Morphogenesis of mechanoreceptor and epidermal cells of crickets during the last instar, and its relation to molting-hormone level.

Abstract

(1) The fine structure of the cercal campaniform sensilla and epidermal cells of Gryllus bimaculatus Deg. (Saltatoria, Gryllidae) was examined, and the ecdysteroid level was monitored throughout the last larval instar. (2) The epidermal cells show changes in shape, cytoplasmic inclusions and differentiation of the apical cell membrane, coupled to the phases of buildup and breakdown of the (cercus) cuticle. (3) The imaginal epicuticle of the epidermal cells begins to form later (by about approximately 6h) than that of the campaniform sensilla. (4) The campaniform sensilla were studied with respect to (a) the morphogenesis of the cuticular apparatus, (b) the inclusion of phenol oxidases in the cuticular apparatus, and (c) changes in the sensory apparatus preparatory to molting. (5) After apolysis the folding of the tormogen-cell wall into microvilli transiently disappears. Microvilli re-form shortly before imaginal ecdysis, and at the same time an outer receptor-lymph space develops. The role of the tormogen-cell "plaques" is discussed. (6) The levels of alpha- and beta-ecdysone were determined separately by radioimmunoassay. (7) At the beginning of the instar the hormone level, especially that of beta-ecdysone, falls. Prior to apolysis, the concentration of alpha-ecdysone rises, reaching an intermediate peak after apolysis is complete. The maximum hormone concentration (approximately 2,000 ng/g) is reached after the cuticulin layer is deposited, primarily due to the increase in beta-ecdysone. While the proecdysial cuticle is forming, the hormone titer is reduced; at this time beta-ecdysone is its chief component. (8) The identification of the ecdysteroids monitored by radioimmunoassay was confirmed by gas chromatography.