Abstract
Microanalytical techniques are used to show that organic molecules play an important role is osmoregulation. Changes in the inorganic ion composition of haemolymph from fourth instar larvae of Aedes aegypti are correlated with the changes in the concentration of organic ions. Free amino acids have a significant role in regulating haemolymph osmotic pressure with respect to the osmotic pressure of the water in which the animal lives. Mechanisms by which amino acid levels could respond to changes in salt concentration are discussed.