Preliminary observations of the gustatory function of sensilla on antennae of the female Spodoptera littoralis (Lepidoptera: Noctuidae): from single compounds to complex natural stimuli

The gustatory system in insects is composed of detectors screening different panels of ligands, which enable or suppress life behaviors depending on the context. Single sensillum electrophysiology recordings were conducted from the antennal sensilla chaetica of adult female moth of Spodoptera littoralis to ascertain whether these sensilla have a gustatory function. Five test stimuli (NaCl, sucrose, ethanol, green cotton leaves extract, and conspecific larval frass extract) were used to stimulate each sensillum. Two types of gustatory sensilla chaetica (Vch and Lch) located on the same flagellomere of the distal third of antenna were characterized, each sensillum enclosed three gustatory receptor neurons (N1, N2, and N3). Overall, responses (spikes/s) were higher in the case of low concentration of sucrose and higher concentration of ethanol than of salt and higher concentration of sucrose. Individual differences were observed in the response patterns of these sensilla to the tested stimuli but functional sensillum types could not be identified. Higher concentration of sucrose, ethanol extracts of green cotton leaves, or conspecific larval frass significantly stimulated the same neuron of ventral (Vch) and lateral (Lch) sensilla chaetica. Response patterns revealed that antennal gustatory sensilla contain gustatory neurons, which are possible receptors for host-plant recognition. Moreover, stimulation of female's antennae with phagostimulative sucrose evoked an activation of the proboscis extension reflex (PER) with dose dependent responses. Differences in sensilla distribution and their response patterns suggest that gustatory sensilla on antennae of the female S. littoralis have a key role in adaptation and host plant recognition.