Behavioral and electrophysiological responses of the blowfly, Phormia regina meigen, to acids

Summary

Flies of a standardized degree of thirst were tested for proboscis extension in response to mono- and dicarboxylic acids applied to the tarsi. Presence or lack of extension depended on pH for each acid, but acids of greater chain length were effective in preventing extension at higher pH values than those with shorter chain length (Fig. 1). Concentrated solutions of monocarboxylic acids at low pH values evoked a strange, delayed, hesitant form of proboscis extension which seemed to signify rejection rather than acceptance. It was not abolished by satiation with either water or sucrose (Fig. 2).

Electrophysiological tests of tarsal hairs with NaCl and sucrose showed that the corresponding hairs of each type on opposite sides of the same individual leg had almost identical neural responses to a given stimulus (Table). This meant that hairs on one side of a leg could be used as controls for hairs on the other side.

Comparisons were made of the neural responses to different pH values of each acid and of the responses to different acids at the same pH. For each acid, as the pH decreased the response increased both in number of neurons firing and in their firing frequency (Fig. 6). There were differences between the responses to different acids at the same pH correlated with the nature of the behavioral response at the pH value tested. In general, responses were greater to mono- than to dicarboxylic acids (Figs. 7–10).

Lack of proboscis extension was correlated with the firing of a small spike of regular frequency, especially prominent in responses of D4 hairs, and with firing of the salt receptor. The strange, hesitant extension was correlated with the indiscriminate firing of a large number of neurons at irregular frequencies. It is concluded that flies have available information enabling them to distinguish acids from salts, sugars, and water. The basis for this distinction is at least in part anatomical. Flies also can distinguish between different specific acids at the same pH, depending on the molecular structure of the acids.