It should be added that Malpighi, Swammerdam, and also Réaumur had detected the rudiments of the wings in the caterpillar just before pupation under the old larval skin. Lyonet (1760) also describes and figures the four wing-germs situated in the second and third thoracic segments, but was uncertain as to their nature. Each of these masses, he says, is “situated in the fatty body without being united to it, and is attached to the skin in a deep fold which it makes there.” He could throw no certain light on their nature, but says: “their number and situation leads to the supposition that they may be the rudiments of the wings of the moth” (pp. 449, 450). 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.) [Illustration: 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. ] Afterwards (1866) Weismann studied the development of the wings in _Corethra plumicornis_, which is a much more primitive and generalized form than Musca, and in which the process of development of the wings is much simpler, and, as since discovered, more as in other holometabolous insects. He also examined those of Simulium (Fig. 141). In Corethra, after the fourth and last larval moulting, there arises at first by evagination and afterwards by invagination a cup-shaped depression on each side in the upper part of the mesothoracic segment within which the rudiment of the wings lies like a plug. The wings without other change simply increase in size until, in the transformation into the pupa by the withdrawal of the hypodermis, the wings project out and become filled with blood, the tracheæ now being wholly wanting, and other tissues being sparingly present. [Illustration: