Prostigmata, Parasitengona. This structure, which appears rather like the eye of a character straight from science fiction, is the genital opening of a terrestrial parasitengone mite and the three oval structures are the genital papillae. Genital papillae are remnants of opisthosomal limb segments in acariform mites (and still retain the segmented condition in some primitive oribatid mites) and are probably homologous with the spinnerets of spiders. Although spiders use their opisthosomal limb remnants for spinning silk, acariform mites probably use theirs for osmoregulatory activities, possibly including drinking. 

Centre for Microscopy and Microanalysis