The labidostommatid palpus: a morphological enigma (Acariformes: Prostigmata)

Hypotheses are reviewed concerning which segments of the primitively five-segmented palpus are coalesced to form the simple, linear, four-segmented form characteristic of labidostommatid mites. Among alternatives, those of either a femurogenu or a tibiotarsus are not based on convincing evidence. Instead, based on ontogeny of the peculiar palpal structure and chaetome of labidostommatids, a case is made that the proximal segment, which bears a single, ventral, postlarval seta, is a basifemur, with the trochanter autapomorphically either coalesced with it or subsumed into the base. Consequently, the second segment is a telofemur, articulating with the third article, which is an unusual, coalescent genuotibia. The phylogenetic significance of various interpretations of segmental coalescence are considered. Ontogenetic studies of palpal musculature and gene expression are needed to provide more substantive evidence.