The pattern of osmotic regulation in larvae of the mosquito Culiseta inornata.

Larvae of Culiseta inornata (Williston) can survive and complete development in dilutions of sea water ranging from 50-700 mosmol kg-1. The larvae hyperregulate with regard to haemolymph osmotic concentration in dilute media (50-400 mosmol kg-1) and osmoconform when external salinities exceed 400 mosmol kg-1. This pattern of osmoregulation is distinct from that observed in freshwater and saline-water mosquito species. We propose that mosquitoes exhibiting this osmoregulatory pattern should be described as 'brackish-water' species. Larvae of Culiseta inornata are able closely to regulate both sodium and chloride ion concentrations in the haemolymph over the full range of salinities tested (50-750 mosmol kg-1). The Malpighian tubules produce an isosmotic, potassium-rich fluid. In vitro and in vivo sampling of rectal fluids demonstrates that rectal secretions are isosmotic or only slightly hyperosmotic to the haemolymph and the surrounding saline media, and that they are isotonic with regard to sodium.