Electrophysiological studies of gustation in lepidopterous larvae

Summary

The electrophysiological responses of the maxillary gustatory sensilla of ten species of lepidopterous larvae to sugars, polyhydric alcohols, amino acids, organic acids, glycosides, and sterols were tested. Of the caterpillars examined three were monophagous (Calpodes ethlius, Ceratomia catalpae, Danaus plexippus), three were oligophagous (Pieris rapae, Papilio polyxenes, Malacosoma americana), and four were polyphagous (Helioihis zea, Estigmene acrea, Isia isabella, Porthetria dispar). The sensilla of all species are structurally similar to the extent of having four chemosensory bipolar neurons in each peg.

It is not possible to account for the wide range of responsiveness to diverse compounds in terms of narrowly specific receptors characterized as water, sugar, salt, inositol, or amino acid receptors. Bach receptive cell was found to have only a limited degree of specificity. Not all amino acids stimulated the same cell. Some amino acids stimulated only one; some stimulated two; some inhibited all cells; some inhibited one while stimulating another. Differential sensitivity to amino acids varied from species to species and even between the two sensilla in an individual.

The diversity of relationships found with amino acids existed also with respect to acids and glycosides. Ascorbic and hydrochloric acids at equivalent pH values each stimulated a different cell. Glycosides stimulated one or two cells depending on the species and sensillum.

Synergism occurred nearly as often as inhibition. Among the instances were synergistic reactions between sucrose and inositol, proline and inositol, sucrose and sinigrin, glycine and sucrose. The occurrence of the phenomenon was specific to certain species and sensilla.

Results point to the conclusion that the basis of difference in sensitivity to different compounds from species to species and from receptor to receptor reflects variations in the molecular structure of the receptor substrate. Whatever the mechanisms involved it is clear that caterpillars are able to detect not only compounds that may be of nutritive value but also non-nutritive compounds some of which are secondary plant substances. When the total taste sensitivities of the various species of caterpillars are compared, there is no obvious difference between polyphagous, oligophagous, and monophagous species.