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Corethra.

Pieris.

Musca.

Extension of the wing; drawing out of the tracheoles.

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.