Humidity Detection and Hygropreference Behavior in Larvae of the Tobacco hornworm, Manduca sexta

Water is a critical resource for any terrestrial animal, especially for a soft-bodied insect such as larvae of the tobacco hornworm, Manduca sexta L. (Lepidoptera: Sphingidae). Strategies for coping with a dry environment might include seeking out regions of high relative humidity that reduce desiccative stress, or to find and imbibe liquid water. Desiccated larvae placed in a linear arena with a humidity gradient preferred the humid end, whereas un-desiccated larvae did not. This behavior was not affected by temperature. Ablation or occlusion of the antennae showed that they are required to mediate this behavior. A series of experiments showed that control larvae oriented towards and imbibed liquid water whereas those whose antennae had been occluded with wax did not. Electrophysiological recordings from the lateral basiconic sensillum of the second antennal segment revealed the presence of at least one hygroreceptive unit that greatly increased its firing rate in response to moist air, decreased firing rates in response to dry air, and showed mild post-stimulatory inhibition.