An electrophysiological investigation was performed to analyse activity rec
orded in response to stimulation with concentrated and diluted solutions of
NaCl, sucrose, formamidine acetate, and sodium acetate from labellar chemo
sensilla of the dipteran Protophormia terraenovae. A computerized analysis
of this activity led to the finding of four kinds of spikes (A, B, C, and D
) which differed in their wave forms. The forms of spikes A, B, and C are d
escribed in the literature as being evoked by activation of the specific sa
lt, water, and sugar neurones respectively. The fourth type of spike (D) re
corded in response to formamidine acetate and sodium acetate is probably ge
nerated by a fourth chemosensorial cell that has not yet been defined funct
ionally. These two acetates evoke activities that are equivalent both in sp
ike frequency and time distribution even at low concentrations. The results
suggest that the acetate ion in the blowfly effectively stimulates the fou
rth chemosensory neuron present in the sensillum. The activation appears to
be involved in signalling acidity to the insect.