Читать книгу A Text-book of Entomology - A. S. Packard - Страница 47

Fig. 140.—Imaginal buds in Musca,—A, in Corethra,—B, in Melophagus,—C, in embryo of Melophagus; dorsal view of the head; b, bud; p, peripodal membrane; c, cord; hy, hypodermis; cl, cuticula; st, stomodæum; v, ventral cephalic, behind are the two dorsal cephalic buds.—After Pratt.

As first observed by Weismann, the buds are, like those of the appendages, simply attached to tracheæ and sometimes to nerves, in the former case appearing as minute folds or swellings of the peritoneal membrane of certain of the tracheæ. In Volucella the imaginal buds were, however, found by Künckel d’Herculais to be in union with the hypodermis. Dewitz detected a delicate thread-like stalk connecting the peripodal membrane with the hypodermis, and Van Rees has since proved in Musca, and Pratt in Melophagus, the connection of the imaginal buds with the hypodermis (Fig. 140). These tracheal enlargements increase in size, and become differentiated into a solid mass which corresponds to the upper part of the mesothorax, while a tongue-shaped continuation becomes the rudiment of the wing. During larval life the rudiments of the wings crumple, thus forming a cavity. While the larva is transforming into the pupa, the sheath or peripodal membranes of the rudimentary wings are drawn back, the blood presses in, and thus the wings are everted out of the peripodal cavities.

Due credit, however, should be given to Herold, as the pioneer in these studies, who first described in his excellent work on the development of Pieris brassicæ (1815) the wing-germs in the caterpillar after the third moult. This discovery has been overlooked by recent writers, with the exception of Gonin, whose statement of Herold’s views we have verified. Herold states that the germs of the wings appear on the inside of the second and third thoracic segments, and are recognized by their attachment to the “protoplasmic network” (schleimnetz), which we take to be the hypodermis, the net-like appearance of this structure being due to the cell-walls of the elements of the hypodermal membrane. These germs are, says Herold, also distinguished from the flakes of the fat-body by their regular symmetrical form. Fine tracheæ are attached to the wing-germs, in the same way as to the flakes of the fat-body. It thus appears that Herold in a vague way attributes the origin of these wing-germs, and also the germs of the leg, to the hypodermis, since his schleimnetz is the membrane which builds up the new skin. Herold also studied the later development of the wings, and discovered the mode of origin of the veins, and in a vague way traced the origin of the scales and hairs of the body, as well as that of the colors of the butterfly.

Herold also says that as the caterpillar grows larger, and also the wing-germs, “the larval skin in the region under which they lie hidden is spotted and swollen,” and he adds in a footnote: “This is the case with all smooth caterpillars marked with bright colors. In dark and hairy caterpillars the swelling of the skin through the growth of the underlying wing-germs is less distinct or not visible at all” (pp. 29, 30).

It should be added that Malpighi, Swammerdam, and also Réaumur had detected the rudiments of the wings in the caterpillar just before pupation under the old larval skin. Lyonet (1760) also describes and figures the four wing-germs situated in the second and third thoracic segments, but was uncertain as to their nature. Each of these masses, he says, is “situated in the fatty body without being united to it, and is attached to the skin in a deep fold which it makes there.” He could throw no certain light on their nature, but says: “their number and situation leads to the supposition that they may be the rudiments of the wings of the moth” (pp. 449, 450).

During the transformation into the pupa the imaginal buds unite and grow out or extend along their edges, while the enveloping membrane disappears. The rudimentary wings are now like little sacs, and soon show a fusion of the two wing-membranes or laminæ with the veins, while the tracheæ disappear, the places occupied by the tracheæ becoming the veins. “Very early, as soon as the scales are indicated, begin in a very peculiar way the fusion of the wing-laminæ. There occur openings in the hypodermis into which the cells extend longitudinally and then laterally give way to each other. Hence no complete opening is found, but the epithelium appears by sections through a straight line sharply bordered along the wingcavity. It is a continuous membrane formed of plasma which I will call the ground membrane of the epithelium. Through this ground membrane pass blood-corpuscles as well as blood-lymph.” (Schaeffer.)

Fig. 141.—Anterior part of young larva of Simulium sericea, showing the thoracic imaginal buds: p, prothoracic bud (only one not embryonic); w, w′, fore and hind wing-buds; l, l′, l″, leg-buds; n, nervous system; br, brain; e, eye; sd, salivary duct; p, prothoracic foot.—After Weismann.

Afterwards (1866) Weismann studied the development of the wings in Corethra plumicornis, which is a much more primitive and generalized form than Musca, and in which the process of development of the wings is much simpler, and, as since discovered, more as in other holometabolous insects. He also examined those of Simulium (Fig. 141).

In Corethra, after the fourth and last larval moulting, there arises at first by evagination and afterwards by invagination a cup-shaped depression on each side in the upper part of the mesothoracic segment within which the rudiment of the wings lies like a plug. The wings without other change simply increase in size until, in the transformation into the pupa by the withdrawal of the hypodermis, the wings project out and become filled with blood, the tracheæ now being wholly wanting, and other tissues being sparingly present.

Fig. 142.—Section through thorax of a Tineid larva on sycamore, passing through the 1st pair of wings (w): ht, heart; i, œsophagus; s, salivary gland: ut, urinary tube; nc, nervous cord; m, recti muscles; a part of the fat body overlies the heart. A, right wing-germ enlarged.

These observations on two widely separate groups of Diptera were confirmed by Landois, and afterwards by Pancritius, for the Lepidoptera, by Ganin for the Hymenoptera, by Dewitz for Hymenoptera (ants) and Trichoptera; also for the Neuroptera by Pancritius. In the ant-lion (Myrmeleon formicarius) Pancritius found no rudiments of the wings in larvæ a year old, but they were detected in the second year of larval life, and do not differ much histologically or in shape from those of Lepidoptera. In the Coleoptera and Hymenoptera the imaginal buds appear rather late in larval life, yet their structure is like that of Lepidoptera. In Cimbex the rudiments of the wings are not found in the young larva, but are seen in the semipupa, which stage lasts over six weeks.

Fig. 143.—Section of the same specimen as in Fig. 142, but cut through the second pair of wings (w): i, mid-intestine; h, heart; fb, fat-body; l, leg; n, nervous cord.

The general relation of the rudiments (imaginal buds) of the wings of a tineid moth to the rest of the body near the end of larval life may be seen in Figs. 142, 143 (Tinea?), the sections not, however, showing their connection with the hypodermis, which has been torn away during the process of cutting. That the wing is but a fold of the hypodermis is well seen in Fig. 144, of Datana, which represents a much later stage of development than in Figs. 142 and 143, the larva just entering on the semipupa stage.

In caterpillars of stage I, 3 to 4 mm. in length, Gonin found the wing-germs as in Fig. 145, A being a thickening of the hypodermis, with the embryonic cells, i.e. of Verson, on the convex border. The two leaves, or sides of the wing, begin to differentiate in stage II (C, D), and in stage III the envelope is formed (E), while the tracheæ begin to proliferate, and the capillary tracheæ or tracheoles at this time arise (Fig. 145, tc). The wall of the principal trachea appears to be resolved into filaments, and all the secondary branches assume the appearance of bundles of twine. Landois regarded them as the product of a transformation of the nuclei, but Gonin thinks they arise from the entire cells, stating that from each cell arises a ball (peloton) of small twisted tubes.

Fig. 144.—Section through mesothoracic segment of Datana ministra, passing through the wings (w): c, cuticula; hyp, hypodermis: ap, apodeme; dm, dorsal longitudinal.—vm, ventral longitudinal. muscles; dmt, depressor muscle of tergum; t, trachea; n, nerve cords; i, intestine; u, urinary tubes; l, insertion of legs.

As the large branches penetrate into the wing, the balls (pelotons) of fine tracheal threads tend to unroll, and each of the new ramifications of the secondary tracheal system is accompanied in its course by a bundle of capillary tubes. This secondary system of wing-tracheæ, then, arises from the mother trachea at the end of the third stage, when we find already formed the chitinous tunic, which will persist through the fourth stage up to pupation. It differs from the tracheoles in not communicating with the air-passage; it possesses no spiral membrane at the origin, and takes no part in respiration.

Gonin thus sums up the nature of the two tracheal systems in the rudimentary wing, which he calls the provisional and permanent systems. “The first, appearing in the second stage of the larva, comprises all the capillary tubes, and arising from numerous branches passes off from the lateral trunk of the thorax before reaching the wing; the second is formed a little later by the direct ramification of the principal branch.

“These two systems are absolutely independent of each other within the wing. Their existence is simultaneous but not conjoint. One is functionally active after the third moult; the other waits the final transformation before becoming active.”

Fig. 145.—A, section of wing-bud of larva of Pieris brassicæ of stage I, in front of the invagination pit. B, section passing through the invagination pit. C, section of same in stage II, through the invagination pit;—D, behind it, making the bud appear independent of the thoracic wall. E, wing-bud at the beginning of the 3d larval stage, section passing almost through the pedicel or hypodermic insertion, the traces of which appear at hi; h, hypodermis; t or tr, trachea; i, opening of invagination; ec, embryonic cells; l, external layer or envelope; in, internal wall of the wing; ex, external wall; s, cell of a tactile hair; tc, capillary tubes; c, cavity of invagination.—After Gonin.

Evagination of the wing outside of the body.—We have seen that the alary germs arise as invaginations of the hypodermis; we will now, with the aid of Gonin’s account, briefly describe, so far as is known, the mode of evagination of the wings. During the fourth and last stage of the caterpillar of Pieris, the wings grow very rapidly, and undergo important changes.

Six or seven days after the last larval moult the chitinous wall is formed, the wing remaining transparent. It grows rapidly and its lower edge extends near the legs. It is now much crumpled on the edge, owing to its rapid growth within the limits of its own segment. Partly from being somewhat retracted, and partly owing to the irregularity of its surface, the wing gradually separates from its envelope, and the cavity of invagination (Fig. 145, c) becomes more like a distinct or real space. The outer opening of the alary sac enlarges quite plainly, though without reaching the level of the edge of the wing.

This condition of things does not still exactly explain how the wing passes to the outside of the body. Gonin compares these conditions to those exhibited by a series of sections of the larva, made forty-eight hours later, on a caterpillar which had just spun its girdle of silk. At this time the wings have become entirely external, but, says Gonin, we do not see the why or the how. The partition of the sac has disappeared, and with it the cavity and the leaf of the envelope.

It appears probable that the partition has been destroyed, because the space between the two teguments is strewn with numerous bits, many of which adhere to the chitinous integument, while others are scattered along the edges of the wings, in their folds, or between the wings and the wall of the thorax.

Another series of sections showed that the exit of the fore wings had been accomplished, while the hinder pair was undergoing the process of eversion. In this case the partition showed signs of degeneration: deformation of the nuclei, indistinct cellular limits, pigmentation, granular leucocytes, and fatty globules.

After the destruction of the partition, what remains of the layer of the envelope is destined to make a part of the thoracic wall and undergoes for this purpose a superficial desquamation. The layer of flattened cells is removed and replaced by a firmer epithelium like that covering the other regions. It is this renewed hypodermis which conceals the wing within, serves to separate it from the cavity of the body, and gives the illusion of a complete change in its situation. Other changes occur, all forming a complete regeneration, but which does not accord with the description of Van Rees for the Muscidæ. Finally, Gonin concludes that the débris scattered about the wing comes from the two layers of the partition of the sac, from the flattened hypodermis of the renewed envelope, from the chitinous cuticle of the wing, and from the inner surface of the chitinous integument.

He thinks that the metamorphosis of Pieris is intermediate between the two types of Corethra and of Musca, established by Weismann, as follows:

Corethra.—The wing is formed in a simple depression of the hypodermic wall. No destruction.

Pieris.—The rudiment is concealed in a sac attached to the hypodermis by a short pedicel. Destruction of the partition and its replacement by a part of the thoracic wall by means of the imaginal epithelium.

Musca.—The pedicel is represented by a cord of variable length, whose cavity may be obliterated (Van Rees). The imaginal hypodermis is substituted for the larval hypodermis, which has completely disappeared, either by desquamation (Viallanes), or by histolytic resorption (Van Rees).

Extension of the wing; drawing out of the tracheoles.—When it is disengaged from the cavity, the wing greatly elongates and the creases on its surface are smoothed out; the blood penetrates between the two walls, and the cellular fibres, before relaxed and sinuous, are now firmly extended.

Of the two tracheal systems, the large branches are sinuous, and they are rendered more distinct by the presence of a spiral membrane; but the two tunics are not separated as in the other tracheæ of the thorax; moreover, the mouth choked up with débris does not yet communicate with that of the principal trunk. The bundles of tracheoles on their part form straight lines, as if the folds of the organ had had no influence on them. As they have remained bound together, apart from the chitinous membrane of the tracheal trunk, they become drawn out with this membrane, at the time of exuviation, i.e. of pupation, and are drawn out of the neighboring spiracle.

Fig. 146.—Full-grown larva of Pieris brassicæ, opened along the dorsal line: d, digestive canal; s, silk-gland; g, brain; st I, prothoracic stigma; st IV, 1st abdominal stigma; a, a′, germs (buds) of fore and hind wings; p, bud of prothoracic segment;—those of the third pair are concealed under the silk-glands; I–III, thoracic rings.—After Gonin.

“This is a very curious phenomenon, which can be verified experimentally: if we cut off the wing, while sparing the larval integument around the thoracic spiracles, we preserve the two tracheal systems; the same operation performed after complete removal of the larval skin does not give the secondary tracheal system.” (Gonin.) Deceived by the appearance of the tracheoles while still undeveloped, Landois and Pancritius, who have not mentioned the drawing out of the capillaries of the larva, affirm that they are destroyed by resorption in the chrysalis.

“The study of the tracheæ is closely connected with that of the veins (nervures). It is well to guard against the error of Verson, who mistakes for these last the large tracheal branches of the wing. This confusion is easily explained; it proves that Verson had, with us, recognized that the secondary system is, in the larva, exempt from all respiratory function. Landois thought that the pupal period was the time of formation of the veins. It seems to me probable that they are derived from the sheath of the peritracheal spaces.” (Gonin, pp. 30–33.)

Fig. 147.—Left anterior wing of a larva 3 days before pupation. The posterior part is rolled up: st, prothoracic stigma; tr. i., internal tracheal trunk; tr. e., tr. e.′, external tracheal trunk; p, cavity of a thoracic leg, with the imaginal bud b.—After Gonin.

The appearance of the wing-germs in the fully grown caterpillar, as revealed by simple dissection, is shown at Fig. 146; Fig. 147 represents a wing of a larva three days before pupation, with the germ of a thoracic leg.

Fig. 148.—Graber’s diagrams for explaining the origin and primary invagination of the hypodermis to form the germs of the leg (b), and wings (f, A-C), and afterwards their evagination D, so that they lie on the outside of the body. E, stage B, showing the hypodermal cavities (f) and stalks connecting the germs with the hypodermis (z).—After Graber.

Fig. 149.—Section lengthwise through the left wing of mature larva in Pieris rapæ: t, trachea; hyp, hypodermis; c, cuticula.—After Mayer.

A. G. Mayer has examined the late development of the wings in Pieris rapæ. Fig. 149 represents a frontal section through the left wing of a mature larva and shows the rudiment of the wing, lying in its hypodermal pocket or peripodal cavity. How the trachea passes into the rudimentary wing, and eventually becomes divided into the branches, around which the main veins afterwards form, is seen in Figs. 144, 147, 159.

The histological condition of the wing at this time is represented by Fig. 151, the spindle-like hypodermal cells forming the two walls being separated by the ground-membrane of Semper.

“While in the pupa state,” says Mayer, “the wing-membrane is thrown into a very regular series of closely compressed folds, a single scale being inserted upon the crest of each fold. When the butterfly issues from the chrysalis, these folds in the pupal wings flatten out, and it is this flattening which causes the expansion of the wings.... It is evident that the wings after emergence undergo a great stretching and flattening. The mechanics of the operation appears to be as follows. The hæmolymph, or blood, within the wings is under considerable pressure, and this pressure would naturally tend to enlarge the freshly emerged wing into a balloon-shaped bag; but the hypodermal fibres (h) hold the upper and lower walls of the wing-membrane closely together, and so, instead of becoming a swollen bag, the wing becomes a thin flat one. And thus it is that the little thick corrugated sac-like wings of the freshly emerged insect become the large, thin, flat wings of the imago.... The area of the wing of the imago of Danais plexippus is 8.6 times that of the pupa. Now, as the wing of the young pupa has about 60 times the area of the wing in the mature larva, it is evident that in passing from the larval state to maturity the area of the wings increases more than 500 times.”

Fig. 150.—Diagrammatic reproduction of Fig. 149 showing the wing-germ in its peripodal cavity (p): h’drm, hypodermis; tr, trachea; cta, cuticula; a, anterior end.—After Mayer.

Fig. 151.—Section of the wing-germ, the upper and lower sides connected by spindle-like hypodermic cells (h), forming the rods of the adult wing; mbr, ground-membrane of Semper.—After Mayer.