Osmotic regulation in mosquito larvae; the role of the Malpighian tubules.

In a recent paper (Ramsay, 1950 a) an investigation of the osmotic relations of the larvae of Aedes species was reported. Measurements of freezing-point depression were made on fluids collected from various parts of the body. In the case of A. aegypti, a fresh-water species, it was found that the fluid eliminated from the anus was strongly hypotonic to the haemolymph; the fluid entering the rectum from the intestine—presumably originating in the Malpighian tubules—was isotonic with the haemolymph. It was therefore concluded that the osmotic work required to produce a hyptonic 'urine' was carried out by the rectal epithelium and that the Malpighian tubules made no significant contribution to osmotic work. There is no reason to doubt this conclusion; but it is not to be taken to imply that the fluid excreted by the tubules is identical with the haemolymph in composition. The total concentration of the various osmotically active substances is virtually the same in both fluids, but their individual concentrations may be widely different. Evidence that the Malpighian tubules of Limnophilus larvae are concerned in ionic regulation has been presented by Bon.6 & Koch (1942), who showed that the concentration of chloride in the intestinal fluid can be very different from that of the haemolymph. It has not proved possible to collect intestinal fluid from a mosquito larva in quantity sufficient for the determination of chloride. But a method has recently been worked out whereby it is possible to determine sodium in small volumes of fluid, of the order of io" cu.mm., and this has afforded a means of attacking the problem. The investigation now to be described is in the nature of an extension of the earlier work, and, to anticipate, it is demonstrated that although the fluid excreted by the Malpighian tubules has virtually the same osmotic pressure as the haemolymph, its sodium concentration can be very much lower.