The integument of Oncopeltus is very different in larvae and adults, and the transformation is sensitive to juvenile hormone. The effects of a synthetic juvenile hormone analog on cellular metamorphosis (here regarded as an example of cellular differentiation) were analyzed at two levels: 1. A. Effects on cuticle secreted by individual cells 1.1. 1. Potent doses (1.25 μg of the crude extract) when injected early in the last molt of Oncopeltus caused complete inhibition of metamorphosis and curtailed the molt cycle. 1.2. 2. After injection of the same dose at 27 and 49 hours after ecdysis, larval cuticle was formed; but injection at 64 and 74 hours produced a series of adult/larval intermediate insects whose abdominal integument consisted of intermediate cells. The whole insect was affected and the epidermal cells responded uniformly. A continuous series of integuments showing increasing cellular metamorphosis was obtained. 1.3. 3. When threshold doses (0.06 μg) of the hormone were injected early in the molt, only the epidermis was affected and the response of the epidermal cells was heterogeneous, resulting in mosaic insects. 2. B. Effects on the hair pattern During metamorphosis in Oncopeltus evenly spaced hairs develop. These structures are distinct from, and much denser than, the larval bristles. Their development is sensitive to the juvenile hormone. 2.1. 1. Injection of potent doses of juvenile hormone within a few hours of the previous ecdysis completely suppressed hair development. After this time and up until 64 hours after ecdysis hair development was only partially reduced, even though the cuticle secreted was completely larval by other criteria. There was no effect on hair density after 64 hours, even though the visible cytological events associated with hair differentiation do not begin until 70 hours. 2.2. 2. The intermediates could be arranged in a continuous series of increasing hair density. The uniformity of distribution was measured quantitatively and found to improve with density. 2.3. 3. Mitomycin C was injected and found to disrupt hair development independently from juvenile hormone, probably by direct interference with hair development. 2.4. 4. The hair and bristle pattern was analyzed quantitatively according to the method of Claxton (1964): the results supported the adoption of the Wigglesworth model for bristle development as a working hypothesis. Within the terms of this model, the juvenile hormone was shown to maintain the threshold of epidermal cells to the bristle-forming substance, a threshold which falls during normal cellular metamorphosis. 2.5. 5. The process of cellular metamorphosis and differentiation in this system is considered to be a gradual one, which can be halted by juvenile hormone. The development of small numbers of evenly spaced hairs in intermediates is regarded as an expression of partial metamorphosis or differentiation. © 1969.
