Electrophysiological responses of taste cells to nutrient mixtures in the polyphagous caterpillar of Grammia geneura

In a normally feeding insect, the taste receptors are exposed to complex mi xtures of chemicals, not single compounds. We investigate the responses of neurons in the galeal sensilla of the caterpillar of Grammia geneura to mix tures of nutrient compounds at concentrations occurring in plants. Compound s that stimulated the same neuron were generally additive in their effects in binary mixtures. Amino acids that did not stimulate usually had no effec t in mixtures with a stimulating compound, but glutamic acid reduced the re sponse to serine in the medial sensillum. Nutrient compounds that stimulate d different cells in a sensillum acted independently of each other. Complex mixtures of amino acids resembling samples of free amino acids from three host plants were less stimulating than expected from their molar concentrat ions. In host plant selection, the response from the medial sensillum is pr obably dominated by sucrose; unless sucrose levels are low, amino acids wil l contribute little to sensory input because they stimulate the same cell a s sucrose. In the lateral sensillum, amino acids act independently of sugar s. The limited contact chemosensory array of caterpillars seems inadequate to allow them to make fine distinctions between plants on the basis of thei r free amino acids.