Comparative morphology of the antennal “release and spread structure” associated with sex pheromone-producing glands in male Cynipoidea

Mating behaviour in insects is largely mediated by chemical signals emitted by one sex and recognized by the other (sex pheromones). A particular type of integumentary gland produces a sex pheromone in the antennae of male Cynipoidea (Hymenoptera), a rich and diverse wasp group consisting of species forming galls on plants, species which act as inquilines or parasitoids of these galls, and non-gall-associated parasitoids. This gland is connected to the surface by a modified structure which releases and spreads the chemicals on females (release and spread structure (RSS)). Here, we performed a scanning electron microscope investigation to evaluate the diversity of RSS in detail through an analysis of 49 species spanning almost all extant lineages, with special emphasis on the family Cynipidae (gall-wasps). Up to three strongly modified flagellomeres harboured the RSS in Cynipoidea, more often the first (proximal) flagellomere (F). The Fwas in most cases cylindrical or slightly flattened laterally, in this case often with a longitudinal ridge present, with few cases of a distinctly excavated (only in Cynipidae and the basal parasitoid family Ibaliidae) or strongly distally inflated (only in the cynipid gall-inquiline tribe Synergini) shape. The RSS ranged from long and narrow to short and wide. Pores of the RSS, more often scattered than closely spaced, were well visible in most of species. The observed conspicuous variability in RSS morphology seemed very weakly associated both with phylogenetic relationships among lineages and with life-history (galler, gall-associated inquilines or parasitoids, and non-gall-associated parasitoids). New studies are necessary to shed light on the evolution of this structure.