Ultrastructure of pheromone-detecting sensillum placodeum of the Japanese beetle, Popillia japonica Newmann (Coleoptera : Scarabaeidae)

The pheromone-detecting sensilla placodea are significantly more numerous t han other sensory structures in the antennae of the Japanese beetle, Popill ia japonica (Coleoptera: Scarabaeidae). Their abundance in males is nearly twice of that in females, showing a clear sexual dimorphism. Externally, th ey have a tortoise shell-like round cuticular plate containing a few polygo nal plates separated by narrow ridges. Internally, they house two long dend rites that branch and terminate near fine cuticular pores. They have a syst em of two bipolar neurons accompanied by three enveloping cells, resembling sensilla trichodea in moths. The conspicuous difference with the latter is that the sensillum-lymph cavity near the outer cuticle is funnel-shaped, i nto which the tormogen cell projects numerous microvilli whose tips approac h the terminal branches of the dendrites. (C) 2000 Elsevier Science Ltd. Al l rights reserved.