THE electrical responses of contact chemoreceptors on the mouthparts of many species of Diptera can determine whether or not a fluid medium is ingested. In the blowfly Phormia terranovae, the responses of the large bristle-like sensilla, which occur in two aboral patches on the labellar lobes, regulate the extension of the proboscis and the opening of the labellar lobes. The responses of a single sensillum may mediate this action if stimulated with water or with certain sugars in aqueous solution1. Without extension and opening, feeding is impossible. Rejection thresholds seem to be determined to some extent by the responses of the type 1 (“salt”) receptor cell, one of the four types of chemoreceptor cell contained within each sensillum2.