LITERATURE OF WALKING ON SMOOTH SURFACES
Blackwell, J. Remarks on the pulvilli of insects. (Trans. Linn. Soc. London, xvi, 1831, pp. 487–492, 767–770.)
Lowne, B. T. On the so-called suckers of Dytiscus and the pulvilli of insects. (Trans. Roy. Micr. Soc., pp. 267–271, 1871, 1 Pl.)
West, Tuffen. On certain appendages to the feet of insects subservient to holding or climbing. (Journ. of the Proceed. Linn. Soc. London, Zoölogy, vi, 1862, pp. 26–88.)
Dewitz, H. Ueber die Fortbewegung der Tiere an senkrechten, glatten Flächen vermittelst eines Sekrets. (Pflüger’s Archiv f. d. ges. Physiologie, xxxiii, 1884, pp. 440–481. 3 Taf. Also Zool. Anzeiger, 1884, pp. 400–405.)
—— Wie ist es den Stubenfliegen und anderen Insekten möglich, an senkrechten Glaswanden emporzulaufen. (Sitzungsb. Ges. naturf. Freunde zu Berlin, 1882, pp. 5–7.)
—— Weitere Mitteilungen über den Klettern der Insekten (Ibid., 1882, pp. 109–113).
—— Die Befestigung durch einen klebenden schleim beim springen gegen senkrechte Flächen. (Zool. Anzeiger, 1883, pp. 273, 274.)
—— Ueber die Wirkung der Haftlappchen toter Fliegen. (Ent. Nachr., x Jahrg., 1884, pp. 286, 287.)
—— Weitere Mitteilungen über das Klettern der Insekten an glatten senkrechten Flächen. (Zoolog. Anzeiger, 1885. viii Jahrg., pp. 157–159.)
—— Richtigstellung der behauptungen des Herrn F. Dahl. (Archiv f. mikroskop. Anat., 1885, xxvi, pp. 125–128.)
Rombouts, J. E. Ueber die Fortbewegung der Fliegen an glatten Flächen. (Zool. Anzeiger, 1884, pp. 619–623.)
—— De la faculté qu’out les mouches de se mouvoir sur le verre et sur les autres corps polis. (Archiv Museum Teyler (2), 4 Part, pp. 16. Fig.)
Simmermacher, G. Untersuchungen über Haftapparate an Tarsalgliedern von Insekten. (Zeitschr. f. wissensch. Zool. xl, 1884, pp. 481–556. 3 Taf., 2 Figs. Also Zoolog. Anzeiger, vii Jahrg., 1884, pp. 225–228.)
—— Antwort an Herrn Dr. H. Dewitz. (Ibid., pp. 513–517.)
Dahl, F. Die Fussdrüsen der Insekten. (Archiv f. mikroskop. Anat., 1885, xxv, pp. 236–263. 2 Taf. See also p. 118.)
Emery, C. Fortbewegung von Tieren an senkrechten und überhangenden glatten Flächen. (Biolog. Centralbl., 1884, 4 Bd., pp. 438–443.)
Léon, N. Disposition anatomique des organes de succion chez les Hydrocores et les Géocores. (Bull. Soc. des Medec. et Natur, de Jassy., 1888.)
d. The wings and their structure
The insects differ from all other animals except birds in possessing wings, and as we at the outset have claimed, it is evidently owing to them that insects are numerically so superior to any other class of animals, since their power of flight enables them to live in the air out of reach of many of their enemies, the greatest destruction to insect life occurring in the wingless larval and pupal stages.
The presence of wings has exerted a profound influence on the shape and structure of the body, and it is apparently due to their existence that the body is so distinctly triregional, since this feature is least marked in the synapterous insects. The wings are thin, broad leaf-like folds of the integument, attached to the thorax and moved by powerful muscles which occupy the greater part of the thoracic cavity. The two pairs of wings are outgrowths of the middle and hinder part of the thorax, the anterior pair being attached to the mesothoracic and the hinder pair to the metathoracic segment. The larger pair is developed from the middle segment of the thorax. The differentiation of the tergites into scutum, scutellum, etc., is the result of the appearance of wings, because these sclerites are more or less reduced or effaced in wingless insects, such as apterous Orthoptera and moths, ants, etc.
The size of the hinder thoracic segments is closely related to that of the wings they bear. In those Orthoptera which have hind wings larger than those of the fore pair, the metathorax is larger than the mesothorax. In such Neuroptera as have the hind wings nearly or quite as large as the anterior pair, or in the Trichoptera and in the Hepialidæ, the metathorax is nearly as large as the mesothorax, while in Coleoptera the metathorax is as large and often much larger. In the Ephemeridæ, Diptera, and Hymenoptera, which have either only rudimentary (halteres) or small hind wings, the metathorax is correspondingly reduced in size.
The wings morphologically, as their development shows, are simple sac-like outgrowths of the integument, i.e. of the free hinder edge of the tergal plates, their place of origin being apparently above the upper edge of the epimera or pleural sclerites. Calvert[[24]] however, regards the upper lamina of the wing as tergal, and the lower, pleural.
The wings in most insects are attached to the thorax by a membrane containing several little plates of chitin called by Audouin articulatory epidemes.
The wings, then, are simple, very thin chitinous lamellate expansions of the integument, which are supported and strengthened by an internal framework of hollow chitinous tubes.
The veins.—The so-called “veins” or “nervures,” which are situated between the upper and under layers of the wing are so disposed as to give the greatest lightness and strength to the wings. Hagen has shown that in the freshly formed wings these two layers can be separated, when it can be seen that the veins pass through each layer.
These veins are in reality quite complex, consisting of a minute central trachea enclosed within a larger tube which at the instant the insect emerges from the nymph, or pupa, as the case may be, is filled with blood (Fig. 136). Since these tubes at first contain blood, which has been observed to circulate through them, and since the heart can be most easily injected through them, they may more properly be called veins than nervures. The shape and venation of the wings afford excellent ordinal as well as family and generic characters, while they also enable the systematist to exactly locate the spots and other markings of the wings. The spaces enclosed by the veins and their cross-branches are called cells, and their shape often affords valuable generic and specific characters.
Fig. 136.—Cross-section of wing of Pronuba.—After Spuler.
Fig. 137.—Cross-section of wing of Pieris: s, insertions of scales.-After Spuler.
The structure of a complete vein is described by Spuler. In a cross-section of a noctuid moth (Triphæna pronuba, Fig. 136) the chitinous walls are seen to consist of two layers, an outer (U) and inner (c), the latter of which takes a stain and lies next to the hypodermis (hy). In the cavity of the vein is the trachea (tr), which shows more or less distinctly the so-called spiral thread; within the cavity are also Semper’s “rib” (r) and blood-corpuscles (bc), which proves that the blood circulates in the veins of the completely formed wing, though this does not apply to all Lepidoptera with hard mature wings. We have been able to observe the same structure in sections of the wing of Zygæna.
A cross-section of a vein of Pieris brassicæ shows that the large trachea is first formed, and that it extends along the track between the protoplasmic threads connecting the two hypodermal layers.
The main tracheæ throw off on both sides a number of secondary branches showing at their end a cell with an intracellular tracheal structure; these accessory tracheæ afterwards branch out. The accessory or transverse tracheæ often disappear, though in some moths they remain permanently. Fig. 137 tr2 represents these secondary veins in the edge of the fore wing of Laverna vanella, arising from a main trachea (tr) passing through vein I (v), two of the twigs extending to the centre, showing that the latter has no homology with a true vein. Only rarely and in strongly developed thick folds are the transverse tracheæ provided with a chitinous thickening, as for example in Cossus ligniperda. Since from such accessory tracheæ the transverse veins in lepidopterous wings are developed, we can recognize in them the homologies of the net-veins in reticulated venations. There is no sharply defined difference between reticulated and non-reticulated venations; no genetic difference exists between the two kinds of venation, since there occur true Blattidæ both with and without a reticulated venation (Spuler).
In the fore wings of Odonata, Psocina, Mantispidæ, and most Hymenoptera is an usually opaque colored area between the costal edge and the median vein, called the pterostigma.
In shape the wings are either triangular or linear oval, and at the front edge the main veins are closer together than elsewhere, thus strengthening the wings and affording the greatest resistance to the air in making the downward stroke during flight. It is noticeable that when the veins are in part aborted from partial disuse of the wings, they disappear first from the hinder and middle edge, those on the costal region persisting. This is seen in the wings of Embiidæ (Oligotoma), Cynipidæ, Proctotrupidæ, Chalcids, ants, etc.
The front edge of the wing is called the costal, its termination in the outer angle of the wing is called the apex; the outer edge (termen) is situated between the apex and the inner or anal angle, between which and the base of the wing is the inner or internal edge.
While in Orthoptera, dragon-flies, Termitidæ, and Neuroptera the wings are not attached to each other, in many Lepidoptera they are loosely connected by the loop and frenulum, or in Hymenoptera by a series of strong hooks. These hooks are arranged, says Newport, “in a slightly twisted or spiral direction along the margin of the wing, so as to resemble a screw, and when the wings are expanded attach themselves to a little fold on the posterior margin of the anterior wing, along which they play very freely when the wings are in motion, slipping to and fro like the rings on the rod of a window curtain.”
At the base of the hind wings of Trichoptera and in the lepidopterous Micropteryx there is an angular fold (jugum) at the base of each wing (Fig. 138); that of the anterior wings is retained in Eriocephala and Hepialidæ.
Fig. 138.—Venation of fore and hind wings of Micropteryx purpurella: j, jugum, on each wing; d, discal vein; the Roman numerals indicate veins I.-VIII. and their branches.
In the wings of Orthoptera as well as other insects, the fore wings, especially, are divided into three well-marked areas, the costal, median, and internal; of these the median area is the largest, and in grasshoppers and crickets is more or less modified to form the musical apparatus, consisting of the drum-like resonant area, with the file or bow.
The squamæ.—In the calyptrate Muscidæ, a large scale-like membranous broad orbicular whitish process is situated beneath the base of the wing, above the halter; (Fig. 94, 10 sq.) it is either small or wanting in the acalyptrate muscids. Kirby and Spence state that when the insect is at rest the two divisions of this double lobe are folded over each other, but are extended during flight. Their exact use is unknown. Kolbe, following other German authors, considers the term squama as applicable to the whole structure, restricting the term alula to the other lobe-like division.
More recently (1890 and 1897) Osten-Sacken recommends “squamæ; in the plural, as a designation for both of these organs taken together; squama, in the singular, would mean the posterior squama alone, and antisquama the anterior squama alone;” the strip of membrane running in some cases between them, or connecting the squama with the scutellum, should be called the post-alar membrane. By a mistake Loew, and others following him, used the word tegula for squama, but this term should be restricted to the sclerite of the mesothorax previously so designated (Fig. 90, A, t). The squama or its two subdivisions has also by various authors been termed alula, calypta, squamula, lobulus, axillary lobe, aileron, cuilleron, schuppen, and scale. (Berlin Ent. Zeitschrift, xli, 1896, pp. 285–288, 328, 338.)
The halteres.—In the Diptera the hind wings are modified to form the halteres or balancers, which are present in all the species, even in Nycteribia, but are absent in Braula.
Meinert finds structures in the Lepidoptera which he considers as the homologues of the halteres of Diptera. “In the Noctuidæ,” he remarks, “I find arising from the fourth thoracic segment (segment médiaire), but covered by hair, an organ like the halter of Diptera.” (Ent. Tidskrift., i, 1880, p. 168.) He gives no details.
In the Stylopidæ, on the contrary, the fore wings are reduced to little narrow pads, while the hind wings are of great size.
The thyridium is a whitish spot marking a break in the cubital vein of the fore wing of Trichoptera; these minute thyridia occur in the fore wings of the saw-flies; there is also an intercostal thyridium on the costal part of the wings of Dermaptera.
The fore wings of Orthoptera are thicker than the hinder ones, and serve to protect the hind-body when the wings are folded; they are sometimes called tegmina. It is noteworthy, that, according to Scudder, in all the paleozoic cockroaches the fore wings (tegmina) were as distinctly veined as the hinder pair, “and could not in any sense be called coriaceous.” (Pretertiary Insects of N. A., p. 39.) Scudder also observes that in the paleozoic insects as a rule the fore and hind wings were similar in shape and venation, “heterogeneity making its appearance in mesozoic times.” In the heteropterous Hemiptera, also, the basal half of the fore wings is thick and coriaceous or parchment-like, and also protects the body when they are folded; these wings are called hemelytra. In the Dermaptera the small short fore wings are thickened and elytriform.
The elytra.—This thickening of the fore wings is carried out to its fullest extent in the fore wings of beetles, where they form the sheaths, shards, or elytra, under which the hind wings are folded. The indexed costal edge is called the epipleurum, being wide in the Tenebrionidæ. During flight “the elytra are opened so as to form an angle with the body and admit of the free play of the wings” (Kirby and Spence). In the running beetles (Carabidæ), also in the weevils and in many Ptinidae, the hind wings are wanting, through disuse, and often the elytra are firmly united, forming a single hard shell or case. The firmness of the elytra is due both to the thickness of the chitinous deposit and to the presence of minute chitinous rods or pillars connecting the upper and lower chitinous surfaces.
Fig. 139.—Longitudinal section through the edge of the elytrum of Lina ænea: gl, glands; r, reservoir; fb, fat-body; m, matrix; u, upper,—l, lower, lamella.—After Hoffbauer.
Hoffbauer finds that in the elytra of beetles of different families the venation characteristic of the hind wings is wanting, the main tracheæ being irregular or arranged in closely parallel longitudinal lines, and nerve-fibres pass along near them, sense-organs being also present. The fat-bodies in the cavity of the elytra, which is lined with a matrix layer, besides nerves, tracheæ, and blood, contain secretory vesicles filled with uric-acid concretions such as occur in the fat-body of Lampyris. There are also a great many glands varying much in structure and position, such occurring also in the pronotum (Fig. 139).
Meinert considers the elytra of Coleoptera to be the homologues of the tegulæ of Lepidoptera and of Hymenoptera. He also calls attention to the alula observed in Dyticus, situated at the base of the elytra, but which is totally covered by the latter. The alulæ of these beetles he regards as the homologues of the anterior wings of Hymenoptera and Diptera. No details are given in support of these views. (Ent. Tidskrift, i, 1880, p. 168.)
Hoffbauer (1892) also has suggested that the elytra are not the homologues of the fore wings of other insects, but of the tegulæ.
Kolbe describes the alula of Dyticus as a delicate, membranous lobe at the base of the elytra, but not visible when they are closed: its fringed edge in Dyticus is bordered by a thickening forming a tube which contains a fluid. The alula is united with the inner basal portion and articulation of the wing-cover, forming a continuation of them. Dufour considered that the humming noise made by these beetles is produced by the alulets.
Hoffbauer finds no structural resemblances in the alulæ of Dyticus to the elytra. He does not find “the least trace of veins.” They are more like appendages of the elytra. Lacordaire considered that their function is to prevent the disarticulation of the elytra, but Hoffbauer thinks that they serve as contrivances to retain the air which the beetle carries down with it under the surface, since he almost always found a bubble of air concealed under it; besides, their folded and fringed edge seems especially fitted for taking in and retaining air. Hoffbauer then describes the tegulæ of the hornet and finds them to be, not as Cholodkowsky states, hard, solid, chitinous plates, but hollow. They are inserted immediately over the base or insertion of the fore wings, being articulated by a hinge-joint, the upper lamella extending into a cavity of the side of the mesothorax, and connected by a hinge-like, articulating membrane with the lower projection of the bag or cavity. The lower lamella becomes thinner towards the place of insertion, is slightly folded, and merges without any articulation into the thin, thoracic wall at a point situated over the insertion of the fore wing. The tegulæ also differ from the wings in having no muscles to move them, the actual movements being of a passive nature, and due to the upward and downward strokes of the wings.
Comstock adopts Meinert’s view that the elytra are not true fore wings, but gives no reasons. (Manual, p. 495.)
Dr. Sharp,[[25]] however, after examining Dyticus and Cybister, affirms that this structure is only a part of the elytron, to which it is extensively attached, and that it corresponds with the angle at the base of the wing seen in so many insects that fold their front wings against the body. He does not think that the alula affords any support to the view that the elytra of beetles correspond with the tegulæ of Hymenoptera rather than with the fore wings.
That the elytra are modified paraptera (tegulæ) is negatived by the fact that the latter have no muscles, and that the elytra contain tracheæ whose irregular arrangement may be part of the modified degenerate structure of the elytra. Kolbe finds evidences of veins. The question may also be settled by an examination of the structure of the pupal wings. A study of a series of sections of both pairs of wings of the pupa of Doryphora and of a Clytus convinces us that the elytra are the homologues of the fore wings of other insects.
e. Development and mode of origin of the wings
Embryonic development of the wings.—The wings of insects are essentially simple dorsal outgrowths of the integument, being evaginations of the hypodermis. They begin to form in the embryo before hatching, first appearing as folds, buds, or evaginations, of the hypodermis, which lie in pouches, called peripodal cavities. They are not visible externally until rather late in larval life, after the insect, such as a grasshopper, has moulted twice or more times; while in holometabolous insects they are not seen externally until the pupa state is attained.
The subject of their origin is in a less satisfactory state than desirable from the fact that at the outset the development of the wings of the most generalized insects, such as Orthoptera, Termes, etc., was not first examined, that of the most highly modified of any insects, i.e. the Muscidæ, having actually been first studied.
In the course of his embryological studies on the Muscidæ (Musca comitoria and Sarcophaga carnaria) Weismann (1864) in examining the larvæ of these flies just before pupation, found that the wings, as well as the legs and mouth-appendages, developed from microscopic masses of indifferent cells, which he called “imaginal discs.” From the six imaginal discs or buds in the lower part of the thorax arise the legs, while from four dorsal discs, two in the meso- and two in the metathoracic segment, arise the fore and hind wings (Fig. 141.) These imaginal buds, as we prefer to call these germs, usually appear at the close of embryonic life, being found in freshly hatched larvæ.
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.
f. The primitive origin of the wings
Farther observations are needed to connect the mode of formation of the wings in the holometabolous insects with the more primitive mode of origin seen in the hemimetabolous orders, but the former mode is evidently inherited from the latter. Pancritius remarks that the development of the rudiments of the wing in a hypodermal cavity is in the holometabolic insects to be regarded as a later inherited character, the external conditions causing it being unknown.
Fritz Müller was the first to investigate the mode of development of the wings of the hemimetabolic insects, examining the young nymphs of Termites. He regards the wings as evaginations of the hypodermis, which externally appear as thoracic scale-like projections, into which enter rather late in nymphal life tracheæ which correspond to the veins which afterward arise.
Fig. 152.—Rudimentary wing of young nymph of Blatta, with the five principal veins developed.
The primitive mode of origin of the wings may, therefore, be best understood by observing the early stages of those insects, such as the Orthoptera and Hemiptera, which have an incomplete metamorphosis. If the student will examine the nymphs of any locust in their successive stages, he will see that the wings arise as simple expansions downward and backward of the lateral edges of the meso- and metanotum. In the second nymphal stage this change begins to take place, but it does not become marked until the succeeding stage, when the indications of veins begin to appear, and the lobe-like expansion of the notum is plainly enough a rudimentary wing.
Graber[[26]] thus describes the mode of development of the wings in the nymph of the cockroach:
“If one is looking only at the exterior of the process, he will perceive sooner or later on the sides of the meso- and metathorax pouch-like sacs, which increase in extent with the dorsal integument and at the same time are more and more separated from the body. These wing-covers either keep the same position as in the flat-bodied Blattidæ, or in insects with bodies more compressed the first rudiments hang down over the sides of the thorax. As soon as they have exceeded a certain length, these wing-covers are laid over on the back. However, if we study the process of development of the wings with a microscope, by means of sections made obliquely through the thorax, the process appears still more simple. The chief force of all evolution is and remains the power of growth in a definite direction. In regard to the skin this growth is possible in insects only in this way; namely, that the outer layer of cells is increased by the folds which are forced into the superficial chitinous skin. These folds naturally grow from one moult to another in proportion to the multiplication of the cells, and are not smoothed out until after the moulting, when the outer resistance is overcome.
Fig. 153.—Partial metamorphosis of Melanoplus femur-rubrum, showing the five nymph stages, and the gradual growth of the wings, which are first visible externally in 3, 3b, 3c.—Emerton del.
“As, however, the first wing-layers depend upon the wrinkling of the general integument of the body through the increase in the upper layer, the further growth of the wings depends in the later stages upon the wrinkling of the epidermis of the wing-membrane even, which fact we also observe under the microscope when the new wings drawn forth from the old covers appear at first to be quite creased together. These wing-like wrinkles in the skin are not empty pouches, but contain tissues and organs within, which are connected with the skin, as the fat of the body, the network of tracheæ, muscles, etc. Alongside the tracheæ, running through the former wing-pouches and accompanied by the nerves, there are canals through which the blood flows in and out.
Fig. 154.—Stages in the growth of the wings of the nymph of Termes flavipes: A, young; a, a wing enlarged. B, older nymph; b, fore wing; n, a vein. C, wings more advanced;—D, mature.
Fig. 155.—Wings of nymph of Psocus.
“After the last moult, however, when the supply of moisture is very much reduced in the wing-pouches, which are contracted at the bottom, their two layers become closely united, and afterward grow into one single, solid wing-membrane.
“These thick-walled blood-tubes arising above and beneath the upper and lower membrane of the wing are the veins of the wings; the development of the creased wings in the pupa of butterflies is exactly like that of cockroaches and bugs. The difference is only that the folds of integument furnishing the wings with an ample store of material for their construction reach in a relatively shorter time, that is the space of time between two moults, the same extent that they would otherwise attain only in the course of several periods of growth in the ametabolous insects.”
Fig. 156.—Nymph of Aphrophora permutata, with enlarged view of the wings and the veins: pro, pronotum; sc, mesoscutum; 1ab, 1st abdominal segment.
Ignorant of Graber’s paper, we had arrived at the same result, after an examination of the early nymph-stages of the cockroach, as well as the locusts, Termites, and various Hemiptera. In all these forms it is plainly to be seen that the wings are simply expansions, either horizontal or partly vertical (where, as in locusts, etc., the body is compressed, and the meso- and metanota are rounded downwards), of the hinder and outer edge of the meso- and metanotum. As will be seen by reference to the accompanying figures, the wings are notal (tergal) outgrowths from the dorsal arch of the two hinder segments of the thorax. At first, as seen in the young pupal cockroach (Fig. 152) and locust (Fig. 153, also Figs. 154 and 156) the rudiments of the wings are continuous with the notum. Late in nymphal life a suture and a hinge-joint appear at the base of the wing, and thus there is some movement of the wing upon the notum; finally, the tracheæ are well developed in the wings, and numerous small sclerites are differentiated at the base of the wing, to which the special muscles of flight are attached, and thus the wings, after the last nymphal moult, have the power of flapping, and of sustaining the insect in the air; they thus become true organs of flight.
It is to be observed, then, that the wings in all hemimetabolous insects are outgrowths from the notum, and not from the flanks or pleurum of the thorax. There is, then, no structure in any other part of the body with which they are homologous.
Fig. 157.—Development of wings of Trichoptera: A, portion of body-wall of young larva of Trichostegia; ch, cuticula, forming at r a projection into the hypodermis, m; r, and d, forming thus the first rudiment of the wing. B, the parts in a larva of nearly full size; a, c, d, b, the well-developed hypodermis of the wing-germ separated into two parts by r, the penetrating extension of the cuticula; v, mesoderm, C, wing-pad of another Phryganeid freed from its case at its change to the pupa: b, d, outer layer of the hypodermis (m) of the body-wall; v, inner layer within nuclei.—After Dewitz, from Sharp.
The same may be said of the true Neuroptera, Trichoptera (Fig. 157), the Coleoptera, and the Diptera, Lepidoptera, and Hymenoptera. As we have observed in the house fly,[[27]] the wings are evidently outgrowths of the meso- and metanotum; we have also observed this to be most probably the case in the Lepidoptera, from observations on a Tortrix in different stages of metamorphosis. It is also the case with the Hymenoptera, as we have observed in bees and wasps;[[28]] and in these forms, and probably all Hymenoptera, the wings are outgrowths of the scutal region of the notum.
With these facts before us we may speculate as to the probable origin of the wings of insects. The views held by some are those of Gegenbaur, also adopted by Lubbock, and originally by myself.[[29]] According to Gegenbaur:
“The wings must be regarded as homologous with the lamellar tracheal gills, for they do not only agree with them in origin, but also in their connection with the body, and in structure. In being limited to the second and third thoracic segments they point to a reduction in the number of the tracheal gills. It is quite clear that we must suppose that the wings did not arise as such, but were developed from organs which had another function, such as the tracheal gills; I mean to say that such a supposition is necessary, for we cannot imagine that the wings functioned as such in the lower stages of their development, and that they could have been developed by having such a function.”
Fig. 158.—Changes in external form of the young larva of Calotermes rugosus, showing, in A and B, the mode of origin of the wing-pads: A, newly hatched, with 9 antennal joints, × 8. B, older larva, with 10 joints, × 8. C, next stage, with 11 joints, × 8. D, larva, with twelve joints; the position of the parts of the alimentary canal are shown: v, crop; m, stomach; b, “paunch”; e, intestine; r, heart, × 16
3.—After Fritz Müller, from Sharp.
If we examine the tracheal gills of the smaller dragon-fly (Agrion), or the May-flies, or Sialidæ, or Perlidæ, or Phryganeidæ, we see that they are developed in a very arbitrary way, either at the end of the abdomen, or on the sternum, or from the pleurum; moreover, in structure they invariably have but a single trachea, from which minute twigs branch out;[[30]] in the wings there are five or six main tracheæ, which give rise to the veins. Thus, in themselves, irrespective of their position, they are not the homologues of the gills. The latter are only developed in the aquatic representatives of the Neuroptera and Pseudoneuroptera, and are evidently adaptive, secondary, temporary organs, and are in no sense ancestral, primitive structures from which the wings were developed. There is no good reason to suppose that the aquatic Odonata or Ephemerids or Neuroptera were not descendants of terrestrial forms.
To these results we had arrived by a review of the above-mentioned facts, before meeting with Fritz Müller’s opinions, derived from a study of the development of the wings of Calotermes (Fig. 158). Müller[[31]] states that “(1) The wings of insects have not originated from ‘tracheal gills.’ The wing-shaped continuations of the youngest larvæ are in fact the only parts in which air tubes are completely wanting, while tracheæ are richly developed in all other parts of the body.[[32]] (2) The wings of insects have arisen from lateral continuations of the dorsal plates of the body-segments with which they are connected.”
Now, speculating on the primary origin of wings, we need not suppose that they originated in any aquatic form, but in some ancestral land insect related to existing cockroaches and Termes. We may imagine that the tergites (or notum) of the two hinder segments of the thorax grew out laterally in some leaping and running insect; that the expansion became of use in aiding to support the body in its longer leaps, somewhat as the lateral expansions of the body aid the flying squirrel or certain lizards in supporting the body during their leaps. By natural selection these structures would be transmitted in an improved condition until they became flexible, i.e. attached by a rude hinge-joint to the tergal plates of the meso- and metathorax. Then by continued use and attempts at flight they would grow larger, until they would become permanent organs, though still rudimentary, as in many existing Orthoptera, such as certain Blattariæ and Pezotettix. By this time a fold or hinge having been established, small chitinous pieces enclosed in membrane would appear, until we should have a hinge flexible enough to allow the wing to be folded on the back, and also to have a flapping motion. A stray tracheal twig would naturally press or grow into the base of the new structure. After the trachea running towards the base of the wing had begun to send off branches into the rudimentary structure, the number and direction of the future veins would become determined on simple mechanical principles. The rudimentary structures beating the air would need to be strengthened on the front or costal edge. Here, then, would be developed the larger number of main veins, two or three close together, and parallel. These would be the costal, subcostal, and median veins. They would throw out branches to strengthen the costal edge, while the branches sent out to the outer and hinder edges of the wings might be less numerous and farther apart. The net-veined wings of Orthoptera and Pseudoneuroptera, as compared with the wings of Hymenoptera, show that the wings of net-veined insects were largely used for respiration as well as for flight, while in beetles and bees the leading function is flight, that of respiration being quite subordinate. The blood would then supply the parts, and thus respiration or aëration of the blood would be demanded. As soon as such expansions would be of even slight use to the insect as breathing organs, the question as to their permanency would be settled. Organs so useful both for flight and aëration of the blood would be still further developed, until they would become permanent structures, genuine wings. They would thus be readily transmitted, and being of more use in adult life during the season of reproduction, they would be still further developed, and thus those insects which could fly the best, i.e. which had the strongest wings, would be most successful in the struggle for existence. Thus also, not being so much needed in larval life before the reproductive organs are developed, they would not be transmitted except in a very rudimentary way, as perhaps masses of internal indifferent cells (imaginal discs), to the larva, being the rather destined to develop late in larval and in pupal life. Thus the development of the wings and of the generative organs would go hand in hand, and become organs of adult life.[[33]]
The development and structure of the tracheæ and veins of the wing.—The so-called veins (“nervures”) originate from fine tracheal twigs which pass into the imaginal discs. A single longitudinal trachea grows down into the wing-germ (Fig. 147), this branch arising through simple budding of the large body-trachea passing under the rudiment of the wing.
Fig. 159.—Germ of a hind wing detached from its insertion, and examined in glycerine: i, pedicel of insertion to the hypodermis; tr, trachea; b, semicircular pad; e, enveloping membrane; c, bundle of capillary tracheoles; the large tracheæ of the wing not visible; they follow the course of the bundles of tracheoles.—After Gonin.
Gonin states that before the tracheæ reach the wing they divide into a great number of capillary tubes united into bundles and often tangled. This mass of tracheæ does not penetrate into the wing-germ by one of its free ends, but spreading over about a third of the surface of the wing, separates into a dozen bundles which spread out fan-like in the interior of the wing. (Fig. 159). These ramifications, as seen under the microscope, are very irregular; they form here and there knots and anastomoses. They end abruptly in tufts at a little distance from the edge of the wing. A raised semicircular ridge (b) surrounds the base of the wing, and within this the capillaries are formed, while on the other side they are covered by a cellular layer.
Landois, he says, noticed neither the pedicel of the insertion of the wing (i) nor the ridge (b). Herold only states that the tracheæ pass like roots into the wing. Landois believed that they formed an integral part of it. Dewitz and Pancritius used sections to determine their situation.
Fig. 160 will illustrate Landois’ views as to the origin of the tracheæ and veins. A represents the germ of a hind wing attached to a trachea; c the elongated cells, in which, as seen at B, c, a fine tangled tracheal thread (t) appears, seen to be magnified at C. The cell walls break down, and the threads become those which pass through the centre of the veins.
Fig. 160.—Origin of the wings and their veins.—After Landois.
Fig. 161.-Section of the “rib” of a vein: c, cord; b, twig.—After Schaeffer.
The wing-rods.—Semper discovered in transverse sections of the wings, what he called Flügelrippen; one such rib accompanying the trachea in each vein. He did not discover its origin, and his description of it is said to be somewhat erroneous. Schaeffer has recently examined the structure, remarking: “I have surely observed the connection of this cellular tube with the tracheæ. It is found in the base of the wing where the lumen of the tracheæ is much widened. I only describe the fully formed rib (rippe). In a cross-section it forms a usually cylindrical tube which is covered by a very thin chitinous intima which bears delicate twigs (Fig. 161). These twigs are analogous to the thickened ridge of the tracheal intima. I can see no connection between the branches of the different twigs. Through the ribs (rippen) extend a central cord (c) which shows in longitudinal section a clear longitudinal streaking. Semper regarded it as a nerve. But the connection of the tube with the trachea contradicts this view. I can only regard the cord as a separation-product of the cells of the walls.”
Fig. 162.—Parts of a vein of the cockroach, showing the nerve (n) by the side of the trachea (tr); c, blood-corpuscles.—After Moseley.
Other histological elements.—These are the blood-lymph, corpuscles, blood-building masses, and nerves. Schaeffer states that in the immature pupal wings we find besides the large tracheæ, which are more or less branched, and in the wing-veins at a later period, blood-corpuscles which are more or less gorged with nutritive material, and also the “balls of granules” of Weismann, which are perhaps the “single fat-body cells” detected by Semper. Schaeffer also states that into the hypodermal fold of the rudiments of the wings pass peculiar formations of the fat-body and tracheal system, and connected with the fat-body are masses of small cells which by Schaeffer are regarded as blood-building masses.
Fine nerves have also been detected within the veins, Moseley stating that a nerve-fibre accompanies the trachea in all the larger veins in the insects he has examined (Fig. 162), while it is present in Melolontha, where the trachea is absent.