A comparative investigation of the antennal sensilla in adult Anisoptera

A fine structural overview of the flagellar sensilla of Onychogomphus forcipatus (Gomphidae), Aeshna cyanea (Aeshnidae), Somatochlora metallica (Cordulidae) and Cordulegaster boltonii (Cordulegastridae) revealed the presence of pits containing sensilla typically located on the latero-ventral side of the first flagellar segments in all four species. These sensilla are represented by coeloconic single-walled olfactory sensilla and deeply sunken sensilla styloconica (type-1 and type-2) sharing common features typical of thermo-hygroreceptors. Sensilla styloconica are located inside deep convoluted cavities. It is suggested that olfactory and thermo-hygrosensory sensilla are the main sensilla on the antennae of all anisopteran families. The attribution of the coeloconic sensilla of dragonflies to single-walled olfactory sensilla (confirmed by the finding of pore tubules in O. forcipatus), together with their common occurrence in the suborder Anisoptera, are relevant for phylogenetic studies.