Mechanisms of Gustatory Coding in Spodoptera Littoralis

Adult Lepidoptera use their antennae to probe their distant and proximate environment with sensory organs which detect volatile and contact chemicals. In this work, we have investigated the role and the sensitivity of antennal contact chemoreceptors of Spodoptera littoralis. Adults of both sexes have long and filiform antennae composed of about 70 flagellomeres. Each flagellomere bears six taste sensilla, except on the basal segments 1-5 where two sensilla are missing and on the terminal segment which has an additional crown of six sensilla. This represents a population of about 420 sensilla which provide taste and mechanosensory information. By mean of tip-recordings, we recorded the responses of these sensilla to aversive stimuli (KCl, NaCl) and to appetitive stimuli (sucrose, fructose, glucose). The questions we addressed were whether the sensitivity of these sensilla depends on (i) their position along the antenna, (ii) their position on each flagellomere, and (iii) sex. In all responsive sensilla, sucrose, fructose and NaCl elicited dose-dependent phasic responses while KCl and glucose did not elicit dose-dependent responses. In some sensilla, we could detect the activity of up to two neurones with different spike amplitudes in response to sucrose and KCl. No differences could be detected between sensilla located at the basal, medial and terminal regions of the antenna. No differences could be detected either between the lateral, dorsal and ventral sensilla in males; in females, however, the dorsal sensilla responded less than ventral and lateral sensilla but with the same response profile. No obvious differences were noted between sexes. These observations suggest that antennal contact sensilla serve as a warning and proprioreceptive system rather than as an elaborate taste system as found on the mouthparts or on the legs.