LITERATURE ON ANCESTRY OF INSECTS, ETC.

Müller, Fritz. Für Darwin, 1869, pp. 144, 67 Figs.

Brauer, Friedrich. Betrachtung ueber die Verwandlung der Insekten im Sinne der Descendenz-Theorie. (Verhandlung d. k.k. zool. bot. Gesell. Wien., 1869, 1 Taf., pp. 1–20.)

Packard, A. S. Amer. Naturalist, iii, p. 45, March, 1869.

—— Proc. Boston Soc. Nat. Hist., xiv, 1870, p. 61.

—— Amer. Nat., iv, Feb. 1871, p. 756; v, 1871, pp. 52, 567.

—— Embryological Studies. (Memoirs Peabody Acad. Sc. Salem, 1871–72.)

—— Our common insects, 1873, Chapter on Ancestry of Insects, pp. 175–178.

—— Third Report U. S. Ent. Commission, 1883, pp. 295–304.

Lubbock, John. On the origin of insects. (Journ. Linn. Soc., London, xl, 1873.)

—— Origin and metamorphoses of insects. (Nature, 1873 [in book form, 1874], pp. 108, 66 Figs.)

Mayer, Paul. Ueber Ontogenie and Phylogenie der Insekten. (Jena. Zeitschr. Wissens., x, 1876, pp. 125–221, 4 Taf.)

Hyatt, A., and Arms, J. M. Insecta. (Bost. Soc. Nat. Hist. Guides for science-teaching, viii.) Boston, 1890, pp. 300, 13 Pls., 223 Figs.

c. Growth and increase in size of the larva

The rapidity of growth and enormous increase in size in early life is especially noticeable in caterpillars and other phytophagous larvæ. The latest observations are those of Trouvelot on Telea polyphemus. When this silkworm hatches, it weighs 1
20 of a grain.

When

10 days old it weighs½ a grain, or10 times the original weight.
20 days old it weighs3 grains60 times the original weight.
30 days old it weighs31 grains620 times the original weight.
40 days old it weighs90 grains1800 times the original weight.
56 days old it weighs207 grains4140 times the original weight.

“When,” he says “a worm is 30 days old, it will have consumed about 90 grains of food; but when 56 days old, it is fully grown and has consumed not less than 120 oak leaves, weighing ¾ of a pound; besides this it has drank not less than ½ an ounce of water. So the food taken by a single silkworm in 56 days equals in weight 86,000 times the primitive weight of the worm. Of this about ¼ of a pound becomes excrementitious matter, 207 grains are assimilated, and over 5 ounces have evaporated.”[[94]]

Dandolo stated that the Asiatic silkworm (Bombyx mori) weighs on hatching not over 1
100 of a grain, but when fully grown about 95 grains. During this period, therefore, it has increased 9500 times its original weight, and has eaten 60,000 times its weight of food. Newport thought this estimate of the amount of food was a little too great. But comparing it with Trouvelot’s estimate for the American silkworm, which weighs when hatched five times as much, it would not appear to be so. Newport found that the larva of Sphinx ligustri at the moment of leaving the egg weighs about 1
80 of a grain, and when fully fed 125 grains, so that in the course of 32 days it increases about 9976 times its original weight. This proportion of increase is exceeded by the larva of Cossus ligniperda, which, boring in the trunks of trees, remains about three years in the larva state, and increases, according to Lyonet, to the amount of 72,000 times its first weight.

Newport adds that those larvæ in which the proportion of increase is the greatest, are usually those which remain longest in the pupa state, as in the silkworm. “Thus Redi observed in the maggots of the common flesh-flies a rate of increase amounting to about 200 times the original weight in 24 hours, but the proportion of increase in these larvæ does not at all approach that of the Sphinx and Cossus.” From his observations on the larva of one of the wild bees (Anthophora retusa) Newport believes that this is also the case with the Hymenoptera. The weight of the egg of this insect is about 1
150 of a grain, and the average of a full-grown larva 68
10 grains, so that its increase is about 1020 times its original weight; “which compared with that of the Sphinx of medium size, is but as 1 to 9¾, and to a Sphinx of maximum size only as 1 to a little more than 11.”

The growth is most rapid after the last moult. “Thus a larva of Sphinx ligustri, which at its last change weighed only about 19 to 20 grains, at the expiration of eight days, when it was fully grown, weighed nearly 120 grains.” (Newport.)

d. The process of moulting (ecdysis)

Insects periodically shed the exoskeleton, together with the chitinous lining of their internal organs of ectodermal origin, which thus sloughed off are called the exuvia. The process in the locust has been described by Riley.[[95]] It occupies from half to three-quarters of an hour (Fig. 565). This process has naturally, from the ease with which it can be observed, been most frequently examined in the Lepidoptera, though careful and detailed observations of the inner and outer changes are still greatly needed, especially in other orders. In the caterpillar of most moths, especially one of the more generalized bombycine moths, on slipping out of its egg-shell the head is of enormous size as compared with the body, but the latter soon fills out after the creature has eaten a few hours; the head, of course, does not during this time increase in size, and the larvæ of different instars may be exactly distinguished, as Dyar has shown, by the measurements of the head.

Fig. 565.—Process of moulting from nymph to imago in the locust (M. spretus): a, nymph with skin just split on the back; b, the imago drawing itself out, at c, nearly free; d, the imago, with wings expanded; e, the same with all parts perfect.—After Riley.

Before the caterpillar moults, it stops feeding, and the head is now small compared with the body; the head of the second instar is now large, situated partly under the much-swollen prothoracic segment, and pushes the head of the first instar forward.

Newport has well described the mode of shedding the skin in Sphinx ligustri, and his detailed description will apply to most lepidopterous larvæ.

The whole body is wrinkled and contracted in length, and there are occasionally powerful contractions and twitchings of its entire body; the skin becomes dry and shrivelled, and is gradually separated from the new and very delicate one of the next instar beneath. After several powerful efforts of the larva the old skin cracks along the middle of the dorsal surface of the mesothoracic segment, and by repeated efforts the fissure is extended into the 1st and 3d segment, while the covering of the head divides along the vertex and on each side of the clypeus. “The larva then gradually presses itself through the opening, withdrawing first its head and thoracic legs, and subsequently the remainder of its body, slipping off the skin from behind like the finger of a glove. This process, after the skin has once been ruptured, seldom lasts more than a few minutes. When first changed the larva is exceedingly delicate, and its head, which does not increase in size until it again changes its skin, is very large in proportion to the rest of the body.” (Art. Insecta, etc.)

Trouvelot’s account is more detailed and an advance on that of Newport’s view. He explicitly states, and we know that he was a very close observer, that the old skin is detached by “a fluid which circulates between it and the worm.” His account is as follows: The polyphemus worm, like all other silkworms, changes its skin five times during its larval life. The moulting takes place at regular periods, which comes around about every 10 days for the first four moultings, while about 20 days elapse between the fourth and fifth moulting. The worm ceases to eat for a day before moulting, and spins some silk on the vein of the under surface of a leaf; it then secures the hooks of its hind legs in the texture it has thus spun, and there remains motionless; soon after, through the transparency of the skin of the neck, can be seen a second head larger than the first, belonging to the larva within. The moulting generally takes place after four o’clock in the afternoon; a little before this time the worm holds its body erect, grasping the leaf with the two pairs of hind legs only; the skin is wrinkled and detached from the body by a fluid which circulates between it and the worm; two longitudinal bands are seen on each side, produced by a portion of the lining of the spiracles, which at this moment have been partly detached; meanwhile the contractions of the worm are very energetic, and by them the skin is pulled off and pushed towards the posterior part; the skin thus becomes so extended that it soon tears just under the neck, and then from the head. When this is accomplished the most difficult operation is over, and now the process of moulting goes on very rapidly. By repeated contractions the skin is folded towards the tail, like a glove when taken off, and the lining of the spiracles comes out in long white filaments. When about one-half of the body appears, the shell still remains like a cap, enclosing the jaws; then the worm, as if reminded of this loose skull-cap, removes it by rubbing it on a leaf; this done, the worm finally crawls out of its skin, which is attached to the fastening made for the purpose. Once out of its old skin, the worm makes a careful review of the operation, with its head feeling the aperture of every spiracle, as well as the tail, probably for the purpose of removing any broken fragment of skin which might have remained in these delicate organs. Not only is the outer skin cast off, but also the lining of the air-tubes and intestines, together with all the chewing organs and other appendages of the head. After the moulting, the size of the larva is considerably increased, the head is large compared with the body, but 8 or 10 days later it will look small, as the body will have increased very much in size. This is a certain indication that the worm is about to moult. Every 10 days the same operation is repeated. From the fourth moulting to the time of beginning the cocoon the period is about 16 days. (Amer. Naturalist, i, pp. 37, 38.)

Little has been recorded as to the exact mode of casting the larval skin in Coleoptera. Slingerland states that Euphoria inda when pupating sheds the larval skin off the anal end in the same way as in caterpillars, while in Pelidnota punctata the larval skin splits down the whole length of the back, retains the larval shape, and forms a covering for the pupa which remains inside. (Can. Entomologist, xxix, p. 52.) The old larval skin in the Coccinellidæ and certain Chrysomelidæ is retained crumpled up at the end of the body, while in Dermestes, Anthrenus, etc., it cloaks the pupa.

Not only is the integument, with its hairs, setæ, and other armatures, as well as the cornea or facets of the eyes, shed, but also all the lining or intima of those internal organs which have been originally derived by an ingrowth or invagination of the ectoderm are likewise cast off, with the probable exception, of course, of the mid-intestine, which is endodermal in its origin. Even so early an observer as Swammerdam noticed that the internal lining of the alimentary canal comes away with the skin. He states that the larva of Oryctes nasicornis sheds both the lining of the colon, and of the smaller as well as the larger branches of the tracheæ.

Careful observations are still needed on the internal changes at ecdysis of most insects. Newport seems to have observed more closely than any one else, notwithstanding the great number who have reared caterpillars but have not carefully observed these points, the extent of the process internally. He informs us: “The lining of the mouth and pharynx, with that of the mandibles, is detached with the covering of the head, and that of the large intestines with the skin of the posterior part of the body, and besides these also the lining of the tracheal tubes. The lining of the stomach itself, or the portion of the alimentary canal which extends from the termination of the œsophagus to the insertion of the so-called biliary vessels, is also detached, and becomes completely disintegrated, and appears to constitute part of the meconium voided by the insect on assuming its imago state.” (Art. Insecta, p. 876.) Newport states on another occasion that he had “noticed the remarkable circumstance [now explained by the fact that the mid-intestine is of endodermal origin] that the mucous lining of the true ventriculus was not cast off with the rest, but was discharged with the fecula.”[[96]] Burmeister also observed that the smaller tracheæ as well as the internal tunic of the colon of Libellulæ are shed.

In the apodous larvæ of Hymenoptera which live in cells, as we have observed in those of Bombus, during the process of moulting, the delicate skin breaks away in shreds, probably owing to the tension due to the unequal growth of the different parts of the body. “Thus after the skin beneath has fully formed, shreds of the former skin remain about the mouth-parts, the spiracles, and anus. Upon pulling upon these, the lining of the alimentary tube and tracheæ can be drawn out, sometimes, in the former case, to the length of several lines.”[[97]] We then added, “As all these internal systems of vessels are destined to change their form in the pupa, it may be laid down as a rule in the moulting of insects and Crustacea, that the lining of the internal organs, which is simply a continuation of the outer tegument, or arthroderm, is, in the process of moulting, sloughed off with that outer integument.” We have satisfied ourself that in the larvæ of the Lepidoptera (e.g. Datana) the tracheæ at the time of ecdysis undergo a complete histolysis, and arise de novo from hypodermal cells, the so-called spiral threads originating from elongated peritracheal nuclei. (See p. 449, Fig. 412.) This is undoubtedly also the case with the salivary ducts, which are strengthened and rendered elastic by tænidia like those of tracheæ. As the urinary tubes are diverticula of the proctodæum, itself an ectodermal invagination, they may also, though not lined with a chitinous intima, be renewed. With little doubt the intima of the ducts of poison, spinning, and most, if not all the other glands, though certainly the dermal glands, is exuviated. We have found that the lobster in moulting sheds, besides the skin with the most delicate setæ, the lining of the proventriculus, and the apodemes of the head and thorax, hence it is most probable that the tentorium of the head of insects as well as the apodemes and phragmas of the thorax are exuviated.

The formation of the inner skin, or that of any succeeding stage (instar), is due to the secretion of the structureless chitinous layer by the cells of the hypodermis, during the process of histogenesis. These cells at this time are very active, and the formation of the new layer of chitin arrests the supply of nourishment to the old skin, so that it dries, hardens, and with the aid of the fluid thrown out at this time separates from the new chitinous layer secreted by the hypodermis.

Mention of this fluid, which Newport was the first to observe, and which he says causes the separation of the old from the underlying fresh integument of the caterpillar, recalls a passage in Hatchett-Jackson’s Studies in the morphology of the Lepidoptera, which we quote on a succeeding page, where he calls attention to the formation of such a liquid, which in the reptiles facilitates the process of moulting, adding, “Whether such is the case with the moult of the caterpillar, I do not know.” Is it not also possible that the growth of the setæ or tubercles on the cuticle of the caterpillar may likewise serve to loosen and detach the overlying skin about to be cast off? After writing the foregoing, we find that Miall and Denny have suggested that the setæ of the cockroach probably serve the same purpose as the casting-hairs of the crayfish and reptiles.

It is well known that in the crayfish and in lizards the skin is first loosened by the growth of temporary hairs or setæ, which locally grow inward from the old cuticle and push the skin away when it is shuffled off by the movements of the body, jaws, and limbs, as well as the body in general.[[98]]

Such spines arise in the pupa of many insects, for Verhoeff finds that the spines and teeth of pupal fossorial and other Hymenoptera, as well as Coleoptera, function as moulting-processes for loosening and pushing off the last larval skin, rather than for locomotion. He also claims that the spines of the pupa of the dipterous Anthrax are both for locomotion and for boring, especially the spines on the head and tail. He therefore divides these pupal spines into helcodermatous (boring or tearing) and locomotor spines.

Gonin has fully confirmed Newport’s discovery of the exuvial fluid. He states that during pupation the outside of the pupa, especially the parts of the head and thorax “is coated with a viscous liquid secreted by special glands.” The parts only harden subsequent to pupation after exposure to the air (p. 41). His observations were made under the direction of Professor Bugnion, who kindly writes us:—

“M. Gonin has proved the formation of a liquid which passes under the cuticle at the time of the last moult and facilitates exuviation. We think that this liquid is secreted by large cells (unicellular glands) which we see especially on the surface of segments 1–3. These cells form part of the hypodermis, and their pores open under the cuticle.”

In a subsequent letter enclosing a sketch kindly made for me by M. Gonin (Fig. 566), Professor Bugnion writes me Aug. 24, 1897, regarding the functions of the large hypodermal cells (l. hy), as follows: “It seems to me, in fact, after having again examined the sections, that the function of these cells is not sufficiently elucidated. Indeed these cells occur only in the section passing through the 1st segment, between the head and 1st thoracic segment. It would seem, if these cells supply the liquid which lubricates the surface at the time of ecdysis, that they should be spread over the entire surface of the body. Moreover, these cells have no distinct orifice, and although there is seen at times to issue streams of a substance (coagulated by the reagents), they cannot be compared with true unicellular glands like those of the epidermis of fishes, amphibians, etc.

“On the other hand, if it is the blood which oozes out on the surface (according to your hypothesis), it would seem that the loss of blood would cause the death of the larva. I believe then it is due to the secretion of the hypodermis which spreads over the whole surface when the cells are still soft (not yet hardened from contact with the air). At all events, there is a liquid spread over the surface; it is this liquid which glues the wings and the legs to the body at the moment the caterpillar issues from the rent in its skin. If at this instant we plunge the pupa in the water the liquid is dissolved, and the feet, wings, etc., are not glued to the body.”

Dr. T. A. Chapman also writes us: “There is no question about the existence of a fluid between the two skins at moulting. In hairy larvae the hairs are always wet at first, or if the skin be renewed rather more quickly than the larva does it naturally, the wetness of both surfaces is obvious. I do not know the nature of the fluid, but it is related to that which hardens into the dense pupal case, and also hardens in a less degree the skin of the larva. I suppose it must contain some chitin in a soluble form. If a newly cast larva skin be taken, there is no difficulty in extending the shrivelled mass to its full length and dimensions, but if a short time elapses, this chitin hardens, and the skin cannot be extended after soaking in water, alcohol, ammonia, or any other solvent I have tried.”

It has been stated that there is a subimaginal pellicle in Lepidoptera, but as Dr. Chapman writes me, “what has been observed has been some of the inner pupal dissepiments, such as the pupal cases of the under wings,” etc. They may be observed in the head of the tineid pupæ, and other small moths. We have thought that the delicate, purplish, powdery layer left in the cast shells of the pupæ of saturnians, Catocalæ, and other moths, might possibly be such a pellicle, but this view has been dispelled by the following statement of Professor Bugnion in a letter answering an inquiry whether he had noticed such a pellicle.

“A liquid which is secreted in a few minutes at the time of the last moult, forms in drying a yellowish layer spotted with black (in Pieris brassicæ). This layer extends around the entire pupa, and serves both to protect it and to glue together the wings, legs, etc., in their new position. The dried liquid on the surface of the pupa, and by means of which the appendages are glued to the surface, very likely corresponds to the pellicle of which you speak.” The newly exposed integument is at first pale and colorless, but soon assumes the hues peculiar to the species, and the insect, at first exhausted, after a short rest becomes active.

Fig. 566.—Transverse section through the prothoracic segment (ventral face) of larva of Pieris brassicæ, about 12 hours before pupation: c, cuticula; l. hy, large glandular (?) hypodermal cells; gradually passing into normal hypodermal cells (hy).—Gonin del.

E. Howgate has noticed under the microscope peculiar internal movements in a small immature transparent geometrid while moulting. “Each separate segment,” he says, “commencing at the head, elongated within the outer skin, whilst the next ones remained in their former state. Each segment in its turn behaved in this curious manner until the last was reached, when the motion was reversed and proceeded toward the head, when it was again reversed.... The whole proceeding appeared as if the larva was gliding within itself, segment after segment, the outer skin remaining as if held by the other segments, whilst the particular one in motion freed itself within. After remaining motionless for a short interval, the skin near the head swelled and burst, open at the back.... Presently out comes the head of the new caterpillar, pushing forward the old one.... After a short struggle the new true legs appear, pushing off and treading under foot the old ones. Then by violent wriggling movements the abdominal legs were extricated. Then all is clear, and the larva, which is quite exhausted, coils itself up and literally pants for breath.” (The Naturalist, November, 1885, No. 124, p. 366, quoted in Psyche, iv, p. 327, 1887.)

Since the worms and most other ametabolous invertebrates are not known to moult their integument, the body steadily increasing in size without frequent changes of skin, it seems that growth may go on and still be accompanied by considerable changes in shape of the body without change of skin. Frequent ecdyses appear, then, to be the result of the great and sudden changes of the body, necessitated by the adaptation of the animal to new or unusual conditions of life. In young Daphnia, a cladocerous crustacean, as many as eight moults were observed in a period of 17 days, and spiders frequently moult even after reaching their full size. The swollen bodies of the gravid female of Gastrophysa, Meloë, or of Termites, and of the honey ant show that the skin can stretch to a great extent, but in the metamorphoses of Crustacea and of insects, whose young are more or less worm-like or generalized in form, with fewer segments and appendages, or with appendages adapted for quite different uses from those of mature life, the necessity for a change of skin is seen to be necessary for mechanical reasons. Hence Crustacea and insects moult most frequently early in life, when the changes of form are most thoroughgoing and radical, while simple growth and increase in size are most rapid at the end of larval life, as seen both in shrimps and crabs, and in insects.

The hibernating caterpillars of certain butterflies are known to moult once oftener than those of the summer brood. Mr. W. H. Edwards has discussed the subject with much detail. “There seems,” he says, “to be a necessity with the hibernators of getting rid of the rigid skin in which the larva has passed the winter; that is, if the hibernation has taken place during the middle stages, as it does in Apatura and Limenitis. In these cases very little food is taken between the moult which precedes hibernation and the one which follows it, and the larva while in lethargy is actually smaller than before the next previous moult. The skin shrinks, and has to be cast off before the awakened larva can grow. Those species (observed) whose larva moults five times in the winter brood require but four moults during the summer.” He adds that while the larva is in lethargy, it is actually smaller than before the next previous moult. Dr. Dyar writes: “I think there is no doubt about the number of stages of arctian larvæ. They seem to have a great capacity of spinning out their life-history by interpolated stages (as regards width of head). I think it is because so many of them hibernate, and only a single brood extends throughout the season.” (Psyche iii, p. 161.)

On the other hand, it is difficult to understand why the caterpillars of arctians moult so frequently, nearly twice as often as in most other caterpillars, though the changes of form and armature are so slight.

Dr. Chapman also writes me: “Arctians resemble bears (Arctos), polar and others, in having long hairs to protect them during winter, and are, in fact, typically hibernators. Many of them have to half-hibernate, having warmth enough to keep them awake, but not enough food for growth, but their tissues, at least the chitinous ones of the cutis, and also probably, and perhaps especially, of the alimentary canal, become old and effete, and require the rejuvenescence acquired by a moult. Other smooth-skinned hibernators have similar capabilities.”

Chapman has shown in his paper on Acronycta that these caterpillars of this genus illustrate how larvæ may lose a moult, and they do so to acquire a sudden change of plumage.

The number of moults in insects of different orders.—It will be seen from the data here presented that the number of moults is as a rule greatest in holometabolic insects with the longest lives, and that an excessive number of ecdyses may at times be due to some physical cause, such as lack of food combined with low temperature.

In Campodea there is a single fragmentary moult (Grassi), while the Collembola (Macrotoma plumbea) shed their skin throughout life. (Sommer.)

In the winged insects, especially Lepidoptera, the number of moults is dependent on climate. Insects of wide distribution growing faster in warmer climates consequently shed their skins oftener; for example, the same species may moult once oftener in the southern than in the northern States, as in the case of Callosamia promethea, which in West Virginia is double-brooded. Hibernating larvæ moult once oftener than those of the summer brood. (W. H. Edwards.) Weniger by rearing the larvæ of Antheræa mylitta and Eacles imperialis, and which, when reared under normal conditions, actually have six stages, found that when reared in a warm moist atmosphere of about 25° C. they have but five stages, i.e. moult but four times. In the hot and moist climate of Ceylon, A. mylitta has but five stages. (Psyche, v, p. 28.)

Among Orthoptera Acrydians moult five times; Diapheromera femorata but twice (Riley); a katydid (Microcentrum retinervis) moults four times (Comstock). Mantis religiosa, according to Pagenstecher, moults seven times, having eight stages, including that before the amnion is cast, but the first “moult” being an exuviation of the amnion, the number of stages is seven. Cockroaches (Periplaneta americana) are said by Marlatt to “pass through a variable number of moults, there being sometimes as many as seven.”

In the Homoptera there are, in general, from two to four moults; thus in Typhlocyba there are five stages, and in Aphis at least three, and in Psylla four during the nymphal state. Psocus has four. Riley states that the nymph of the female coccid, Icerya purchasi, sheds its skin three times, and that of the male twice. Notwithstanding its slow growth, Riley says, the 17–year Cicada moults oftener than once a year, and the number of larval stages probably amounts to 25 or 30 in all. The bed-bug sheds its skin five times; and with the last moult appear the minute wing-pads characteristic of the adult. In Conorhinus sanguisuga there are “at least two larval stages and pupal stages.” (Marlatt.)

In the dragon-flies moulting occurs, Calvert thinks, many times, since the rudiments of wings are said by Poletaiew to only appear in odonate nymphs after the third or fourth moult.

In the May-fly, Chloëon, the number of ecdyses is 20. The neuropterous Ascalaphus (Helecomitus) insimulans of Ceylon moults three times before pupating. Among the Mecoptera Felt has shown that Panorpa rufescens moults seven times.

In Coleoptera the normal or usual number is not definitely known; Meloë moults five times, but this is a hypermetamorphic insect; Tribolium confusum has been carried by Mr. Chittenden through seven moults. Phytonomus punctatus, the clover-leaf weevil, moults three times, according to Riley, who has observed that Dermestes vulpinus passes through seven larval stages.

In the breeding jars, with plenty of food and a constant temperature of from 68° to 78° F., the larvæ cast their 1st skin in from four to nine days, the great majority moulting at seven days. Under the same conditions the 2d skin was cast at from four to seven days, the majority moulting at six days; the 3d skin at from three to six days, the majority moulting at five days; and the 4th skin at from three to six days, the majority moulting at five days; the 5th skin at from five to seven days, and the 6th skin at six days. There are thus seven larval stages. (Report for 1885, p. 260.)

Riley has ascertained that by rearing isolated larvæ of Tenebrio molitor, one after being kept nearly a year had moulted 11 times, when it died. A second larva, hatched June 5, had moulted 12 times by June 10 of the following year, (1877), when it also died. Of T. obscurus three larvæ were reared to the imago state. One moulted 11 times by Aug. 30 of the same year, pupated Jan. 20, 1877, and finally became a beetle Feb. 7, 1877. The other two both moulted 12 times, and reached the imago stage Feb. 18 and March 9, respectively. “All were, as nearly as possible, under like conditions of food and surroundings, and in all cases the moult that gave the pupa is not considered among the larval moults.”

Two larvæ of the museum pest (Trogoderma tarsale) were kept by Riley in a tight tin box with an old silkworm cocoon. “They were half-grown when placed in the box. On Nov. 8, 1880, there were in the box 28 larva skins, all very much of a size, the larva having apparently grown but little. The skins were removed and the box closed again as tightly as possible. Recently, or after a lapse of two years, the box was again opened and we found one of the larvæ dead and shrivelled up; but the other was living and apparently not changed in appearance. There were 15 larva skins in the box. He could not tell when the one larva died, but it is certain that within a little more than three and a half years, two larvæ shed not less than 43 skins, and that one larva did not, during that time, appreciably increase in size. We know of no observations which indicate the normal or average length of life, or number of moults in either Tenebrio or Trogoderma, but it is safe to assume from what is known, in these respects, of allied species, that in both the instances here referred to, but particularly in the case of Trogoderma, development was retarded by insufficient nutrition, and that the frequent moulting and slow growth resulted therefrom, and were correlated.”[[99]] Further observations such as these are greatly needed.

Of the Siphonaptera the common cat and dog flea (Pulex serraticeps) moults three times before pupating. (Howard.)

In Lepidoptera the usual or average number of moults is four, but the number varies considerably, the greatest number yet known occurring in Phyrrarctia isabella, which, Dr. Dyar informs me, moults 10 times.

From Dyar’s observations it appears that there are usually five larval stages, but six and seven stages are not infrequent, while there are seven in Seirarctia echo, eight in Ecpantheria scribonia, Scepis, and Apatelodes, and nine and ten in arctians, while the European Nola centonalis moults nine times, other species of this genus shedding their skins six times. (Buckler.) (Psyche, v, pp. 420–422.) Callosamia promethea appears, as a rule, to moult but three times. Orgyia antiqua was found by Hellins to moult from three to five times. Riley found that in O. leucostigma the males moult four times, the female four, but sometimes five times, while Dyar states that in O. gulosa the male larvæ moult three or four times, the female always four times; in O. antiqua, however, there are six stages, and in the female seven. Lithocolletis, Chambers thinks, as a rule, moults eight times, and Comstock thinks that L. hamadryadella casts its skin seven or eight times.

In the blow-fly (Calliphora) Leuckart and Weismann have inferred at least two moults, while Weismann suspected that there are as many as four. In Musca domestica we have observed that the larva moults three times; in Œstridæ there are three larval stadia. (Brauer.) In Corethra there are four larval moults, and Miall thinks there are probably as many in Chironomus. Passing to the phytophagous Hymenoptera, there are three moults or four larval stages in Nematus erichsonii, but Dyar informs us that less than four stages in saw-fly larvæ is very rare, that he has only one record of less than five, and that that is doubtful; “five for nematid, six and seven for others, is certainly the rule. The highest I have is the indication of 11 stages for Harpiphorus varianus, but this again is an inference only, and attended with doubt.” (Can. Ent., xxvii, p. 208.) In Bombus we have observed five different sizes of larvæ, and hence suppose the least number of ecdyses is five, while we are disposed to believe that this insect, as well as wasps and bees, in general shed their skins as many as eight times during their entire existence.

The honey-bee, Cheshire thinks, since he has found the old and ruptured pellicles, probably moults six times before it spins its cocoon, or passes into the semipupa condition. (Bees and Bee-keeping, p. 20.)

As to the cause of the great number of moults in the arctians and in the beetles experimented with by Riley, it would seem that cold and the lack of food during hibernation were the agents in arctians, and starvation or the lack of food in the case of the beetles, such cause preventing growth, though the hypodermis-cells retained their activity.

Reproduction of lost limbs.—Here might be discussed the subject of the renovation or renewal of maimed or lost limbs, or the reparation of other injuries. As is well known, the cœlenterates, echinoderms, and worms under certain circumstances multiply by self-division, or if artificially mutilated, the parts are gradually restored by cell-proliferation or histogenesis. It is so with the antennæ and legs of crustaceans as well as the digits and tail of salamanders. The experiments first made by Le Pelletier[[100]] on spiders, and later by Heineken,[[101]] and others after him, on different spiders, as well as on Orthoptera and Hemiptera (Blatta, Reduvius, etc.), have proved that antennæ and legs and other external parts which have been injured or shortened, or entirely cut off in young individuals, are replaced at the next, or after successive moults, though generally in diminished size. This does not usually occur in adult life, and the process of reparation of lost parts is apparently due to the active growth of the cells of the parts affected during the process of moulting, when the histolysis of the maimed or diseased parts is succeeded by the rapid development of new cells, not only of the hypodermis, but also of the more specialized tissues within. And this tends to prove that such histolysis and making over of the muscles and other structures within occur especially in all metamorphic insects, and also in ametabolous forms, though the process has been most thoroughly examined in the Diptera, where these changes are more marked.

Gonin has found that the thoracic legs of the caterpillar correspond only to the tarsi of the imago (Fig. 608). It results, he says, from this fact that in accordance with the observations of Réaumur (which were wrongly interpreted by Newport and Künckel D’Herculais) that the amputation of the legs of the larva does not involve the entire leg, but only the extremity of the leg of the imago.

Formation of the cocoon.—While the larvæ of many insects, as those of the butterflies, suspend themselves before transforming, and spin no cocoon, or dig into the earth for protection and to secure an immunity from too great changes of temperature, a large proportion of the larvæ of metabolous insects which lead an inactive pupal life, line their earthen cells with silk, or spin a more or less elaborate case of silk, called the cocoon. We have seen that the inactive pupa of the male scale-insects is covered by the scale itself, or even in one case the insect forms a true cocoon of fibres of wax. The aquatic larvæ of the Neuroptera and Coleoptera creep out of the water, and by the movements of their bodies make a rude earthen cell in the bank, while that of Donacia spins a dense, leathery cocoon (Fig. 567) in the earth. The larvæ of the Embiidæ are protected by a cocoon, which they renew at each moult. Coniopteryx spins an orbicular cocoon, the Hemerobiidæ a spherical, dense, whitish one. The Trichoptera transform within their larval cases, which thus serve as cocoons, as do certain case-bearing Lepidoptera, notably the Psychidæ.

Fig. 567.—Cocoon (natural size) of Donacia proxima.

Fig. 568.—Cocoon and larva of Lucanus dama.

The pupa of certain leaf-eating beetles (Chrysomelidæ), as well as the Coccinellidæ, Dermestidæ, Hister, etc., are usually protected by the cast larval skin, which is retained, forming a rude shelter. While many beetles spin an oval cocoon (Gyrinus, Silphidæ), the wood-boring species make one of chips glued together, and that of Lucanus, which feeds on decayed wood, is lined with silk (Fig. 568). Anobium constructs a silken cocoon, interweaving the fine particles of its thin castings; the larvæ of weevils also usually spin silken cocoons.

Fig. 569.—Larva (a), puparium (b), and imago (c) of Sarcophaga, enlarged.

Fig. 570.—a, Erax bastardi; b, pupa.—After Riley.

The larval skin of the coarctate Diptera is retained as a protection for the soft-bodied pupa within, the old larval skin separating from the integument of the semipupa. To this cocoon-like covering of the coarctate pupa we have restricted the term puparium, originally used by Kirby and Spence to designate the pupa. The puparium is usually cylindrical or barrel-shaped, rounded at each end.

Fig. 571.—Puparium of Hypoderma bovis: a, side; b, ventral view, showing exit hole of adult; c, cap which splits off for exit of fly.—After Clark, from Osborn, Bull. 5, Div. Ent. U. S. Dept. Agr.

In the Diptera cyclorhapha, or common house and flesh flies, etc., the puparium remains in vital connection, by means of four tracheæ, with the enclosed pupa, which escapes from the case through a curved seam or lid at the anterior end and not by a slit in the back, as do the orthoraphous families, represented by the horse-fly (Tabanidæ, Asilidæ, Fig. 570), etc., where in some cases the obtected pupa remains within the loose envelope formed by the old larval skin, which Brauer calls a false puparium. The dry, hard puparium is burst open at the cephalic end when the fly emerges, by means of the frontal vesicle, which is distended with fluid (Fig. 571).

The exact mode of spinning the cocoon by caterpillars has been carefully observed by L. Trouvelot in the case of the polyphemus silkworm.

“When fully grown, the worm, which has been devouring the leaves so voraciously, becomes restless and crawls about the branches in search of a suitable place to build up its cocoon; before this it is motionless for some time, holding on to the twig with its front legs, while the two hind pair are detached; in this position it remains for some time, evacuating the contents of the alimentary canal until finally a gelatinous, transparent, very caustic fluid, looking like albumen, or the white of an egg, is ejected; this is a preparation for the long catalepsy that the worm is about to fall into. It now feels with its head in all directions, to discover any leaves to which to attach the fibres that are to give form to the cocoon. If it finds the place suitable, it begins to wind a layer of silk around a twig, then a fibre is attached to a leaf near by, and by many times doubling this fibre and making it shorter every time, the leaf is made to approach the twig at the distance necessary to build the cocoon; two or three leaves are disposed like this one, and then fibres are spread between them in all directions, and soon the ovoid form of the cocoon distinctly appears. This seems to be the most difficult feat for the worm to accomplish, as after this the work is simply mechanical, the cocoon being made of regular layers of silk united by a gummy substance. The silk is distributed in zigzag lines of about one-eighth of an inch long. When the cocoon is made, the worm will have moved his head to and fro, in order to distribute the silk, about 254,000 times.

“After about half a day’s work, the cocoon is so far completed that the worm can hardly be distinguished through the fine texture of the wall; then a gummy resinous substance, sometimes of a light-brown color, is spread all over the inside of the cocoon. The larva continues to work for four or five days, hardly taking a few minutes of rest, and finally another coating is spun in the interior, when the cocoon is all finished and completely air tight. The fibre diminishes in thickness as the completion of the cocoon advances, so that the last internal coating is not half so thick and so strong as the outside ones.” (Amer. Naturalist, i, p. 86.)

The mode of spinning the cocoon of an ichneumon (Microgaster) parasitic on Philampelus has been well described by John P. Marshall, as follows:—

Fig. 572.—Microgaster larvæ; spinning their cocoons: a, enlarged view of 5.—After Marshall.

The first appearance of the parasite is represented in Fig. 572, 1. A warty excrescence appears on the back of the caterpillar, which slowly emerges until it is seen to be a larva enclosed in a delicate transparent membrane, as represented in 2. This it soon succeeds in bursting, and, rising to its full length, balances itself a moment as in 3, then, bending double, it ejects from its mouth a glairy liquid, which instantly changes to silk, and fastens the posterior end to the skin of the caterpillar, as shown in 4, side view. It now begins to spin its cocoon by attaching a silken thread to the silky mass by which it had previously fastened itself to the caterpillar, and forming a series of loops of uniform size, first from right to left, and then back again from left to right, as represented in the front view, 5, and better in the enlarged view, 5a, the arrow heads showing the direction in which the head of the larva moved while forming the loops. The ends of the series, numbered 1, 2, 3, 4, are fastened to the edges of the ventral side of the body, which thus serves as a measure of the width of the cocoon, and also acts as a support for the frail fabric in the first stages of spinning. After the larva has fastened the fabric as far up on its ventral surface as it can, conveniently, it then begins to spin free, as shown in the side view, 6, where it is represented as just completing the first half of its cocoon, which resembles in form a slipper. This accomplished, the larva ceases to spin for the time being, bends its head, as in 7, towards its ventral surface, and pushes the half cocoon free from its body. The form of the silken fabric enables it to stand unsupported, while the larva, sliding its head down to the base, holds on firmly until it swings its posterior end into the toe of the slipper.

Figure 572, 8, shows it in the act of changing end for end, and in 9 the larva is seen erect, beginning at the base to complete the other half of its cocoon; 10 shows the larva contracting its body as it spins upward for about half the length of the cocoon, when it again changes end for end, as shown in 11, where it is beginning at the upper part to unite the two sides, finally enclosing itself as represented in 12.

It may now be seen, under the microscope, through the meshes of its cocoon actively engaged in lining the interior with layers of very fine silk ejected from its mouth in great abundance. One half of the cocoon is first lined by a forward and back movement of its head, and then reversing its position, it lines the other half in a similar manner.

In one case the larva was disengaged from the skin of the caterpillar, after beginning its cocoon. It, however, began again, and spun a portion while lying on the table. This was removed, when it began a third time, and completed its cocoon.

In about 10 days the insect made its appearance through a hole in the upper end, as represented in 13. The top was eaten off in a perfect circle and hung by a few threads, so as to resemble a lid as it was thrown back.

One caterpillar observed had between 300 and 400 cocoons on its back and sides, and another was dissected after more than 30 larvæ had escaped, and 130 were discovered in the soft integuments of the back.

The figures from 1 to 13 are magnified five diameters, but in order to observe the spinning of the cocoon a power of 50 is required. (Amer. Naturalist, xii, pp. 559, 560.)

Certain differences observed by W. A. Buckhout in a Microgaster parasitic on the different species of Macrosila, are referred to in the same volume, p. 752.

Fig. 573.—Body of larva of Lithocolletis. swollen and filled with cocoons of Copidosoma, enlarged.

While those chalcidid larvæ which feed internally on their host, as a rule, transform into naked, more or less coarctate pupæ, Howard states that the larvæ of Copidosoma, Bothriothorax, Homalotylus, and perhaps others, which are much crowded within their host, cause a marked inflation of the body of the latter (Figs. 573, 574). The nature of this cocoon-like cell, and how it is produced, is unknown. “Its structure shows it not to be silk, nor yet the last larval skin of the parasite, and whether it is an adventitious tissue of the host-larva or a secretion of the parasite, or is explicable upon other grounds, I cannot say.”

The silken cocoon of an aphidiid ichneumon has been found by Miss Murtfeldt, and also by Dr. Riley, under a rose aphid in which it had lived, and referred by Howard to the genus Praon (Fig. 575).

Sanitary conditions observed by the honey-bee larva, and admission of air within the cocoon.—Cheshire has observed that after the larva of the honey-bee has spun its cocoon or silken lining of its cell, it observes the following means of preserving cleanliness. The food given to the larva, especially during the latter part of the growing period, contains much pollen, the cases of the grains of which consist of cellulose, which is indigestible.

Fig. 574.—Coccinellid larva infested by Homalotylus obscurus, enlarged.

Fig. 575.—Cocoon of Praon under the body of a dead Aphis, enlarged.—This and Figs. 573 and 574 after Howard, from Insect Life.

Fig. 576.—Pupation of Proctotrupes in the body of a larva of a beetle, representing a case mentioned by Dr. Sharp, where the parasites have pupated on the outside of the host, a pair of each attached to nearly each segment of the body of their host.—After Sharp.

“These cases, with other refuse matters, collect in quantity within the bowel, which becomes distended, since it has no opening. The imprisoned larva, having little more than enough room for turning, must be freed of these objectionable residua.... In a word, the larva turns its head upon its stomach, and pushes the former towards the base of the cell until its position is reversed, the tail being outwards; and, thus placed, it laps up all residue of food, especially from its old clothes previously referred to, until they are dried, and practically occupy no space. It now throws up its stomach and bowel, with all their contents, and without detaching them from its outer skin, which is moulted as before, but in this instance to be pressed against the cell, so as to form for it an interior lining. The dejectamenta of the bowel in this way lie between the cast skin and cell-wall (as seen at e, Fig. 577), and so the larva remains absolutely unsoiled. It now turns its head and resumes its old position, joining its cocoon to the edges of its last cast skin, so that its habitation is relined, it is cleansed, and air can still pass to it through the imperceptible openings left by the bees in the sealing. This point is of radical importance, since breathing is carried on pretty rapidly during the latter part of its subsequent transformations, the absorbed oxygen permitting then of a production of heat, and causing also considerable diminution in weight.”

Fig. 577.—Larva and pupa of honey-bee in their cell: SL, spinning-larva; N, pupa; FL, young feeding larva; co, cocoon; sp, spiracles; t, tongue; m, mandible; an, antenna; w, wing; ce, compound eye; e, excrement; ex, exuvium.—After Cheshire.

As to the passage of air into the bee’s cocoon, Cheshire states that before the cocoon can be built, a cover, technically called sealing, is put over the larva by its nurses. These covers are made of pollen and wax, and are pervious to the air. They are more convex and regular in form than those sealing in the honey.[[102]]