The idiosomal chaetotaxy of astigmatid mites

Within the Astigmata, setal homologies with the general chaetotaxic systems developed for acariform mites by F. Grandjean have never been convincingly established. This study deals with all body regions, exclusive of legs and gnathosoma, mostly utilizing as models astigmatid species from three different families. Six hypotheses which attempt to explain the ontogeny of segments, setae and cupules in the caudal bend region are compared using three‐dimensional views obtained by scanning electron microscopy. The hypotheses are evaluated by in‐group comparisons testing their consistency with presumed segmental boundaries, and by out‐group comparisons with oribatid mites. The strongest, most parsimonious hypothesis suggests that larval astigmatid mites possess segment F, but not its setae, and the two pairs posterior to segment F are hl and h2, whilst the five setae added in the protonymph are h3, f2 and ps1–3. Six pairs of structures on the deutonymphal anal plate are identified as modified setae, and evidence supporting their homologies with setae of other instars is discussed for the first time. A comparison of the principal chaetotaxic systems used for the dorsal and anal regions of the Astigmata is presented. Application of Grandjean's setal signatures to these regions, as well as to the coxisternal and genital regions, is discussed and illustrated by examples.