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.