The Loci of Contact Chemoreceptors in Insects. A Review with New Evidence

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Abstract
The literature is reviewed critically (142 refs.). Experimentally the loci of these receptors were determined for 23 spp. of insects from the orders Thysanura, Orthoptera, Neuroptera, Odonata, Coleoptera, Hemiptera, Lepidoptera, Diptera and Hymenoptera. Local stimulation and extirpation expts. were performed on living insects immobilized by being mounted on wax blocks. In the mandibulate insects studied, the receptors were found generally at the tips of the maxillary and labial palpi, the sensilla involved being basiconic or trichoid in type. Other contact chemoreceptors were found to be present on the mouth-parts; where located accurately, these were on the ligula and hypopharynx (roaches) or the hypopharynx alone (grasshopper). The antennae, tarsi and cerci, where present, in mandibulate insects seem not to bear contact chemoreceptors. In most of the haustellate insects tested, tarsal chemoreceptors were found to be present, the end-organs being thin-walled trichoid sensilla. Antennal receptors were found only in the honeybee and a wasp. The receptors on the mouth-parts of haustellate insects are usually at or near the tip of the proboscis. These were located accurately in a few spp. of Diptera, and were found to be thin-walled trichoid sensilla. The differences in distr. and structure of contact chemoreceptors in different spp. seem to be related to food habits. Much further work is necessary, however, before unequivocal generalizations will be possible.