Antennal sensillar equipment in the Apoidea: morphology, diversity and proposition of a consistent terminology

Insect sensilla are the structural units responsible for the perception of chemical, hygro-thermal and mechanical stimuli, and they are especially abundant in the antennae. A number of studies have reported that the antennal sensilla play a critical role in kin recognition, detection of natural enemies, foraging activity and mating activity. Given the wide range of ecological diversity of insects, it is not surprising that differences in antennal sensory system were reported between species with different life-history traits and within species between sexes. In the Apoidea (Hymenoptera: Aculeata), a large group including bees and stinging wasps, the morphology and distribution of antennal sensilla have been investigated in several species, though the level of detail is very variable and quantitative analyses are rare. Moreover, entire lineages have not yet been investigated. We present here new data from a Scanning Electron Microscopy (SEM) analysis of antennal sensilla in 14 species of bees, spanning six of the seven known bee families (Andrenidae, Halictidae, Colletidae, Apidae, Megachilidae and Melittidae), including lineages that have been particularly underrepresented in previous studies. A total of six types of sensilla are present in the studied bee species: sensilla trichoidea (ST, with three subtypes), s. basiconica (SB), s. placodea (SP), s. coeloconica (CO), s. ampullacea (AM) and s. campaniformia (CA). All these sensillar types occurred in the studied species. However, there was considerable variation in their density, which can be partly explained by body size variation. Indeed, head width (a proxy for body size) positively affected both antennal length and density of several sensillar types. Some differences among species, however, seem to be unrelated with variation in body size and may be linked to differences in life-history traits such as pollen specialization. These findings provide a foundation for more detailed investigations into the ecological pressures that may have influenced the evolution of antennal sensillar equipment in the Apoidea. Furthermore, this work has also given us the opportunity to review the currently available information on the antennal sensilla of Apoidea with the aim to address issues, such as the lack of a standardised terminology for sensillar types, which is essential for future comparative studies.