Antennal sensilla in male gall-wasps (Hymenoptera: Cynipidae) and insights on the evolution of sexual dimorphism in cynipoid sensory equipment

The diversity of insect antennal structures involved in communication is still poorly known because of the limited number of comparative studies and as such studies often exclusively focus on one sex. Within Cynipoidea, a recent study on female gall-wasps (Hymenoptera: Cynipidae) and some of their non-gall associated relatives (Ibaliidae and Figitidae) showed a great diversity of the antennal sensillar equipment both between and within lineages. In order to better understand possible patterns of this diversity, we here present a study in males. By Scanning Electron Microscopy analysis of 45 species of Cynipoidea (30 of them gall-wasps) from most known lineages, we found that the male antennal flagellum is generally filiform and bears overall eight types of sensilla: s. placoidea (SP), s. coeloconica (SCo-A), s. campaniformia (SCa), s. basiconica (SB) and s. trichoidea (ST-A, ST-B, ST-C, ST-D) (5–8 types per species). The number, size and arrangement of sensilla greatly varied among and within cynipoid lineages, with only a partial effect of phylogeny on this variation. By using data of 27 species for which both sexes were analysed, we found that males generally possess a lower number of sensillar types than females in gall-inducers and a greater number of sensillar types than females in non-gall-inducers. Sexes shared 40–100% of sensillar types independently from their biology. Males seem to possess an overall greater number of SP than females, with a weak tendency of such difference to be larger in gall-inducers. A possible hypothesis is that gall-inducing female cynipids may have evolved a richer sensillar equipment in response to female-only activities (e.g. plant host recognition), while cynipid males may have evolved higher density of sensilla devoted to mate recognition.