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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.