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During the transformation into the pupa the imaginal buds unite and grow out or extend along their edges, while the enveloping membrane disappears. The rudimentary wings are now like little sacs, and soon show a fusion of the two wing-membranes or laminæ with the veins, while the tracheæ disappear, the places occupied by the tracheæ becoming the veins. “Very early, as soon as the scales are indicated, begin in a very peculiar way the fusion of the wing-laminæ. There occur openings in the hypodermis into which the cells extend longitudinally and then laterally give way to each other. Hence no complete opening is found, but the epithelium appears by sections through a straight line sharply bordered along the wingcavity. It is a continuous membrane formed of plasma which I will call the ground membrane of the epithelium. Through this ground membrane pass blood-corpuscles as well as blood-lymph.” (Schaeffer.)

Fig. 141.—Anterior part of young larva of Simulium sericea, showing the thoracic imaginal buds: p, prothoracic bud (only one not embryonic); w, w′, fore and hind wing-buds; l, l′, l″, leg-buds; n, nervous system; br, brain; e, eye; sd, salivary duct; p, prothoracic foot.—After Weismann.