Towards an Understanding of Molecule Capture by the Antennae of Male Beetles Belonging to the Genus Rhipicera (Coleoptera, Rhipiceridae).

Working on the hypothesis that an important function of the lamellate antennae of adult male beetles belonging to the genus Rhipicera is to detect scent associated with female conspecifics, and using field observations, anatomical models derived from X-ray microcomputed tomography, and scanning electron microscopy, we have investigated the behavioral, morphological, and morphometric factors that may influence molecule capture by these antennae. We found that male beetles fly upwind in a zigzag manner, or face upwind when perching, behavior consistent with an animal that is tracking scent. Furthermore, the ultrastructure of the male and female antennae, like their gross morphology, is sexually dimorphic, with male antennae possessing many more of a particular type of receptor-the sensillum placodeum-than their female counterparts (approximately 30,000 vs. 100 per antenna, respectively). Based on this disparity, we assume that the sensilla placodea on the male antennae are responsible for detecting scent associated with female Rhipicera beetles. Molecule capture by male antennae in their alert, fanned states is likely to be favoured by: (a) male beetles adopting prominent, upright positions on high points when searching for scent; (b) the partitioning of antennae into many small segments; (c) antennal morphometry (height, width, outline area, total surface area, leakiness, and narrow channels); (d) the location of the sensilla placodea where they are most likely to encounter odorant molecules; and (e) well dispersed sensilla placodea. The molecule-capturing ability of male Rhipicera antennae may be similar to that of the pectinate antennae of certain male moths.