THE ABDOMEN AND ITS APPENDAGES
Fig. 176.—Abdomen of Termes flavipes: 1–10, the ten tergites; 1–9, the nine urites; c, cercopod.
Fig. 177.—End of abdomen of Panorpa debilis drawn out, the chitinous pieces shaded: L, lateral, D, dorsal view; c, jointed cercopoda.—Gissler del.
In the abdomen the segments are more equally developed than elsewhere, retaining the simple annular shape of embryonic life, and from their generalized nature their number can be readily distinguished (Fig. 176). The tergal and sternal pieces of each segment are of nearly the same size, the tergal often overlapping the sternal (though in the Coleoptera the sternites are larger than the tergites), while there are no pleural pieces, the lateral region being membranous when visible and bearing the stigmata (Fig. 177, L). In the terminal segments beyond the genital outlet, however, there is a reduction in and loss of segments, especially in the adults of the metabolous orders, notably the Panorpidæ (Fig. 177), Diptera, and aculeate Hymenoptera; in the Chrysididæ only three or four being usually visible, the distal segments being reduced and telescoped inward.
The typical number of abdominal segments (uromeres), i.e. that occurring in each order of insects, is ten; and in certain families of Orthoptera, eleven. In the embryos, however, of the most generalized winged orders, Orthoptera (Fig. 199), Dermaptera, and Odonata, eleven can be seen, while Heymons has recently detected twelve in blattid and Forficula embryos, and he claims that in the nymphs of certain Odonata there are twelve segments, the twelfth being represented by the anal or lateral plates. It thus appears that even in the embryo condition of the more generalized winged insects, the number of uromeres is slightly variable.
We have designated the abdomen as the urosome; the abdominal segments of insects and other Arthropods as uromeres, and the sternal sclerites as urosternites, farther condensed into urites. (See Third Report U. S. Entomological Commission, 1883, pp. 307, 324, 435, etc.)
Fig. 178.—Nymph of the pear tree Psylla, with its glandular hairs.—After Slingerland. Bull. Div. Ent. U. S. Dep. Agr.
The reduction takes place at the end of the abdomen, and is usually correlated with the presence or absence of the ovipositor. In the more generalized insects, as the cockroaches, the tenth segment is, in the female, completely aborted, the ventral plate being atrophied, while the dorsal plate is fused during embryonic life, as Cholodkowsky has shown, with the ninth tergite, thus forming the suranal plate.
In the advanced nymph of Psylla the hinder segments of the abdomen appear to be fused together, the traces of segmentation being obliterated, though the segments are free in the first stage and in the imago (Fig. 178). It thus recalls the abdomen of spiders, of Limulus, and the pygidium of trilobites.
The median segment.—There has been in the past much discussion as to the nature of the first abdominal segment, which, in those Hymenoptera exclusive of the phytophagous families, forms a part of the thorax, so that the latter in reality consists of four segments, what appearing to be the first abdominal segment being in reality the second.
Latreille and also Audouin considered it as the basal segment of the abdomen, the former calling it the “segment médiaire,” while Newman termed it the “propodeum.” This view was afterward held by Newport, Schiödte, Reinhard, and by the writer, as well as Osten-Sacken, Brauer, and others. The first author to attempt to prove this by a study of the transformations was Newport in 1839 (article “Insecta”). He states that while the body of the larva is in general composed of thirteen distinct segments, counting the head as the first, “the second, third, fourth, and, as we shall hereafter see, in part also the fifth, together form the thorax of the future imago” (p. 870). Although at first inclined to Audouin’s opinion, he does not appear to fully accept it, yet farther on (p. 921) he concludes that in the Hymenoptera the “fifth” segment (first abdominal) is not in reality a part of the true thorax, “but is sometimes connected more or less with that region, or with the abdomen, being intermediate between the two. Hence we have ventured to designate it the thoracico-abdominal segment.” Had he considered the higher Hymenoptera alone, he would undoubtedly have adopted Latreille’s view, but he saw that in the saw-flies and Lepidoptera the first abdominal segment is not entirely united with the thorax, being still connected with the abdomen as well as the thorax. Reinhard in 1865 reaffirmed Latreille’s view. In 1866 we stated from observations on the larvæ made three years earlier, that during the semipupa stage of Bombus the entire first abdominal segment is “transferred from the abdomen to the thorax with which it is intimately united in the Hymenoptera,” and we added that we deemed this to be “the most essential zoölogical character separating the Hymenoptera from all other insects.” (See Fig. 93, showing the gradual transfer and fusion of this segment with the thorax.) In the saw-flies the fusion is incomplete, as also in the Lepidoptera, while in the Diptera and all other orders the thorax consists of but three segments. (See also pp. 90–92.)
Fig. 179.—Abdomen of Machilis maritima, ♀, seen from beneath: the left half of the 8th ventral plate removed; I-IX, abdominal segments; c, cercopoda; cb, coxal glands; hs, coxal stylets; lr, ovipositor.—After Oudemans, from Lang.
The cercopoda.—We have applied this name to the pair of anal cerci appended to the tenth abdominal segment, and which are generally regarded as true abdominal legs. As is now well known, the embryos of insects of different orders have numerous temporary pairs of abdominal appendages which arise in the same manner, have the same embryonic structure, and are placed in a position homologous with those of the thorax. In the embryo of Œcanthus rudimentary legs appear, as shown by Ayers, on the first to tenth abdominal segment, the last or tenth pair becoming the cercopoda; and similar rudimentary appendages have been detected in the embryos of Coleoptera, Lepidoptera, and Hymenoptera (Apidæ). Cholodkowsky has observed eleven pairs of abdominal appendages in Phyllodromia.
They are very long and multiarticulate in the Thysanura (Fig. 179). In the Dermaptera they are not jointed and are forcep-like. It should also be observed that in the larva or Sisyra (Fig. 181) there are seven pairs of 5–jointed abdominal appendages, though these may be secondary structures or tracheal gills. In the Perlidæ and the Plectoptera (Ephemeridæ), they are very long, sometimes over twice as long as the body, and composed of upward of 55 joints; they also occur in the Panorpidæ (Fig. 177). In the dragon-flies the cerci are large, but not articulated, and serve as claspers or are leaf-like[[35]] (Fig. 180). In a few Coleoptera, as the palm-weevil (Rhynchophorus phœnicis), Cerambyx, Drilus, etc., the so-called ovipositor ends in a hairy, 1–jointed, palpiform cercus. Short 25–jointed cercopoda are present in Termitidæ, and 2–jointed ones in Embiidæ.
Fig. 180.—End of abdomen of Æschna heros, ♀: ur, urosternite; or, outer, ir, inner styles of the ovipositor; 11, 11th abdominal segment; c, cercopod.
Fig. 181.—Larva of Sisyra, from beneath. B, an abdominal appendage.—After Westwood, from Sharp.
Fig. 182.—Cercopoda (P) of Mantis.—After Lacaze-Duthiers.
The anal cerci are present in the Orthoptera and, when multiarticulate, function as abdominal antennæ. They are longest in the Mantidæ (Fig. 182); they also occur in the larva of the saw-fly, Lyda (Fig. 183). Dr. A. Dohrn has stated that the cerci of Gryllotalpa are true sensory organs, and we have called those of the cockroach abdominal antennæ, having detected about ninety sacs on the upper side of each joint of the stylets, which are supposed to be olfactory in nature, and which are larger and more numerous than similar sacs or pits in the antennæ of the same insect.[[36]] From his experiments upon decapitated cockroaches, Graber concluded that these cerci were organs of smell.
Fig. 183.—Lyda larva: a, head; b, end of body seen from above; c, from side, with cercopod.
Haase regarded these appendages, from their late development and frequent reduction, as old inherited appendages which are approaching atrophy through disuse.
Cholodkowsky states that Tridactylus, a form allied to Gryllotalpa, bears on the tenth abdominal segment two pairs of cerci (ventral and dorsal), and that the ventral pair may correspond to the atrophied appendages of the tenth embryonic segment of Phyllodromia, with which afterward the eleventh segment becomes fused.
The cercopods are not necessarily confined to the eleventh or to the tenth segment, for when there are only nine segments, with the vestige of a tenth, as in Xiphidium, they arise from the ninth uromere, and in the more modern cockroaches, as Panesthia, in which there are but seven entire segments, they are appended to the last or eighth uromere.
Fig. 184.—Anabrus, ♀, side-view, dissected; showing the relative size of the ovipositor: c, the minute cercopod.—Kingsley del.
As to the homology and continuity of these cercopods with the ventral outgrowths of the embryo, several embryologists, notably Wheeler, are emphatic in regarding them as such. It thus appears that either the embryonic appendages of the seventh or eighth, ninth or tenth uromere may persist, and form the cercopoda of the adult.
The ovipositor.—The end of the oviduct is guarded by three pairs of chitinous, unjointed styles closely fitted together, forming a strong, powerful apparatus for boring into the ground or into leaves, stems of plants, the bodies of insects, or even into solid wood, so that the eggs may be deposited in a place of safety. In the ants, wasps, and bees the ovipositor also functions as a sting, which is further provided with a poison-sac.
Morphologically, the ovipositor is composed of three pairs of unjointed styles (rhabdites of Lacaze-Duthiers, gonapophyses of Huxley), which are closely appressed to or sheathed within each other, the eggs passing out from the end of the oviduct, which lies, as Dewitz states, between the two styles of the lowest or innermost pair, and under the cross-bars or at the base of the stylets mentioned; the styles or blades spreading apart to allow of the passage of the egg.
Fig. 185.—Saw of Hylotoma: a, lateral scale; i, saw; f, gorget; 7t, 7th tergite; 6s, 6th sternite; ov, oviduct; in, intestine.—After Lacaze-Duthiers.
The ovipositor is best developed in the Thysanura (Fig. 179, Campodea excepted), in Orthoptera (Fig. 184), in the Odonata, Hemiptera, certain Physapoda, Rhaphiidæ, and in the phytophagous Hymenoptera, where it is curiously modified to form a rather complicated saw for cutting slits in wood or leaves (Fig. 185). It is wanting or quite imperfect in Coleoptera, Diptera, and Lepidoptera.
Morphologically, the ovipositor appears to be formed out of the abdominal appendages of the seventh, eighth, and ninth segments of the female, which, instead of disappearing in the orders first mentioned, persist as permanent styles.
Wheeler asserts from his study of the embryonic development of Xiphidium “there can be no doubt concerning the direct continuity of the embryonic appendages with the gonapophyses.” He goes on to say:—
“One embryo, which had just completed katatrepis, still showed traces of all the abdominal appendages. The pairs on the eighth, ninth, and tenth segments were somewhat enlarged. In immediately succeeding stages the appendages of the second to sixth segments disappear; the pair on the seventh disappear somewhat later. Up to the time of hatching the gonapophyses could be continuously traced, since in Xiphidium there is no flexure of the abdomen, as in other forms, to obscure the ventral view of the terminal segments. From the time of hatching Dewitz has traced the development of the ovipositor in another locustid (Locusta viridissima), so that now we have the complete history of the organ.”
Heymons, however, is inclined to believe that they are simply hypodermal outgrowths.
Fig. 186.—Ideal plan of the structure of the ovipositor to illustrate Lacaze-Duthiers’ view: b, 8th tergite; c, epimerum; a′, a, two pieces forming the outer pair of rhabdites; i, the 2d pair, or stylets; and f, the inner pair, or sting; d, support of sting; e, piece supporting the stylet; R, anus; o, outlet of oviduct. The 7th, 8th, and 9th sternites are aborted.—After Lacaze-Duthiers.
The first to study the morphology of the ovipositor was Lacaze-Duthiers, who referred their origin to the partially atrophied dorsal or ventral sclerites of one of the last abdominal segments; a view accepted by Gerstaecker[[37]] (Figs. 186, 187). The present writer (1866), however, showed that the sting of Bombus was not formed of the reduced pieces of the segments themselves, but arose from special outgrowths on the ventral side of the eighth and ninth abdominal segments. These appendages he did not at first regard as the homologues of the limbs, until in 1871, after studying the origin of the spring of the Podurans (Isotoma), he found that it was a true jointed appendage and therefore a homologue of a pair of the styles forming the ovipositor of the winged insects, and that the three pairs of styles of the latter were homologues of the thoracic legs and cephalic appendages. The view was stated in the Guide to the Study of Insects. (See also Amer. Nat., March, 1871, p. 6.) Kraepelin also affirms that the styles of the ovipositor are segmental appendages and homologues of the antennæ, wings (sic), and legs.
Fig. 187.—1, abdomen of Cynips, showing the great dorsal segment, the peduncle, and the position of the ovipositor within; 2, the entire ovipositor; a, lateral scale; a′, its valve; b, anal scale; b′, stylet; c, support of the stylet; e, base or support of sting (fi); 3, profile showing the relation of the genital armature to the rest of the abdomen, the 6th sternite having been drawn to show its full size; 4, anal scale (b) and stylet; e, i, supports and body of the stylet; c, piece uniting the two scales; 5, lateral scale (a), and a′ sheath; d, support of the sting (f); 6, transverse section of the body through the sting (diagrammatic); R, internal armature; o, oviduct; a, lateral scale; a′ its valve; e, support of the stylets (i); b, anal scale; c, piece uniting two scales; f, sting; d, its support; 7, a second section simpler and more theoretical than the first; 8, diagrammatic, all the elements of the sting have been reduced to pieces of the same form.—After Lacaze-Duthiers.
An objection to this view is the fact that the posterior pairs of styles appear to arise both from one and the same segment,—the ninth. Dewitz questions whether the four appendages of the ninth segment represent two pairs of limbs, or one pair split into two branches, and prefers the latter view, but leaves it as a point to be settled by future investigations. As will be seen below, both Kraepelin and Bugnion observed a pair of rudiments to each of the three penultimate segments, those of the middle pair splitting in two. Wheeler maintains, erroneously we think, that the inner of the two pairs on the ninth segment represents the tenth pair of abdominal appendages; but in reality this latter pair become the cercopods. That there are probably originally in insects of all the orders provided with an ovipositor three distinct pairs of appendages, one to each segment, is proved, or at least strongly suggested, by Ganin’s researches on the three pairs of abdominal imaginal discs of the third larva of Platygaster and Polynema (Fig. 188), which are transformed into the ovipositor. He remarks that these imaginal discs have the same origin and pass through the same changes as those in front, i.e. those destined to form the thoracic legs. Dewitz has shown that the germs of the ovipositor of the honey-bee arise as buds on the two segments before the last (Fig. 189).
Fig. 188.—Third larva of Polynema: at, antenna; fl, imaginal buds of the wing; l, of the legs; tg, buds of the middle pair of stylets of the ovipositor; fk, fat-body; eg, ear-like process.—After Ganin.
Fig. 189.—Imaginal buds and papillæ of the ovipositor of the honey-bee attached to tracheæ; at different stages: b′, 1st; b″, 2d or middle; and c′, 3d pair of papillæ.—After Dewitz.
Kraepelin also detected in the larva of the honey-bee a pair of what he regarded as genuine imaginal buds on abdominal segments eight, nine, and ten; the buds on the tenth segment are divided each in two; of these four appendages the two median ones form the barbed sting (gorgeret or stachelrinne), and the two lateral stylets, the valves (stachelscheiden). The two buds of the ninth segment give rise to the vagina and to the oviducts, and these unite secondarily with the posterior end of the ovaries. The genital appendages of the male correspond to those of the female, and arise from four imaginal buds situated on the under side of the tenth abdominal segment.
Fig. 190.—1, sting and poison sac of the honey-bee: GD, poison gland; Gb, poison reservoir; D, accessory gland; sh, sheathing style or sting-“feeler”; Str sting; Ba, sheath; Q, quadrate plate; O, oblong piece; W, angular piece; B, base of the sting and stylets; Stb′, Stb″, the two barbed stylets or darts. 2, sting seen from the ventral face: lettering as in the other figure.—After Kraepelin, from Perrier.
In the ants, according to Dewitz, the genital armature is derived from imaginal buds situated on the under side of the seventh, eighth, and ninth abdominal segments. Bugnion has observed the formation of six imaginal buds of the genital armature in the larva of a chalcid (Encyrtus, Figs. 41, 42, 191, q1, q2, q3), the transformation of the central part of these structures into small digitiform pads, then the division of the two intermediate buds into four (?) (Fig. 191, B, q2), but was unable to trace their farther development.
The subject still needs farther investigation, since certain observers, as Haase, and, more recently, Heymons, do not believe that they are homologues of the legs, but integumental structures, though of somewhat higher value than the style of the base of the legs of Scolopendrella and Thysanura; but it is to be observed that as yet we know but little of the embryological history of these styles.
Those authors who have examined the elements of the ovipositor, and regard them as homologues (homodynamous) of the limbs, are Weismann (1866), Ganin (1869), Packard (1871), Ouljanin (1872), Kraepelin, Kowalevsky (1873), Dewitz (1875), Huxley (1877), Cholodkowsky, Bugnion (1891), and Wheeler (1892).
As shown, then, by our observations and those of Dewitz (Figs. 189 and 192), the rudiments of the ovipositor consist of three pairs of tubercles, arising, as Kraepelin and also Bugnion (Fig. 191) have shown, from three pairs of imaginal discs, situated respectively on the seventh, eighth, and ninth uromeres, or at least on the three penultimate segments of the abdomen. With the growth of the semipupa, the end of the abdomen decreases in size, and is gradually incurved toward the base (Fig. 193), and the three pairs of appendages approach each other so closely that the two outer ones completely ensheath the inner pair, until a complete extensible tube is formed, which, by the changes in form of the muscles within, is gradually withdrawn entirely within the body.
Fig. 191.—A, end of larva of Encyrtus of 2d stage, showing the three pairs of imaginal buds of the ovipositor q1, q2, q3. B, the same in an older larva ready to transform; i, intestine; x, genital gland; a, anus.—After Bugnion.
An excellent account of the honey-bee’s sting is given by Cheshire (Figs. 194, 195). The outermost of the three pairs of stylets forming the apparatus is the two thick, hairy “palpi” or feelers (P), these being freer from the sting proper than in the ovipositor of Orthoptera. The sting itself is composed of the two inner pairs of stylets; one of these pairs is united to form the sheath (sh), while the other pair form the two barbed darts. The sheath has three uses: first, to open the wound; second, to act as an intermediate conduit for the poison; and third, to hold in accurate position the long barbed darts. The sheath does not enclose the darts as a scabbard, but is cleft down the side presented in Fig. 194, which is below when the sting points backward. But, says Cheshire, as the darts move up and down, they would immediately slip from their position, unless prevented by a mechanical device, exhibited by B and C, giving in cross-section sheath and darts near the end, and at the middle of the former. “The darts (d) are each grooved through their entire length, while upon the sheath (sh) are fixed two guide rails, each like a prolonged dovetail, which, fitted into the groove, permits of no other movement than that directly up and down.” The darts are terminated by ten barbs of ugly form (D, Fig. 194), and much larger than those of the sheath, and as soon as the latter has established a hold, first one dart and then the other is driven forward by successive blows. These in turn are followed by the sheath, when the darts again more deeply plunge, until the murderous little tool is buried to the hilt. But these movements are the result of a muscular apparatus yet to be examined, and which has been dissected away to bring the rigid pieces into view. The dovetail guides of the sheath are continued far above its bulbous portion, as we see by E, Fig. 195; and along with these the darts are also prolonged upward, still held to the guides by the grooved arrangement before explained; but both guides and darts, in the upper part of their length, curve from each other somewhat like the arms of a Y, to the points c, c′ (A, Fig. 194), where the darts make attachment to two levers (i, i′). The levers (k, l and k′, l′) are provided with broad muscles, which terminate by attachment to the lower segments of the abdomen. These, by contraction, revolve the levers aforesaid round the points f, f′, so that, without relative movement of rod and groove, the points c, c′ approach each other. The arms of the Y straighten and shorten, so that the sheath and darts are driven from their hiding-place together and the thrust is made by which the sheath produces its incision and fixture. The sides being symmetrical, we may, for simplicity’s sake, concentrate our attention on one, say the left in the figure. A muscular contraction of a broad strap joining k and d (the dart protractor) now revolves k on l, so that a is raised, by which clearly c is made to approach d; i.e. the dart is sent forward, so that the barbs extend beyond the sheath and deepen the puncture. The other dart, and then the sheath, follow, in a sequence already explained, and which G, Fig. 195, is intended to make intelligible, a representing the entrance of the sheath, b the advance of the barbs, and c the sheath in its second position. The barb retractor muscle is attached to the outer side of i, and by it a is depressed and the barbs lifted. These movements, following one another with remarkable rapidity, are entirely reflex, and may be continued long after the sting has been torn, as is usual, from the insect. By taking a piece of wash-leather, placing it over the end of the finger, and applying it to a bee held by the wings, we may get the fullest opportunity of observing the sting movements, which the microscope will show to be kept up by continued impulses from the fifth abdominal ganglion and its multitudinous nerves (n, Fig. 194, A), which penetrate every part of the sting mechanism, and may be traced even into the darts. These facts, together with the explanation at page 49, will show why an abdomen separated many hours may be able to sting severely, as I have more than once experienced.
Fig. 192.—Base of the ovipositor of Locusta viridissima seen from beneath: c′, sheath, or outer and lower pair of stylets turned to one side to show the others; b′, upper and inner pair; b″, third or innermost, smallest pair of stylets. A, the same on one side, in section. The shaded parts show the muscular attachments. The muscles which extend the apparatus and are attached to ν, δ, and η, as also the membranes which unite the pieces from η; to γ with each other and the body, are removed, so that only the chitinous parts remain.—After Dewitz.
Fig. 193.—Development of the sting in Bombus: A, a, 1st pair on 8th sternite; b, 2d inner pair forming the darts; c, outer pair. B-E, more advanced stages. F, x, y, z, three pairs of tubercles, the germs of the male organs.
Fig. 194.—Sting of bee × 30 times: A, sting separated from its muscles; ps, poison sac; pg, poison gland; 5th g, 5th abdominal ganglion; n, n, nerves; e, external thin membrane joining sting to last abdominal segment; i, k, l, and i′, k′, l′, levers to move the darts; sh, sheath; v, vulva; p, sting-palpus or feeler, with tactile hairs and nerves. B and C, sections through the darts and sheath, × 300 times: sh, sheath; d, darts; b, barbs; p, poison-channel. D, end of a dart, × 200: o, o, openings for poison to escape into the wound.—After Cheshire.
Fig. 195.—Details of sting of bee: E, darts, sheath, and valves; pb, poison-bag duct; fo, fork; s, slide piece; va, valve; b, barbs. F, terminal abdominal segments; w, worker’s sting; q, queen’s sting; r, r′, anal plate; G, sting entering skin; sh, sheath; a, b, c, positions in first, second, and third thrusts with the sting. H, portion of poison gland, × 300; cn, cell nucleus; n, nerve; g, ganglionic cell. I, portion of the poison gland, cells removed; cd, central duct; d, individual small ducts; pr, tunica propria. K, gland of Formica rufa; cd, central duct; d, small ducts; sc, secreting cells. L, valve and support; t, trachea; va, valve; tr, truss or valve-prop.—After Cheshire.
Fig. 196.—A, rudimentary ovipositor of nymph of Æschna. B, the corresponding ♂ structures; a, enlarged. C, ovipositor of nymph of Agrion; d, gill.
The male genital armature in the bees is originally composed of three pairs of tubercles, homologous with those of the female, all originally arising from three abdominal segments, two afterward being anterior, and the third pair nearer the base of the abdomen.
The ovipositor of the dragon-flies (Odonata) is essentially like that of the Orthoptera and Hymenoptera. Thus in Æschna (Fig. 196), Agrion (Fig. 196, C), and also in Cicada it consists of a pair of closely appressed ensiform processes which grow out from under the posterior edge of the eighth uromere and are embraced between two pairs of thin lamelliform pieces of similar form and structure.
The styles and genital claspers (Rhabdopoda).—Other appendages of the end of the abdomen of pterygote insects, and generally, if not always, arising from the ninth segment, are the clasping organs, or rhabdopoda as we may call them, of Ephemeridæ (Fig. 197), Neuroptera (Corydalus [Fig. 198], Myrmeleon, Rhaphidia), Trichoptera, Lepidoptera, Diptera, and certain phytophagous Hymenoptera. They do not appear to occur in insects which are provided with an ovipositor. In Thysanura the styles are present on segments 1–9 (Fig. 179). Those of the male Ephemeridæ, of which there are two pairs arising from the ninth segment, are remarkable, since they are jointed, and they serve to represent or may be the homologues of two of the pairs of stylets composing the ovipositor of insects of other orders. The lower pair (Fig. 197, rh) are either 2–, 3–, or 4–jointed (in Oniscigaster 5–jointed), while those of the upper pair are 2–jointed (rh′). These rhabdopods in the ephemerids are evidently very primitive structures, since they approach nearest in shape and in being jointed to the abdominal legs of Scolopendrella and the Myriopoda. The styles of the Orthoptera are survivals of the embryonic appendages of the ninth segment (Wheeler, etc.). In Mantis they are seen to have the same relations as the cerci, as shown by Heymons (Fig. 200).
Fig. 197.—Abdomen of Ephemera (Leptophlebia) cupida, ♂: c, base of cercopoda; rh, outer 3–jointed claspers or rhabdopods; rh′, inner pair. A, side view.
Fig. 198.—End of abdomen of Corydalus cornutus, ♂: vh, rhabdopod; c, cercopod.
In the Phasmidæ, in Anabrus, and in the Odonata the cercopods, which are not jointed, are converted into claspers, and in the Odonata the claspers are spiny within, so as to give a firmer hold. The suranal plate is apparently so modified as to aid in grasping the female. In nearly all the Trichoptera there are, besides the suranal plate, which is sometimes forked (Nosopus), a pair of superior and of inferior claspers, and in certain genera (Ascalaphomerus, Macronema, Rhyacophila, Hydropsyche, Amphipsyche, Smicridea, and Ganonema) the lower pair are 2–jointed like those of Ephemeridæ. The number of abdominal segments in the adult Trichoptera is nine, and McLachlan states that the genital armature consists of three pairs of appendages, i.e. a superior, inferior, and intermediate pair, besides the suranal plate (vestige of a tenth segment) and the penis. Judging by his figures, these three pairs of appendages arise from the last or ninth uromere, and the upper pair seem to be the homologue of the cercopoda of ephemerids. It needs still to be ascertained whether the intermediate pair is a separate set, or merely subdivisions of the upper or lower, and whether one of the latter may not arise from the penultimate segment, because we should not expect that the last segment should bear more than one pair of appendages, as we find to be the case in arthropods in general, and in the Neuroptera, from which the Trichoptera may have originated.
Fig. 199.—End of abdomen of embryo of Mantis: r, rhabdopod; c, cercopod; sp, suranal plate; st, stigma on 8th segment.—After Heymons.
Fig. 200.—End of abdomen of Periplaneta americana, ♂, side view: c, cercopod; st, stilus; p, penis; t, titillator; d, “bird’s head” (clasper?); i, “oblong plate”; IX-XI, terminal segments; X, suranal plate; XI′, 11th sternite.—After Peytoureau.
In most larvæ of the Trichoptera, especially the Rhyacophilidæ and Hydropsychidæ, the last abdominal segment bears a pair of 2–jointed legs (cercopoda), ending in either one or two claws, which under various forms, sometimes forming long processes, persist in the pupa; and there appears to be a suranal plate, the vestige of the tenth uromere. In the pupa, judging by Klapálek’s figure of Leptocerus (248,9, 25), a pair of lateral spines arise which may in the imago form one of the pairs of appendages or styles. In the pupa of Œcetis furva his figure 289 shows two pairs of 1–jointed appendages arising from the last segment; whether the long dorsal or upper styles arise from the vestige of a more distal segment is not distinctly shown in Klapálek’s sketch. The origin of these elements of the genital armature evidently needs further study.
Whether the abdominal legs or so-called false or prop-legs of lepidopterous larvæ are genuine legs, homologous with those of the thorax and with the cephalic appendages, or whether they are secondary adaptive structures, is a matter still under discussion. That, however, they are true legs is shown by the embryology of the Lepidoptera, where there is a pair to each abdominal segment. It may also be asked whether the anal legs of lepidopterous larvæ are not the homologues of the 2–jointed anal appendages of caddis-worms.
Fig. 201.—Eriocephala calthella, ♂, side view: t, palpiform suranal plate; cl, claspers; s, inferior claspers; mxp, maxillary palpi; cx. coxa; tr, trochanter; sc, scutum; sc′, scutellum.
In Lepidoptera, notably the male of the very generalized Eriocephala calthella (Fig. 201), besides the broad unjointed claspers, which are curved upward and provided with a brush of stiff hooked setæ (this upper pair being perhaps modified cercopods), there is an accessory lower slenderer pair, while the suranal plate (t) is palpiform or clavate and also adapted to aid in the action of the claspers. The examination of the cercopods and rhabdopods in the Trichoptera and in a generalized lepidopterous form like this enables one to understand the morphology of the genital armature, since it consists, besides the suranal plate, which is often deeply forked (in Sphingidæ, Smith), of a pair of modified hook-like cercopoda, and in some cases (Eriocephala) of an additional pair of claspers which may be the homologues of the ephemerid rhabdopods. A pair of hooks, often strong and claw-like (harpes), are situated, one near the base on the inside of each clasper; they are especially developed in the Noctuidæ (Smith), and appear to be present in certain Trichoptera, but this remains to be proved. This complicated apparatus of claspers and hooks is utilized by those insects which pair while on the wing, and is wanting in such forms as Coleoptera and Hemiptera. Besides the forceps of Panorpa, there are two pairs of slender filiform appendages which need farther examination. In the Diptera, especially Tipulidæ, there is a pair of 2–jointed appendages or forceps, as in Limnophila (Osten Sacken). The male genital armature of Diptera appears to be on the same general plan as in Lepidoptera, but more complicated.
Fig. 202.—Male organs of generation of Athalia.—After Newport.
Notice should also be taken of the paired uncinate hooks which are modifications of the penis-sheath of the male of cockroaches (Phyllodromia), which Haase states appear to originate on the tenth ventral plate, and which probably “serve to open and dilate the vagina of the female, especially as a perforated penis, which is highly developed in Machilis, seems to be wanting in the Blattidæ.” (Haase.)
The penis.—This is a single or double median style-like structure either hollow and perforated, or solid, very variable in shape, receiving the end of the ejaculatory duct. It is usually enclosed between two lateral plates, the homologues perhaps of the inner pair of sheaths of the ovipositor. In the Coleoptera, as in Carabidæ and Melolonthidæ, the penis is a long chitinous tube, “retractile within the abdomen on the under surface as far as the anterior segments.” (Newport.) In the Hymenoptera, of which that of the saw-flies is a type, Newport states that it “consists of a short valvular projectile organ, covered externally by two pointed horny plates (i) clothed with soft hairs.” Above these are two other irregular double-jointed plates (Fig. 202, l) surrounded at their base by a chitinous ring (k); they are edged with prehensile hooked spines (i). Between these in the middle line are two elongated muscular parts (m) which enclose the penis (h), and which, like those in beetles, perhaps aid in dilating the vulva of the female.
An examination of Figs. 203–207 will aid in understanding the various modifications in beetles, etc., of this organ.
A general study of the anatomy and homologies of the male genital armature, from a developmental point of view, together with a comparison of them with the corresponding female organs, is still needed.
Fig. 203.—End of abdomen situated under the anal lobes of Hydrophilus piceus, drawn out, seen from the ventral side: 6, sternal region of 6th segment; 7, 8, 9, segments telescoped, when retracted, in 6th segment; zw, membrane connecting 6th and 7th segments; G, intromittent apparatus; vl, external lobes; vlu, inner lobes; pn, penis.
Fig. 204.—The same as in Fig. 203, seen from the side: 6, the free 6th segment; 7–10, the four last, when at rest, retracted and telescoped within the 6th segment, with the copulatory apparatus (g); vl, outer, vlu, inner lobe; 10s, tergite of 10th segment; 10i, sternite of the same; an, anal opening.
Fig. 205.—Terminal parts of the male copulatory apparatus of Hydrophilus piceus, torn apart: vlu, the two inner lodes; pn, penis; x, membrane torn from under side of penis; ej, ejaculatory duct; os, its opening on the under side of the penis, directly under its tip. The muscles, tracheæ, and nerves are not drawn.
Velum penis.—In the locusts (Acrydiidæ) the penis is concealed by a convex plate, flap, or hood, free anteriorly and attached posteriorly and on the sides to the ridge forming the upper edge of the tenth sternite. When about to unite sexually, the tip of the abdomen is depressed, the hood is drawn backward, uncovering the chitinous penis.
The suranal plate.—This is a triangular, often thick, solid plate or area, the remnant of the tergum of the last, usually tenth, segment of the abdomen, the supra-anal or suranal plate, or anal operculum (lamina supra-analis) of Haase. In most lepidopterous larvæ this plate is well marked; in those of the Platypteridæ it is remarkably elongated, forming an approach to a flagellum-like terrifying appendage, and in that of Aglia tau it forms a long, prominent, sharp spine. In the cockroach, both Cholodkowsky and Haase maintain that the tenth abdominal segment is suppressed in the female, the tergal portion being fused with the suranal plate (the latter in this case, as we understand it, being the remnant of the eleventh segment of the embryo). As to the nature of the middle jointed caudal appendage in Thysanura and May-flies Heymons has satisfactorily shown that it is a hypertrophied portion of the suranal plate, being in Lepisma but a filamental elongation of the small eleventh abdominal tergite.
Fig. 206.—Copulatory organ of a weevil, Rhychophorus phœnicis, seen from above. A, vl, the lobes united into a capsule; pp, torn membrane which connects the capsule with the 9th abdominal segment; ej, ejaculatory duct. B, the same seen from the side; mu, end of the muscle of the penis. C, the same as B, without the capsule; os, opening of the ejaculatory duct (ej). Other letters as in A.
Fig. 207.—A, penis (pn) of Carabus hortensis: bl, wrinkled membranous vesicle; vlu, the valves; g, part of 9th segment. B, end of penis of the same, enlarged; os, cleft-like opening; also a wrinkled vesicle, as at bl.—This and Figs. 203–205 after Kolbe.
At the base of the suranal plate of locusts (Acrydiidæ) is the suranal fork or suranal furcula (furcula supra-analis, as we have called it) (Fig. 88, 89, f).
The podical plates or paranal lobes.—In the cockroach and other insects, also in the nymphs of Odonata, the anus is bounded on each side by a more or less triangular plate, the two valves being noticeable in lepidopterous larvæ. They are the valvulæ of Burmeister, and podical plates of Huxley, who also regarded them as the tergites of an eleventh abdominal segment;[[38]] and the subanal laminæ of Heymons. They are wanting in Ephemeridæ.
The infra-anal lobe.—Our attention was first called to this lobe or flap, while examining some geometrid larvæ. It is a thick, conical, fleshy lobe, often ending in a hard, chitinous point, and situated directly beneath the vent. Its use is evidently to aid in tossing the pellets of excrement away so as to prevent their contact with the body. The end may be sharp and hard or bear a bristle. Whether this lobe is the modified ventral plate of the ninth urite, we will not undertake at present to say.
The egg-guide.—In the Acrydiidæ the external opening of the oviduct is bounded on the ventral side by a movable, triangular, acute flap, the egg-guide (Fig. 88, B, eg). Whether this occurs in other orders needs to be ascertained.