March, 1905.] Powell: Wings of Certain Beetles. THE DEVELOPMENT OF WINGS OF CERTAIN BEETLES, AND SOME STUDIES OF THE ORIGIN OF THE WINGS OF INSECTS. By P. B. Powell. Stanford University, Calif. ( Continued from Vol. XII, page 24J. ) The downward growth of the wing continues until early in the prepupal period, when it reaches the base of the leg bud, which pre-vents further growth in this direction (Figs. 8, 21). The wing now exists as a double-walled pad lying between the hypoderm and cuti-cle, extending downward and backward, as far as the base of the leg and with the basement membranes of the two layers usually closely pressed together, except at the places where tracheae have entered the wing, forming vein cavities. The nuclei of the two walls still lie at several different planes and the outer (upper) wall is somewhat the thicker, while the bases of the cells are abruptly narrowed and thread-like. The later stages in the development of the wings of T. plasto-graphus and of D. va/ens are very much as have been described by various authors for the Coleoptera and Lepidoptera. With the begin-ning of the prepupal period the growth of the wing becomes acceler-ated. With the lengthening of the wing the nuclei begin to be drawn up into one row near the outer ends of the cells, while the cells them-selves become very much elongated and drawn out in places (Figs. 9, 10). Shortly before pupation this process becomes completed and the cells have the characteristic "fringed" appearance of the pupal hypodermis described by Verson (1904). The wing is now greatly wrinkled and folded, the vein cavities are greatly enlarged and filled with haemolymph and leucocytes, both of which also circulate in the spaces between the elongated cells. The basement membrane, which throughout the development of the wing is very thin and not easily discernable, becomes more or less degenerated during the prepupal period and in places the bases of the cells either end free (Fig. 19), or become fused and anastomosed with each other (Figs. 11, 22). Early in the last stage there is secreted all around the body, under-neath the dense outer chitin, an inner layer of soft spongy or stringy
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