Structure, distribution and number of surface sensilla and their receptor cells on the olfactory appendage of the male mothManduca sexta

Abstract

Distribution and neuronal organization of sensilla on the surface of the annulate flagellar segment of the antenna of the male Manduca sexta were studied by scanning and transmission electron microscopy. Nine types of sensilla were identified and their bipolar neurons ascribed to specific sensory modalities on the basis of their cuticular and dendritic morphology. Cuticle morphology identifies two types of sensilla trichodea, two types of sensilla basiconica and one type of sensillum coeloconicum. Certain of these olfactory sensilla are further subdivided on the basis of their dendritic structures. One type of sensillum chaeticum was interpreted as a contact chemoreceptor. A second type of sensillum coeloconicum and a styliform sensilla complex were interpreted as bimodal hygro- and thermosensilla. A second species of sensillum chaeticum serves mechanosensation. Counts from annuli situated about midway along the flagellum revealed a total of about 2200 sensilla supplied by approximately 5160 sensory neurons. A conservative estimate suggests that a male antenna with 85-90 annuli provides the flagellar nerve with at least 3.6 x 10(5) receptor axons, a number that exceeds previous estimates by almost 50% Each species of receptor has a characteristic location on the annulus. Of the 2100 or so sensilla situated on the dorsal, ventral and the leading edge surfaces, about 800 consist of male-specific type-I trichoids containing pheromone-sensitive receptors. Arciform arrays of these sensilla on the upper and lower surfaces of each annulus presumably optimize the capture and absorption of odour molecules. The trailing edge of the flagellum, which is thickly covered by scales and was assumed until now to lack receptors, contains both mechanosensitive and contact chemoreceptors. The modality of non-olfactory receptors is considered with respect to similar elements that have been functionally identified in other species. The coexistence of non-olfactory sensilla with olfactory elements is discussed with respect to current knowledge of the organization of olfactory centres in the brain.