Summary
The integument of the woodlouse,Oniscus asellus, consists of a two-layered epicuticle, a largely lamellate procuticle — itself divided into two regions (pre-and postecdysial cuticles), and the epidermis. At the initiation of new cuticle production the epidermal cells become vacuolated and retract away from the cuticle. Apolysis occurs immediately after the cessation of postecdysial cuticle production. The formation of the epicuticle is unique among the arthropods since material aggregates along the distal epidermal membrane. By indenting, doubling back on itself, and incorporating septa, the epicuticle forms surface structures such as plaques and tricorns.
The innervation, and so the receptive function of the tricorns is confirmed, but since there is no connection between the old and new receptors during premoult, sensory information from these exoreceptors must be severely curtailed. This may explain the biphasic moult in all isopods since it ensures that only half the body experiences this sensory deprivation at any one time. In terrestrial species there is the additional advantage of restricting the area of permeable new cuticle. The frequency of moulting may be due to the need to renew disrupted receptor surfaces.
Tricorns do not appear to be the mechanoreceptors involved in the marked thigmotactic response of woodlice since they do not have the typical internal structure of such receptors; rather, the dendrite —which extends into the lumen of the tricorn —is protected from deformation by the previously unreported combination of a dendritic sheath and a cuticular tube. The modality of tricorns is possibly one of hygro-perception. One of the behavioural responses of woodlice to desiccation is aggregation. The numerical distribution of tricorns over the body surface is admirably suited to assist in the formation and maintenance of such aggregates during desiccation and to their observed dispersal when the relative humidity rises.
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Price, J.B., Holdich, D.M. The formation of the epicuticle and associated structures inOniscus asellus (Crustacea, Isopoda). Zoomorphologie 94, 321–332 (1980). https://doi.org/10.1007/BF00998208
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DOI: https://doi.org/10.1007/BF00998208