An Inventory of Taste in Caterpillars: Each Species Its Own Key*

Food plant recognition in lepidopterous larvae is predominantly governed by the activity of eight taste neurones present in two sensilla styloconica located on each maxilla. This paper reviews the results of electrophysiological and behavioural studies made on various caterpillar species during the last 40 years. It appears that all species, even closely related ones, have different taste systems. Taste cells responding to general phagostimulants (e.g., carbohydrates) have been found in all species studied. In some species, highly specialized taste cells have been found that respond to plant taxon-specific secondary plant substances that act as ‘token’ stimuli for plant recognition. Taste cells responding to many different secondary plant substances occur in most species studied. Their activity deters feeding. Though the response profile of these taste cells is best described as generalized, they nevertheless show species-specific stimulus spectra. These generalist deterrent cells often play a crucial role in feeding behaviour, a conclusion which confirms JERMY’s (1966) earlier inference on the preponderant role of inhibitory secondary plant substances in food-plant selection by herbivorous insects. The two most frequently studied neural coding mechanisms, ‘labelled lines’ and ‘across-fibre patterning’ have been inferred to operate in caterpillars. The first type is a likely coding mode in oligophagous species employing token stimulus receptors, whereas ‘acrossfibre patterning’ most probably operates in all species confronted with choices between plant food of varying quality. The responses of each of the taste cell types to their specific stimuli may be modified by the presence of other plant constituents, indicating that a complex stimulus (plant sap) evokes a response that is unpredictable from knowledge of responses to single compounds. Variability in taste cell responsiveness is dependent on developmental stage, time of day, and feeding history. This indicates that caterpillar taste cells are not rigid systems, and even possess a ‘peripheral memory’.