The daily growth-layer formation in the tibial endocuticle of Blaberus craniifer and Leucophaea maderae was investigated. Up to 3 weeks after the final moult one double layer was deposited per day, consisting (in sections cut transverse to the leg axis) of an iso- and an anisotropic layer. This cuticle growth rhythm continued under constant conditions (continuous white and red light, and constant darkness) with a period at or close to 24 h, indicating its endogenous origin. Only at temperatures below 25°C was it possible to differentiate the free-running rhythm from a 24-h period. The Q-10 of the rhythm was 1.02 - 1.39, depending on the temperature range inspected (B. craniifer). Light did not affect the cuticle rhythm in either species: direct effects as well as zeitgeber function have been ruled out. The layer deposition of B. craniifer did not react to light-dark cycles of 13:13, 11:11, 10:10 h but showed instead a period close to 24 h. In L. maderae neither isotropic nor anisotropic bands could be related to a certain part of the light regime. Shifted light-dark cycles or light pulses of high intensity had no effect on the deposition rhythm. The same was true for a light-dark cycle of 6:6 h. From our experiments we conclude, that the endocuticle growth rhythm in cockroaches is governed by a temperature compensated circadian system which lost the ability to use light as a zeitgeber. © 1986.
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