to the coelom and the chief muscles. The reproductive organs arise as an unpaired diverticulum of the alimentary canal, which also gives origin to the Malpighian tubules. The development is thus very primitive and simple, and affords no evidence of degeneration.
With regard to their position in the animal kingdom, writers on the Tardigrada are by no means agreed. O. F. Müller placed them with the Mites; Schultze and Ehrenberg near the Crustacea; Dujardin and Doyère with the Rotifers near the Annelids; and von Graff with the Myzostomidae and the Pentastomida. Plate regards them as the lowest of all air-breathing Arthropods, but he carefully guards himself against the view that they retain the structure of the original Tracheates from which later forms have been derived. He looks upon Tardigrades as a side twig of the great Tracheate branch, but a twig which arises nearer the base of the branch than any other existing forms. These animals seem certainly to belong to the Arthropod phylum, inasmuch as they are segmented, have feet ending in claws, Malpighian tubules, and an entire absence of cilia. The second and third of these features indicate a relationship with the Tracheate groups; on the other hand there is an absence of paired sensory appendages, and of mouth-parts. Von Erlanger has pointed out that the Malpighian tubules, arising as they do from the mid-gut, are not homologous with the Malpighian tubules of most Tracheates, and he is inclined to place this group at the base or near the base of the whole Arthropod phylum. They, however, show little resemblance to any of the more primitive Crustacea. The matter must remain to a large extent a matter of opinion, but there can be no doubt that the Tardigrades show more marked affinities to the Arthropods than to any other group of the animal kingdom.
Biology.—Spallanzani, who published in the year 1776 his Opuscules de physique animale et végétale, was the first satisfactorily to describe the phenomena of the desiccation of Tardigrades, though the subject of the desiccation of Rotifers, Nematodes, and Infusoria had attracted much notice, since Leeuwenhoek had first drawn attention to it at the very beginning of the century. In its natural state and in a damp atmosphere Tardigrades live and move and have their being like other animals, but if the surroundings dry up, or if one be isolated on a microscopic slide and slowly allowed to dry, its movements cease, its body shrinks, its skin becomes wrinkled, and at length it takes on the appearance of a much weathered grain of sand in which no parts are distinguishable. In this state, in which it may remain for years, its only vital action must be respiration, and this must be reduced to a minimum. When water is added it slowly revives, the body swells, fills out, the legs project, and gradually it assumes its former plump appearance. For a time it remains still, and is then in a very favourable condition for observation, but soon it begins to move and resumes its ordinary life which has been so curiously interrupted.
All Tardigrades have not this peculiar power of revivification—anabiosis, Preyer calls it—it is confined to those species which live amongst moss, and the process of desiccation must be slow and, according to Lance,[[379]] the animal must be protected as much as possible from direct contact with the air.
According to Plate, the Tardigrada are free from parasitic Metazoa, which indeed could hardly find room in their minute bodies. They are, however, freely attacked by Bacteria and other lowly vegetable organisms, and these seem to flourish in the blood without apparently producing any deleterious effects on the host. Plate also records the occurrence of certain enigmatical spherical bodies which were found in the blood or more usually in the cells of the stomach. These bodies generally appeared when the Tardigrades were kept in the same unchanged water for some weeks. Nothing certain is known as to their nature or origin.
Systematic.—A good deal of work has recently been done by Mr. James Murray on the Polar Tardigrades and on the Tardigrades of Scotland, many of which have been collected by the staff of the Lake Survey.[[380]] Over forty species have been described from North Britain.
The following table of Classification is based on that drawn up by Plate:—
Table of Genera.
I. The claws of the legs are simple, without a second hook. If there are several on the same foot they are alike in structure and size.
A. The legs are short and broad, each with at least two claws.
2–4 claws Gen. 1. ECHINISCUS, C. Sch. (Fig. [249]).
7–9 claws Sub-gen. 1a. ECHINISCOIDES, Plate.
B. The legs are long and slender; each bears only one small claw.
Gen. 2. LYDELLA, Doy.
II. The claws of the legs are all or partly two- or three-hooked. Frequently they are of different lengths.
A. There are no processes or palps around the mouth.
I. The muscular sucking pharynx follows closely on the mouth-tube.
α. The oral armature consists on each side of a stout tooth and a transversely placed support.
Gen. 3. MACROBIOTUS, C. Sch. (Fig. [252]).
β. The oral armature consists on each side of a stylet-like tooth without support.
Gen. 4. DOYERIA, Plate.
II. The mouth-tube is separated from the muscular sucking pharynx by a short oesophagus.
Gen. 5. DIPHASCON, Plate (Fig. [255]).
B. Six short processes or palps surround the mouth, and two others are placed a little farther back.
Gen. 6. MILNESIUM, Doy.
1. Genus ECHINISCUS (= EURYDIUM, Doy.).—The dorsal cuticle is thick, and divided into a varying number of shields, which bear thread- or spike-like projections. The anterior end forms a proboscis-like extension of the body. Two red eye-spots. There are many species, and the number has increased so rapidly in the last few years that specialists are talking of splitting up the genus. E. arctomys, Ehrb.; E. mutabilis, Murray; E. islandicus, Richters; E. gladiator, Murray; E. wendti, Richters; E. reticulatus, Murray; E. oihonnae, Richters; E. granulatus, Doy.; E. spitzbergensis, Scourfield;[[381]] E. quadrispinosus, Richters; and E. muscicola, Plate, are all British. More than one-half of these species are also Arctic, and E. arctomys is in addition Antarctic. In fact, the group is a very cosmopolitan one. The genus is also widely distributed vertically, specimens being found in cities on the sea level and on mountains up to a height of over 11,000 feet.
Fig. [255].—Diphascon chilenense, Plate, × about 100. (From Plate.) ce, Brain; k, thickening of the epidermis above the mouth; o, egg; oe, oesophagus; p,?salivary glands; ph, pharynx; sa, blood corpuscles; st, stomach.
1a. Sub-genus ECHINISCOIDES differs from the preceding in the number of the claws, the want of definition in the dorsal plates, and in being marine. The single species E. sigismundi, M. Sch., is found amongst algae in the North Sea (Ostend and Heligoland).
2. Genus LYDELLA.[[382]]—The long, thin legs of this genus have two segments, and in other respects approach the Arthropod limb. Marine. Plate suggests the name L. dujardini for the single species known.
3. Genus MACROBIOTUS has a pigmented epidermis, but eye-spots may be present or absent. The eggs are laid one at a time, or many leave the body at once. They are either quite free or enclosed in a cast-off cuticle. The genus is divided into many species and shows signs of disruption. They mostly live amongst moss; but M. macronyx, Doy., is said to live in fresh water. The following species are recorded from North Britain: M. oberhäuseri, Doy.; M. hufelandi, Schultze; M. zetlandicus, Murray; M. intermedius, Plate; M. angusti, Murray; M. annulatus, Murray; M. tuberculatus, Plate; M. sattleri, Richters; M. papillifer, Murray; M. coronifer, Richters; M. crenulatus, Richters; M. harmsworthi, Murray; M. orcadensis, Murray; M. islandicus, Richters; M. dispar, Murray; M. ambiguus, Murray; M. pullari, Murray; M. hastatus, Murray; M. dubius, Murray; M. echinogenitus, Richters; M. ornatus, Richters; M. macronyx?, Doy.
4. Genus DOYERIA.—The teeth of this genus have no support, and the large salivary glands of the foregoing genus are absent; in other respects Doyeria, with the single species Doyeria simplex, Plate, resembles Macrobiotus, and is usually to be found in consort with M. hufelandi, C. Sch.
5. Genus DIPHASCON resembles M. oberhäuseri, Doy., but an oesophagus separates the mouth-tube from the sucking pharynx, and the oral armature is weak. The following species are British, the first named being very cosmopolitan, being found at both Poles, in Chili, Europe, and Asia: D. chilenense, Plate; D. scoticum, Murray; D. bullatum, Murray; D. angustatum, Murray; D. oculatum, Murray; D. alpinum, Murray; D. spitzbergense, Murray.
6. Genus MILNESIUM has a soft oral armature, and the teeth open straight into the mouth. A lens can usually be distinguished in the eyes. Two species have been described, M. tardigradum, Doy., British, and M. alpigenum, Ehrb. Bruce and Richters consider that these two species are identical.
CHAPTER XX
PENTASTOMIDA[[383]]
OCCURRENCE—ECONOMIC IMPORTANCE—STRUCTURE—DEVELOPMENT AND LIFE-HISTORY—SYSTEMATIC
Pentastomids are unpleasant-looking, fluke-like or worm-like animals, which pass their adult lives in the nasal cavities, frontal sinuses, and lungs of flesh-eating animals, such as the Carnivora, Crocodiles, and Snakes; more rarely in Lizards, Birds, or Fishes. From these retreats their eggs or larvae are sneezed out or coughed up, or in some other way expelled from the body of their primary host, and then if they are eaten, as they may well be if they fall on grass, by some vegetable-feeding or omnivorous animal, they undergo a further development. If uneaten the eggs die. When once in the stomach of the second host, the egg-shell is dissolved and a larva emerges (Fig. [260], p. 494), which bores through the stomach-wall and comes to rest in a cyst in some of the neighbouring viscera. Here, with occasional wanderings which may prove fatal to the host, it matures, and should the second host be eaten by one of the first, the encysted form escapes, makes its way to the nasal chambers or lungs, and attaching itself by means of its two pairs of hooks, comes to rest on some surface capable of affording nutriment. Having once taken up its position the female seldom moves, but the males, which are smaller than the females, are more active. They move about in search of a mate. Further, should the host die, both sexes, after the manner of parasites, attempt to leave the body. Like most animals who live entirely in the dark they develop no pigment, and have a whitish, blanched appearance.
The only species of Pentastomid which has any economic importance is Linguatula taenioides of Lamarck, which is found in the nose of the dog, and much more rarely in the same position in the horse, mule, goat, sheep, and man. It is a comparatively rare parasite, but occurred in about 10 per cent of the 630 dogs in which it was sought at the laboratory of Alfort, near Paris, and in 5 out of 60 dogs examined at Toulouse. The symptoms caused by the presence of these parasites are not usually very severe, though cases have been recorded where they have caused asphyxia. The larval stages occur in the rabbit, sheep, ox, deer, guinea-pig, hare, rat, horse, camel, and man, and by their wandering through the tissues may set up peritonitis and other troubles.
As in the Cestoda, which they so closely resemble in their life-history, the nomenclature of the Pentastomids has been complicated by their double life. For long the larval form of L. taenioides was known by different names in different hosts, e.g. Pentastoma denticulatum, Rud., when found in the goat, P. serratum, Fröhlich, when found in the hare, P. emarginatum when found in the guinea-pig, and so on. In the systematic section of this article some of the species mentioned are known in the adult state, some in the larval, and in only a few has the life-history been fully worked out.
Structure.[[384]]—The body of a Pentastomid is usually white, though in the living condition it may be tinged red by the colour of the blood upon which it lives. The anterior end, which bears the mouth and the hooks (Fig. [256]), has no rings; this has been termed the cephalothorax. The rest of the body, sometimes called the abdomen, is ringed, and each annulus is divided into an anterior half dotted with the pores of certain epidermal glands and a hinder part of the ring in which these are absent.
On the ventral surface of the cephalothorax, in the middle line, lies the mouth, elevated on an oral papilla, and on each side of the mouth are a pair of hooks whose bases are sunk in pits. The hooks can be protruded from the pits, and serve as organs of attachment. Their shape has some systematic value.
Fig. [256].—Porocephalus annulatus, Baird. A, Ventral view of head, × 6; B, ventral view of animal, × 2.
There are a pair of peculiar papillae which bear the openings of the “hook-glands,” lying just in front of the pairs of hooks, and other smaller papillae are arranged in pairs on the cephalothorax and anterior annuli. The entire body is covered by a cuticle which is tucked in at the several orifices. This is secreted by a continuous layer of ectoderm cells. Some of these subcuticular cells are aggregated together to form very definite glands opening through the cuticle by pores which have somewhat unfortunately received the name of stigmata. Spencer attributes to these glands a general excretory function. There is, however, a very special pair of glands, the hook-glands, which extend almost from one end to the other of the body; anteriorly these two lateral glands unite and form the head-gland (Fig. [257]). From this on each side three ducts pass, one of which opens to the surface on the primary papilla; the other two ducts open at the base of the two hooks which lie on each side of the mouth. Leuckart has suggested that these important glands secrete some fluid like the irritating saliva of a Mosquito which induces an increased flow of blood to the place where it is of use to the parasite. Spencer, however, regards the secretion as having, like the secretion of the so-called salivary cells of the Leech, a retarding action on the coagulation of the blood of the host.
The muscles of Pentastomids are striated. There is a circular layer within the subcuticular cells, and within this a longitudinal layer and an oblique layer which runs across the body-cavity from the dorso-lateral surface to the mid-ventral line, a primitive arrangement which recalls the similar division of the body-cavity into three chambers in Peripatus and in many Chaetopods. Besides these there are certain muscles which move the hooks and other structures.
The mouth opens into a pharynx which runs upwards and then backwards to open into the oesophagus (Fig. [257]). Certain muscles attached to these parts enlarge their cavities, and thus give rise to a sucking action by whose force the blood of the host is taken into the alimentary canal. The oesophagus opens by a funnel-shaped valve into the capacious stomach or mid-gut, which stretches through the body to end in a short rectum or hind-gut. The anus is terminal.
Fig. [257].—Diagrammatic representation of the alimentary, secretory, nervous, and reproductive systems of a male Porocephalus teretiusculus, seen from the side. The nerves are represented by solid black lines. (From W. Baldwin Spencer.)
1, Head-gland; 2, testis; 3, hook-gland; 4, hind-gut; 5, mid-gut; 6, ejaculatory
duct; 7, vesicula seminalis; 8, vas deferens; 9, dilator-rod sac; 10, cirrus-bulb;
11, cirrus-sac; 12, fore-gut; 13, oral papillae.
There appears to be no trace of circulatory or respiratory organs, whilst the function usually exercised by the nephridia or Malpighian tubules or by coxal glands, of removing waste nitrogenous matter, seems, according to Spencer, to be transferred to the skin-glands.
The nervous system is aggregated into a large ventral ganglion which lies behind the oesophagus. It gives off a narrow band devoid of ganglion-cells, which encircles that tube. It also gives off eight nerves supplying various parts, and is continued backward as a ninth pair of prolongations which, running along the ventral surface, reach almost to the end of the body (Fig. [257]). The only sense-organs known are certain paired papillae on the head, which is the portion that most closely comes in contact with the tissues of the host.
Pentastomids are bisexual. The males are as a rule much less numerous and considerably smaller than the females, although the number of annuli may be greater.
The ovary consists of a single tube closed behind. This is supported by a median mesentery. Anteriorly the ovary passes into a right and left oviduct, which, traversing the large hook-gland, encircle the alimentary canal and the two posterior nerves (Fig. [258]). They then unite, and at their point of union they receive the ducts of the two spermathecae, usually found packed with spermatozoa. Having received the orifices of the spermatheca, the united oviducts are continued backward as the uterus, a highly-coiled tube in which the fertilised eggs are stored. These are very numerous; Leuckart estimated that a single female may contain half a million eggs. The uterus opens to the exterior in the mid-ventral line a short distance—in P. teretiusculus on the last ring but seven—in front of the terminal anus. In L. taenioides the eggs begin to be laid in the mucus of the nose some six months after the parasite has taken up its position.
Fig. [258].—Diagrammatic representation of the alimentary, secretory, nervous, and reproductive systems of a female Porocephalus teretiusculus, seen from the side. The nerves are represented by solid black lines. (From W. Baldwin Spencer.)
1, Head-gland; 2, oviduct; 3, hook-gland; 4, mid-gut; 5, ovary; 6, hind-gut; 7,
vagina; 8, uterus; 9, accessory gland; 10, spermatheca.
The testis is a single tube occupying in the male a position similar to that of the ovary in the female. Anteriorly it opens into two vesiculae seminales, which, like the oviducts, pierce the hook-glands and encircle the alimentary canal (Fig. [257]). Each vesicula passes into a vas deferens with a cuticular lining. Each vas deferens also receives the orifice of a muscular caecal ejaculatory duct, which, crowded with mature spermatozoa, stretches back through the body. Anteriorly the vas deferens passes into a cirrus-bulb, which is joined by a cirrus-sac on one side and a dilator-rod sac on the other, structures containing organs that assist in introducing the spermatozoa into the female. The two tubes then unite, and having received a dorsally-placed accessory gland, open to the exterior by a median aperture placed ventrally a little way behind the mouth.
Life-history.—The egg undergoes a large portion of its development within the body of the mother. In Linguatula taenioides, which lives in the nasal cavities of the dog, the eggs pass away with the nasal excretions. If these, scattered about in the grass, etc., be eaten by a rabbit, the egg-shell is dissolved in the stomach of the second host and a small larva is set free. In Porocephalus proboscideus and others, which inhabit the lungs of snakes, the eggs pass along the alimentary canal and leave the body with the faeces. They also must be eaten by a second host if development is to proceed.
Fig. [259].—A late larval stage of Porocephalus proboscideus, seen from the side. Highly magnified. (From Stiles.) 1, primordium of first pair of chitinous processes; 2, primordium of second pair of chitinous processes; 3, mouth; 4, ventral ganglion; 5, receptaculum seminis; 6, oviduct; 7, ovary; 8, anus; 9, vagina.
The larva which emerges when the egg-shell is dissolved has a rounded body provided with two pairs of hooked appendages, and a tail which is more or less prominent in different species (Figs. [259], 260). Each appendage bears a claw, and is strengthened by a supporting rod or skeleton. Anteriorly the head bears a boring apparatus of several chitinous stylets. The various internal organs are in this stage already formed, though in a somewhat rudimentary state, and it is doubtful if the anus has yet appeared.
Fig. [260].—Larva of Porocephalus proboscideus, seen from below. Highly magnified. (From Stiles.) 1, Boring, anterior end; 2, first pair of chitinous processes seen between the forks of the second pair; 3, ventral nerve-ganglion; 4, alimentary canal; 5, mouth; 6 and 7, gland-cells.
By means of its boring apparatus, and aided by its hooked limbs, the larva now works its way through the stomach-walls of its second host, and comes to rest in the liver or in some other viscus. Its presence in the tissues of its second host causes the formation of a cyst, and within this the larva rests and develops. In man, at least, the cysts often undergo a calcareous degeneration, and Virchow states “dass beim Menschen das Pentastomum am häufigsten von allen Entozoen zu Verwechselungen mit echten Tuberkeln Veranlassungen giebt.” The larva moults several times, and loses its limbs, which seem to have no connexion with the paired hooks in the adult (Fig. [256]). The internal organs slowly assume the form they possess in the adult. The larva is at first quite smooth, but as it grows the annulations make their appearance, arising in the middle and spreading forward and backward (Fig. [259]). In this encysted condition the larva remains coiled up for some months, according to Leuckart; six in the case of L. taenioides, and a somewhat shorter period, according to Stiles,[[385]] in the case of P. proboscideus.
Fig. [261].—Encysted form of Porocephalus protelis, × 1, lying in the mesentery of its host. (From Hoyle.)
The frequency of what used to be called Pentastoma denticulatum (= the larval form of L. taenioides) in the body of man depends on the familiarity of man with dogs. Klebs and Zaeslin found one larva in 900 and two in 1914 autopsies. Laenger[[386]] found the larva fifteen times in about 400 dissections, once in the mesentery, seven times in the liver, and seven times in the wall of the intestine. After remaining encysted for some time it may escape, and begins wandering through the tissues, aided by its hooks and annulations, a proceeding not unaccompanied by danger to its host. Should the latter be eaten by some carnivorous animal, the larva makes its way into the nasal cavities or sinuses, or into the lungs of the flesh-eating creature, and there after another ecdysis it becomes adult. If, however, the second host escapes this fate, the larvae re-encyst themselves, and then if swallowed they are said to bore through the intestine of the flesh-eater, and so make their way to their adult abode.
Systematic.[[387]]—The Pentastomida are a group much modified by parasitism, which has so deeply moulded their structure as to obscure to a great extent their origin and affinities. The larva, with its clawed limbs, recalls the Tardigrades and certain Mites, e.g. Phytoptus, where only two pairs of limbs persist, and where the abdomen is elongated and forms a large proportion of the body. The resemblances to a single and somewhat aberrant genus must not, however, be pressed too far. The striated muscles, the ring-like nature of the reproductive organs and their ducts, perhaps even the disproportion both in size and number of the females to the males, are also characters common to many Arachnids.
The Pentastomida include three genera, Linguatula, Fröhlich, Porocephalus, Humboldt, and Reighardia, Ward.[[388]] The first two were regarded by Leuckart as but sub-genera, but Railliet[[389]] and Hoyle[[390]] have raised them to the rank of genera. They are characterised as follows:—
Linguatula, body flattened, but dorsal surface arched; the edges of the fluke-like body crenelated; the body-cavity extends as diverticula into the edges of the body.
Porocephalus, body cylindrical, with no diverticula of the body-cavity.
Reighardia, devoid of annulations, transparent, with poorly developed hooks and a mouth-armature.
The following is a list of the species with their primary and secondary or larval hosts:—
i. Linguatula pusilla, Diesing, found in the intestine of the fresh-water fish Acara, a South American genus of the Cichlidae. This is possibly the immature form of L. subtriquetra.
ii. L. recurvata, Diesing, found in the frontal sinuses and the trachea of Felis onca.
iii. L. subtriquetra, Diesing, found in the throat of Caiman latirostris and C. sclerops, perhaps the mature form of L. pusilla.
iv. L. taenioides, Lamarck, found in the frontal sinuses and nasal chambers of the dog and ounce, and in the nasal cavities of the wolf, fox, goat, horse, mule, sheep, and man, and in the trachea of the ounce. The immature form has been found in or on the liver of the cat, guinea-pig, and horse; in the lungs of the ox, cat, guinea-pig, porcupine, hare, and rabbit; in the liver and connective tissue of the small intestine of man; and in the mesenteric glands of the ox, camel, goat, sheep, antelope, fallow-deer, and mouse.
v. Porocephalus annulatus, Baird, found in the lungs of the Egyptian cobra, Naja haje; the immature form is thought to live encapsuled in a species of Porphyrio[[391]] and in the Numidian Crane.
vi. P. aonycis, Macalister, from the lungs of an Indian otter taken in the Indus.
vii. P. armillatus, Wyman, found in the adult state in the lungs of certain African pythons, and in the lion; in the larval form it occurs encysted in the abdomen of the Aard-wolf, the mandril, and man—usually in negroes. Its migrations in the body of its second host sometimes cause fatal results.
viii. P. bifurcatus, Diesing, found in the body-cavity of certain snakes, and in the lungs of boa-constrictors and the legless lizard, Amphisbaena alba. Possibly an immature form.
ix. P. clavatus, Lohrmann, found in the lungs of the Monitor lizard.
x. P. crocidura, Parona, found in the peritoneum of the “musk-rat” Crocidura in Burmah. Probably a larval form.
xi. P. crotali, Humboldt, found in the lungs, body-cavity, kidneys, spleen, and mesentery of many snakes and lizards, and of the lion and leopard. The immature forms occur in the liver and abdominal cavity of species of opossum, armadillo, mouse, raccoon, bat, and marmoset.
xii. P. geckonis, Dujardin, found in the lungs of a Siamese gecko.
xiii. P. gracilis, Diesing, found free in the body-cavity or encapsuled on the viscera and mesenteries of South American fishes, snakes, and lizards,
xiv. P. heterodontis; Leuckart, found encapsuled in the abdominal muscles and mesentery of a species of Heterodon.
xv. P. indicus,[[392]] v. Linst., found in the trachea and lungs of Gavialis gangeticus.
xvi. P. lari, Mégnin, found in the air-sacs of the Burgomaster or Glaucous gull, Larus glaucus of the Polar seas.
xvii. P. megacephalus, Baird, found embedded in the flesh of the head of an Indian crocodile, C. palustris, the “Mugger.” Probably a larval form.
xviii. P. megastomus, Diesing, found in the lungs of a fresh-water tortoise, Hydraspis geoffroyana.
xix. P. moniliformis, Diesing, found in the lungs of pythons.
xx. P. najae sputatricis, Leuckart, found encapsuled in the abdominal muscles and peritoneum of the cobra, Naja tripudians. Probably a larval form.
xxi. P. oxycephalus, Diesing, found in the lungs of crocodiles and alligators.
xxii. P. platycephalus, Lohrmann, habitat unknown.
xxiii. P. subuliferus, Leuckart, in the lungs of the cobra Naja haje.
xxiv. P. teretiusculus, Baird, found in the lungs and mouth of certain Australian snakes.
xxv. P. tortus, Shipley, found in the body-cavity of a snake, Dipsadomorphus irregularis, taken in New Britain.
xxvi. Reighardia, sp., Ward, found in the air-sacs of Bonaparte’s gull and the common North American tern.