The membranous wing of Tribolium confusum is subject to specific gross deformity induced by X-irradiation of 1000 to 2000 r during the first 37 hours of the 6-day pupal stage. The survival curve for this effect has a shoulder to 1000 r and a Do of 110 r. The membranous wing contains a synchronous population of hypodermal cells which form a cuticle with an ordered array of projections (spines) which are in one-to-one correspondence with the cells. The cells[long dash]or cellular material[long dash]leave the wing, probably randomly, after eclosion. The radiation damage is manifest microscopically and chemically as a morphological transformation of some of the hypodermal cells; disorder of the spine array in regions; malformation, absence, and iteration of spines; delays in some developmental events; and disruption of the normal loss of cellular material from the wing.