Variance: A Possible Coding Mechanism for Gustatory Sensilla on the Labellum of the Fleshfly Sarcophaga Bullata

Abstract

In behavioural tests, 2-day-old female Sarcophaga bullata consumed more liver or fish powder in solution than 100 mmol l−1 sucrose. We investigated the chemosensory basis of this discrimination by recording electrophysiological responses of 177 medium-length labellar taste sensilla from 10 different flies to two applications of each of these three solutions. Responses from three chemosensory cells were evident in most records. Cell 1 produced a mean response of 37.6 impulses s−1, and similar responses to all three stimuli. It was the most active of the three cells. Cell 2 produced a significantly greater response to fish than to liver or sucrose in one of the two stimulus applications. Cell 3, the least active, responded with twice the firing rate to fish than to liver or sucrose. However, the mean firing rates did not provide information that could account for the observed behavioural discrimination. The only difference in the electrophysiological responses to the three stimuli which correlated with the behavioural discrimination was the variance of the response of cell 1, which was much higher to sucrose than to either fish or liver. We propose that variance itself could provide the necessary information to allow the fly's nervous system to distinguish between a ‘simple’ stimulus such as sucrose and a ‘complex’ stimulus such as fish or liver.