Most extant Chelicerata are characterized by external digestion and the ingestion of fluid food. Exceptions include the marine taxa, most Opiliones, and the mite groups Opilioacarida (Parasitiformes) and Sarcoptiformes (Acariformes), which ingest particulate food. This leads to different physiological and morphological adaptations for food processing, including the production and extrusion of solid fecal pellets, which are rather large in sarcoptiform mites. Few studies have investigated the defecation of such large fecal pellets, and available information is contradictory. We use a combination of non invasive microscopical techniques and in vivo examination to investigate the complex functional morphology of the anal region of the oribatid mite Archegozetes longisetosus Aoki. The opening of the anus is at least initiated by indirect muscular action via an increase of hemolymph pressure, through the action of dorsoventral muscles (dvm). Extrusion of the fecal pellet is accomplished by the prerectal muscle collar, with full opening of the anus and rotation and bowing of the plates probably resulting from pressure of the pellet. The sequential nature of these actions was demonstrated by many observations in which the anus opened only partially; these were concomitant with dmv contraction but pellets were not being extruded. All muscles directly connected to the anal and adanal regions assist in keeping the anus closed; they are antagonists to hydrostatic forces that are necessary for normal activity. Based on the literature, no obvious similarities were noted with defecation musculature in other particle-feeding chelicerates, but most muscles can be homologized with those of more specialized oribatid mites. The function of the outer anal muscles has been modified in both Euphthiracaridae and Brachypylina to assist in providing general hemolymph pressure.
2008 Wiley-Liss, Inc.