In dehydrating conditions late instar nymphs lose weight more slowly and survive longer than adult Periplaneta americana. This appears to be due, at least in part, to the larger lipid stores which are found in the fat bodies of the nymphs. When the water loss from an animal is greater than the amount of metabolic water obtained from the catabolism of stored foods, water is removed from the haemolymph in order to maintain water balance in the tissues. Dehydration for 6 days causes the haemolymph volume to decrease markedly in most adults, but the haemolymph Na+ and K+ concentrations increase only slightly. During dehydration the mean Na+/K+ ratio of the fat body tissue increased in adults, except in those which had been fed on pure carbohydrate prior to dehydration. Although not always statistically significant on account of the large variances, the changes in mean Na+, K+ and in the Na+/K+ ratio suggest there is an increase in Na+ and decrease in K+ in the fat body of animals where the haemolymph volume is markedly reduced by dehydration.

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