Functional units in the labellar chemosensory hairs of the mosquito Culiseta inornata (Williston)

Electron micrographs of the labellar chemosensory hairs of the mosquito show that these sensilla have either three or four dendrites extending the length of the shaft. At the tip of each hair is a circular pore. The dendrites are enclosed within a scolopoid structure. At the distal end of the dendrites the microtubules are arranged in a random manner, but, in the proximal area, they are grouped into nine pairs. Near the base the pairs assume a spiral pattern giving rise to a three‐compartmented structure which forms the root proper. Action potentials were recorded from the hairs, using NaCl at concentrations of 0.002 M to 1.0 M. Four different spikes were recognized — water and three salt. The salt spikes were labeled Types a, b, and c. None of these is identical with the L spike from the blowfly. All four spikes were recorded from some hairs which were certainly those with four dendrites. Behavioral studies demonstrated that the rejection threshold of NaCl for labellar hairs was 0.28 M. This coincides with a rapid increase in the electrical activity of the Type c salt cell. From behavioral and electrophysiological evidence it was concluded that acceptance by Culiseta of NaCl in aqueous solution is the result of activity of the water receptor plus input from the salt receptors Types a and b, and that rejection results from activity of the Type c salt cell.