Stages of the intermoult cycle in the terrestrial isopod Oniscus asellus and their relation to biphasic cuticle secretion

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Abstract
The intermoult cycle of Oniscus has been divided into 15 stages, recognizable by changes in the appearance of the anterior sternites. The stages are related to concurrent microscopical changes in the integument. The normal stimulus which initiates premoult is followed by a period when no tissue changes are discernible. Apolysis occurs about 7 days after premoult initiation. Thus, use of apolysis to recognize premoult (as in many crustaceans) would result in 44% of premoult passing undetected. Storage of calcium in sternal deposits and the general cuticle of the anterior region during the latter half of premoult is associated with an arrest of cuticle secretion in this region; at the time of posterior ecdysis (Ep) the anterior region has no new exocuticle. The anterior region is thus functionally similar to the decapod gastrolith. Stored calcium is resorbed and the anterior exocuticle secreted during the 24 h following Ep, enabling ecdysis of the anterior region to follow. The accumulation of calcium in the anterior cuticle while the posterior cuticle is undergoing resorption and the secretion of exocuticle at different times in the two regions both imply that the integuments of anterior and posterior regions differ in their responses to the moulting hormone.