The Regulation of Osmotic Pressure and Chloride Concentration in the Haemo- Lymph of Mosquito Larvae

IN an earlier paper (Wigglesworth, 19336) it was shown that the cuticle of the mosquito larva Aedes aegypti (= argenteus) is permeable to water only at the anal papillae, and that water is taken in continuously by these organs and excreted by the Malpighian tubules; observations which were confirmed by Pagast (1936). But a more important function of the anal papillae has recently been made clear by Koch (1938), who has shown that in larvae of CMronomus and Culex they absorb chloride ions. They are able to take up chloride from very dilute solutions (o-ooi M) and behave in this respect like the skin of frogs and other fresh-water animals as demonstrated by Krogh (1937). The extent to which this function is called into play will depend upon the concentration of chloride in the external medium. One might therefore expect it to be better developed in a species like Aedes aegypti which breeds in small collections of rain water in rot-holes in trees, coconut husks and such like, than in Culex pipiens which breeds often in contaminated water, in small pools, rain-water barrels, cesspits. The first object of the present work was therefore to compare the capabilities of these two species for absorbing chloride and to see how far this function is affected when the chloride content of the medium is changed. In another paper (Wigglesworth, 1933c) the adaptation of Aedes larvae to high concentrations of salt was studied, but no direct observations were made on the changes in composition of the blood. The second object of the present work was, therefore, to follow the changes in chloride concentration and total osmotic pressure in the haemolymph of larvae exposed to different media and to relate these changes with the movements of fluid in the tracheal endings.