The hormonal control of the development of hairs and bristles in the milkweed bug, Oncopeltus fasciatus, Dall.

The insect cuticle has long been appreciated as excellent material for the study of cellular responses to hormones. Not only is the insect epidermis a single layer of cells, but also ' the tiny fragment of cuticle laid down by a single cell may possess morphological characters controlled by the activities of that cell alone' (Wigglesworth, 1940 a) and cell differences will thus be rendered visible in the types of cuticle they secrete. The hairs and bristles formed by the integument are also indicators of certain otherwise invisible properties of cells. For instance, the scales of moths develop even on small pieces of implanted integument provided that the epidermal cells are in an appropriate hormonal milieu (Piepho, 1938 a). When the types of scales or bristles differ in subsequent stages in the development of the insect, then the form of the scale or the bristle may indicate the developmental state of a piece of experimental integument. The larval Oncopeltus is furnished with bristles which are sparsely distributed on the abdomen, being on the average ten epidermal cells apart. The mature bristle is an innervated sensillum and comprises four cells: the trichogen, the tormogen, nerve, and neurilemma cell; at every moult subsequent to its formation the trichogen and tormogen cells secrete a new bristle shaft and socket. At each moult, just as in Rhodmus (Wigglesworth, 1940 a) new bristles appear amongst the old ones, so that the density of their distribution remains more or less constant throughout growth. During the final moult some of the bristle cell groups degenerate and others form cuticular bristles once again. These adult bristles, as in Rhodmus, differ in form from their larval predecessors ; in this case the ratio of the diameter of the socket to the length of the bristle (length/socket ratio) is smaller than in the larval type (Fig. 1). In addition to these adult bristles there is, during metamorphosis, the development of a large new population of densely distributed adult hairs. The spacing of these adult hairs varies, but in some areas they are only one or two epidermal cells apart. These hairs are not innervated and in the mature form consist of but one cell. The hairs, as the bristles, arise from mother cells, which undergo differentiative divisions to produce the four distinct cells which together form the complete hair. Three of these cells degenerate during the maturation of the hair. A cytological account of the development of hairs and bristles in Oncopeltus is in preparation.