The permeability to water of the cuticles of some adult water bugs.

Studies of the water balance in adult water bugs have been reported in previous papers (Staddon, 1963, 1964). The adults lose water in the rectal fluid and gain water as a result of drinking and by osmotic uptake through the cuticle. The rate of osmotic uptake was estimated indirectly, from a knowledge of the rates of rectal fluid production and drinking, to lie between 2 and 7% of the body weight per day at 18 C. Similar values for the osmotic water uptake have been found in several aquatic insect larvae. For example, Shaw (1955) gives a value of 4% of the body weight per day for the osmotic uptake in Sialis lutaria larvae; SutclifFe (1961) a value of 7% for the uptake in several caddis larvae. Thus it would appear that adult water bugs may be at least as permeable to water as these aquatic larvae; possibly more permeable, since the possession of gas films and elytra must restrict the area of cuticle through which water absorption occurs. On the other hand, the figures given by Holdgate (1956) and Beament (1961) for evaporation rates from a wide variety of aquatic insects, including adult water bugs, caddis and Sialis larvae, indicate that the adult forms are less permeable to water than the larvae. Thus a need for further quantitative information on the cuticular permeability and osmotic water influx in adult water bugs is indicated. The work to be described in this paper goes some way towards supplying this information.