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The influence of temperature and salinity on the cuticular permeability of some Corixidae Cannings, Sydney Graham

Abstract

Most terrestrial, and many aquatic insects are made waterproof by a layer of lipid in or on the epicuticle. At a specific temperature, which is determined by their composition, these lipids undergo a phase transition which markedly increases the permeability of the integument. The major purpose of this study was to assess the possibility that epicuticular wax transition could differentially affect the distribution of four species of water boatmen: Cenocorixa bifida hungerfordi Lansbury, Cenocorixa expleta (Uhler), Cenocorixa blaisdelli. (Hunger-ford , and Callicorixa vulnerata (Uhler). The rates of water loss and cuticle temperatures of adult corixids were measured in a stream of dry carbon dioxide in steps of increasing temperature. The temperatures at which transition occurred in these species were all approximately the same. Although they ranged from 30.3 to 32.6 C there were no major differences between the two genera, among congeners with different distributions, or between two coexisting congeners. This was true, however, only for corixids which had been acclimated to the same temperature: a positive correlation between transition temperature and acclimation temperature was demonstrated in C. befida. Both the short-term and long-term effects of transition on these insects were examined. Immersion of live C. bifida adults in water warmer than their transition temperature did not appear to cause any irreversible changes in cuticular permeability. Survival tests at various temperatures showed that the survival time of C. bifida adults decreased with increased temperature. However, insects placed in warm water did not show any outward signs of osmoregulatory failure or loss of surface wax as a result of transition. In addition, C. bifida placed in water as warm as or several degrees warmer than their transition temperature survived much longer than the length of time that these insects would .be exposed to these temperatures in the field. The present study on transition effects suggests that the transition of epicuticular lipids does not affect the distribution of these corixids in the field. However, it appears that salinity has a pronounced effect on the permeability of C. bifida adults. Individuals from fresh water habitats and highly saline ponds exhibited roughly equal cuticular permeability, but those from lakes of intermediate salinities were up to twice as permeable. This phenomenon was shown to be one of physiological acclimation, since individuals placed into distilled water showed a significant decrease in permeability after five days. It is possible that this influence of salinity on cuticular permeability may affect the relative dispersal success of the corixids which inhabit inland saline lakes.

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