I find the Raphignathoidea (Acariformes, Prostigmata, Raphignathina) one of the more fascinating superfamilies of mites. Many of its members are bizarrely attractive, for example species of Xanthodasythyreus and Dasythyreus (Dasythyreidae) that look like an ambulatory pincushions.
The elongate setae probably act as a defence against predators and are produced in members of several other families. In other raphignathoid families increased degrees of sclerotization are the most obvious trend in predator defence.
Armour reaches it apogee in groups such as a the Homocaligidae or the Cryptognathidae. In the latter, the body is completely encased in a dorsal and a ventral shield that fit together rather like a tin and its lid, but with a tube at the anterior end into which the mouthparts can be withdrawn (and hence crypto = hidden, gnatho = jaws).
Currently, 11 families of Raphignathoidea are recognized and they are highly variable in form from soft-bodied to fully armoured. They all share some characters in common, for example the chelicerae have a needle-like movable digit used to stab and a sheath-like fixed digit that protects the needle when not in use, the genital papillae are not expressed, an ovipositor is not present, prodorsal trichobothria are absent, and the pad between their claws bears tenent hairs. Seven families have stigmatal openings near the bases of their chelicerae and simple to elaborate peritremes, but four families do not, and in any case stigmata and peritremes occur in other superfamilies as do the other character states that have been mentioned. As a result, the superfamily is difficult to key – it shows up at two spots in the current edition of the Manual of Acarology and the second couplet is one of those messy, exception-filled, paragraph-long abominations that are embarrassing to write and a headache to use.
The 7 families with stigmata and peritremes mostly have fewer than 50 described species and are characteristic of dry habitats (grasslands, deserts) or dry microhabitats (tree trunks, logs) in moister places. In contrast, the four families without stigmata, including the obscurely named Stigmaeidae with well over 300 described species, are found from dry to wet habitats and all types in between. In fact, the largest genus in the family Eustigmaeus (eu = good) Berlese, 1910, repeats this act, being found from deserts to submerged lake margins. Most stigmaeids that have been studied are predators of small arthropods and some are important biocontrol agents. However, Uri Gerson of the Hebrew University of Jerusalem demonstrated that at least some Canadian species of Eustigmaeus feed on mosses.
This seems to be true of the species that inhabits the mosses growing along the margin of the lake at my research site: E. frigida (Habeeb, 1958). The cheliceral digits are difficult to observe on a light microscope, even under oil immersion, so I thought I’d check them with the SEM, but this is one of those cases where more magnification is of no help. The mouthparts seem well along the road to becoming a gnathosomal capsule and only the tips of the cheliceral digits protrude from their wrapping. I wonder if the mite simply rasps the leaves of the mosses with the protruding tips of the chelicerae and sucks up the juices? Seems like that would work even under water.