Adults of Oncopeltus fasciatus, the milkweed bug, are covered with an evenly spaced mat of hairs. Each of these hairs develops from a single selected epidermal cell which divides and differentiates in situ. The process which picks out the particular epidermal cell is a simple example of pattern formation. Using a measure of uniformity of distribution an attempt was made to analyse the development of the hair pattern in time and space. Three methods were used: (i) Direct mapping of epidermis undergoing hair differentiation which allowed analysis of the spatial and temporal order in a single developing pattern: (ii) The injection of an agent which killed cells in DNA synthesis and which allowed the development of only those hairs that had completed DNA synthesis at the time of injection. Thus adults developed in which only the first-formed hairs differentiated, (iii) Treatment with the insect moulting hormone ecdysone which led to premature deposition of cuticle and curtailed hair development.

Results from these 3 approaches showed that the first hairs, far from being a random sample of the complete hair population in the normal adult, were a highly dispersed group. It is concluded that the hair pattern is built up in a precise order, with the centre of the largest available spaces being the sites where differentiation begins.

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