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.