ABSTRACT
The anal papillae of the aquatic larva of Aëdes aegypti are responsible for 90 % of the steady-state exchange of chloride.
The relationships between chloride flux and external chloride concentration are approximately described by the Michaelis equation.
There is net uptake of chloride, independent of uptake of sodium, from KC1, CaCl2 and NH4C1, probably in exchange for OH’ or
, but the rate is much slower than from NaCl. The following ions stimulate influx of chloride from 0·1 mM/1. KC1: H+ = Na+ > K+. The following ions inhibit it: 
Movements of sodium and chloride ions are explicable in terms of an anionic and a cationic carrier located in an osmotic barrier in the papillae, the carriers being functionally coupled to sodium and chloride pumps located at the inner surface of the barrier.
An attempt is made to relate these findings to recent electron microscopical studies of the papillae.
Subtraction of the chloride outflux curve from the chloride influx curve at the appropriate external concentrations gives the net chloride uptake at these concentrations (Fig. 1) and similar subtraction of the sodium influx curves (Fig. 1 ; and Stobbart, 1965, fig. 5) and the sodium outflux curves gives the changes in sodium fluxes due to the net chloride uptake.
Recent brief unpublished measurements of mine show that increasing the osmotic pressure of external NaCl solutions with sucrose to twice that of the haemolymph reduces the Na influx into the haemolymph by 50%. The data at present are inadequate to show whether the smaller reductions observed at lower osmotic pressures are significant.
