Fine structure of the genital papillae of Hormosianoetus mallotae (Fashing) (Astigmata: Histiostomatidae), an obligatory inhabitant of water-filled treeholes

Abstract

Scanning and transmission electron microscopy (SEM and TEM) reveal that the genital papillae (= ring organs) of Hormosianoetus mallotae (Fashing) consist of a central disk of modified cuticle surrounded by a thickened ring of normal cuticle. Beneath the disk is an electron-lucent chamber separating it from modified cells below that contain numerous mitochondria in close association with plasma membrane plications, a fine-structural characteristic of cells with an active transport function. Hormosianoetus mallotae is an obligate inhabitant of water-filled treeholes and a completely aquatic species. Evidence points toward osmoregulation and/or ion regulation as the probable function of genital papillae in freshwater mites, and, based on fine structure, this is their probable function in Hormosianoetus mallotae as well. To date, the genital papillae of only one other species of histiostomatid, Histiostoma feroniarum (Dufour), have been investigated. Although both species share many fine-structural characteristics, they also differ in a number. Histiostoma feroniarum is found in a wide variety of habitats (e.g., stable manure, decaying vegetation, rotting mushrooms, fungal beds, under tree bark), and is usually only partially submerged and wading in the semi-aquatic environment. Differences in fine-structural characteristics of the genital papillae between Hormosianoetus mallotae and Histiostoma feroniarum are thought to be associated with habitat differences.