The Effect of Salts on the Anal Gills of the Mosquito Larva

Abstract

The so-called anal gills of the mosquito larvae (Aedes argenteus) are delicate chitinous papillae lined by flattened cells and filled with haemolymph. Externally the cells rest directly upon the chitinous cuticle. Internally they are bounded by a continuous elastic membrane, apparently composed of some "scleroprotein." The faintly granular cytoplasm is crossed vertically by elastic fibrils or membranes. If the gills are cut off in various salt solutions, which can then come in contact with the cells on both their surfaces, the cells swell or contract like other tissues, depending on whether the solutions are hypo- or hypertonic. But if the same solutions are applied to the intact larva, so that they come in contact with the outer surface while the inner surface of the cells is still in contact with the haemolymph, the effects are altogether different: Hypotonic solutions have no visible effect. Hypertonic solutions of salts like NaCl, KBr, etc., in which both ions are monovalent, cause enormous swelling of the cells. This is probably because these salts diffuse through the outer membrane (the cuticle) of the gills into the cells, which then absorb water from the haemolymph by osmosis. If the larva so treated is soon restored to fresh water, the action is reversible; but after a time the elastic filaments in the cells are dissolved and these can no longer contract again. These effects occur equally in the presence of salts with divalent cations. Hypertonic solutions of salts like CaCl2, Na2SO4, etc., in which one or both ions are divalent, extract water from the larva but do not cause swelling of the cells. These salts do not dissolve the elastic filaments. In the presence of hypotonic NaCl, etc. (which by itself has no visible effect), they cause temporary swelling followed by contraction. The cause of the difference between these two groups of salts is discussed. Dilute alkalis (N/50 NaOH) applied to the isolated gill or the intact larva dissolve the cells and cause extreme swelling, but do not dissolve the cuticle or the inner membrane. This action is accentuated by NaCl, etc., and by Na2SO4, etc., but is partially inhibited by CaCl2. Dilute acids (N/100 HCl) cause precipitation of the nuclei, slight swelling of the cells, and complete separation from the cuticle. In the presence of hypertonic Na2SO4 or CaCl2 the separation does not occur. All these effects are peculiar to the gills; no other part of the surface of the larvae is affected by these reagents.1