Laboratory of Chemical Prospecting, National Institute of Sericultural and Entomological Science, 1-2 Ohwashi, Tsukuba, Ibaraki 305-8634, Japan.
Ultrastructure of a pheromone-detecting sensillum placodeum of the Japanese beetle antennae was studied by transmission electron microscopy. The sensillum placodeum has two closely-bound dendritic segments, each of which runs from the cuticular pores to its soma. The outer dendritic segments are surrounded by a large sensillum lymph cavity formed by tormogen and trichogen cells. These accessory cells bear numerous microvilli extending to right below the cuticular surface. The inner dendritic segments ensheathed by thecogen show a ciliary region with a microtubular configuration of 9´ 2+0 near basal bodies. Thus, besides some morphological difference in near-cuticle structures, the general tissue organization of the scarab sensillum placodeum is similar to that of lepidopteran two-dendrite sensilla trichodea.