Tarsal Taste Discrimination in the Blowfly, Calliphora Vicina Robineau-Desvoidy

Abstract

This paper deals with the problem of the specificity of the sense cells in terminal tarsal taste sensilla on the forelegs of the blowfly, Calliphora vicina Rob.-Desv. (= Calliphora erythrocephala Meigen), in relation to the discrimination these flies are capable of making, as judged by their behavioural responses, between water and weak sugar (sucrose) solutions. First, electrophysiologically recorded responses to pure water are described. As many as three receptor cells in the blowfly terminal tarsal taste hairs can be stimulated by water. Next, the reactions of sense cells to both water and weak sucrose solutions are analysed, showing that up to concentrations of several hundredths of Molar sucrose the responses are indistinguishable from those to water. It is therefore surmised that water and sucrose stimulate the same sense cells in these taste hairs, which raises doubts concerning the specificity generally ascribed to these cells. These doubts are confirmed by the results of experiments showing that water and sucrose adapt, and therefore stimulate, the same sense cells. This finding implies that some mechanism other than specificity of sugar- and water-sensitive cells is required to explain the behavioural distinction shown by blowflies between water and sucrose. Before an alternative was formulated, the comparability of the proboscis response in fixed and in free-moving flies was discussed, and also to what extent single-hair stimulations (such as are usually applied in electrophysiological methods) are comparable with multiple-hair stimulations (as usually applied in behavioural experiments). Both questions could be answered satisfactorily and consequently an alternative for cellular specificity is given. It involves the concept that discrimination occurs in the intact animal on the basis of the input from many tarsal hairs, the cells of each of which respond with different relative impulse frequencies to stimulation with water and to stimulation with weak sucrose solutions.