Abstract
ABSTRACT A study has been made of water balance in two closely related species of aquatic bug, Notonecta glauca L. and N. marmorea Fabr., and the mechanism appears to be the same in both. Ammonia is a major nitrogenous excretory compound and the bulk of the ionic material in the rectal fluid is in the form of ammonium bicarbonate. Water is eliminated through the excretory system and the output of water appears to be related to the output of ammonia. The relationship is such that the concentration of ammonia in the rectal fluid tends to a limiting value of about 120 mm./l. Water is gained by uptake through the cuticle and gut. The volume of water ingested is closely related to the volume of water eliminated through the excretory system and the uptake of water through the cuticle appears to be constant at about 7 % of the body weight per day. At the time of moulting the increase in weight is in part due to retention of water in the tissues and in part due to the retention of water in the rectum. It is pointed out that the mechanism by which water balance is maintained in Notonecta may not be of wide application to other aquatic insects.
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