The permeability to water of the cuticle of the larva of Opifex fuscus (Hutton) (Diptera, Culicidae).

Mosquito larvae have been extensively used as models for the investigation of osmotic and ionic regulation in insects. They are phylogenetically well understood, and have colonized a wide variety of different environments, so that comparative studies have been possible on related species from widely differing habitats. Larval mosquitoes are easily obtained and reared in large numbers, are of convenient size for experimental treatment, and the structures considered important in osmoregulation are relatively easily isolated. As a result of the detailed studies carried out by a number of workers a tolerably complete knowledge of the mechanisms involved in salt and water balance has emerged, at least for the freshwater larva of Aedes aegypti (Stobbart, 19710-^). In this insect most of the transepithelial exchange of salts and water between medium and haemolymph occurs through the anal papillae (Wigglesworth, 1933a, b; Treherne, 1954). The papillae are also the sites of active uptake of sodium, potassium and chloride ions into the haemolymph against a steep concentration gradient (Koch, 1938; Ramsay, 1953; Treherne, 1954; Stobbart, 1959, i960, 19716). Less information is available regarding the mechanisms underlying the welldeveloped powers of hypo-osmotic regulation of mosquito larvae inhabiting salt water. The cuticle of Aedes detritus was regarded as being almost impermeable to water by Beadle (1939), and any osmotic loss of water which took place by this route was thought to be replaced by drinking the medium and the production of a concentrated rectal fluid (Ramsay, 1950). Shaw & Stobbart (1963) have suggested that the cuticle of marine insects would prove to have a permeability to water at least an order of magnitude less than that of related freshwater forms. As yet, however, no data have emerged to support this hypothesis. Previous experiments (Nicolson, 1972) established that the general pattern of salinity tolerance and osmotic regulation in larvae of the New Zealand marine mosquito, Opifex fuscus, was very similar to that of Aedes detritus. The present paper reports the results of experiments to test the validity of the hypothesis of Shaw & Stobbart (1963) by measuring the permeability of the cuticle of this larva to water.