Sexual dimorphism in the sensory system of Myrmilla capitata (Hymenoptera: Mutillidae), a bee-attacking velvet ant with both sexes apterous

The insect sensory system was extensively explored in the aculeate Hymenoptera. However, there are entire lineages with only a few studies on the topic, limiting our understanding on its evolution. Here, we studied the sensory system in females and males of the bee-attacking velvet ant Myrmilla capitata, which belongs to a previously neglected family of ectoparasitoid wasps (Mutillidae). Through a Scanning Electron Microscopy analysis, we first described the M. capitata sensory systems. We then tested the hypothesis that males and females may diverge in some traits even if both are apterous and confined in the same restricted area (host bee nesting site), likely as an adaptation to the divergent stimuli they respond to (hosts for females, sexual partners for males). Despite males being larger than females, many of the analyzed traits were sexually dimorphic after having taken into account size difference. Females had slightly lower ommatidial density (worsen resolution) but larger ommatidia (better light capture) than males, which also displayed three small ocelli. The antennal flagellum harbour overall 12 types of sensilla: placoid sensilla (SP), coeloconic sensilla (SCo), sensilla campaniformia (SA), trichoid sensilla (ST) (a total of six types) and basiconic sensilla (SBa) (a total of three types). Some of the ST types and SBa types were found exclusively in either females or males, while both total ST density and SP density and size (better olfaction) were higher in females. Our results preliminarily indicate that males may take advantage of a better visual trait related with acuity to detect females, while females may more heavily rely on odours to detect the host.