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.)
