An active reduction of water level in insect cuticle.

The control of water movement through the cuticle of terrestrial arthropods is demonstrated by a number of phenomena. For example, Lees (1946) has shown an active retention of water, Winston & Nelson (1965) indicate an active regulation of water loss, and Beament (1961) reviews the evidence for active uptake of atmospheric water. These phenomena are best explained by a water pump of some kind in the cuticle (Beament, 1964). It is difficult to envisage a pump which does not, at least periodically, reduce the level of water activity in the cuticle with consequent exposure of the epidermal cells to osmotic gradients. But a pump, for example, of the kind suggested by Beament (1964, 1965) would not lead to such serious conditions for the epidermal cell as a directly acting osmotic system which is evisaged by Edney (1957). If the 'capillary suction pressure' proposal of Wharton & Kanungo (1962) is viable, it might not involve the epidermal cell in osmotic problems. All the observed phenomena would come closer to an explanation if it could be demonstrated that the cuticle is not in equilibrium with the blood, so far as level of water activity is concerned, and much would depend on the actual level of reduction which might exist. The evidence based upon the water exchange of living animals would also lead one to suppose that the water level in the cuticle might be related to the humidity to which the animal is exposed. A preliminary report (Winston, 1967) suggests that the cuticle may not be in osmotic equilibrium with the blood; further evidence forms the substance of this paper.