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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.