For today's post, I'd like to introduce you to the Eupodoidea. These are a group of mites in the Prostigmata (you can find a whirlwind tour of the Prostigmata here) that can be found in soil or vegetation. Various species of eupodoids are predators or plant-eaters; as far as is known, the group doesn't include members symbiotic with other animals. They are mostly soft-bodied (except for members of the family Penthalodidae) and generally small, though members of the Australian genus Eriorhynchus can grow to nearly two millimetres in length (yes, that's big for a mite). Eupodoids are particularly diverse in cooler climates, though once again there are notable exceptions: the Hawaiian species Hawaiieupodes thermophilus inhabits volcanic steam vents with temperatures of up to 41°C (Walter et al. 2009). The first pair of legs are often used as sensory organs, being held in front of the body while the mite walks on the remaining three pairs.
The eupodoids are currently divided between nine families, some of which have only been established recently. The Cocceupodidae was establised by K. Jesionowska in 2010 for the genera Cocceupodes, Linopodes and Filieupodes, and the South African species Dendrodus acarus was described as its own family by P. A. S. Olivier in 2008 (unfortunately, he named this family the 'Dendrochaetidae', which is an invalid name because it was not based on an included genus). The genera Pilorhagidia (known from Hawaii and Europe) and Eriorhynchus are also placed in separate small families, as is the South African species Pentapalpus unguempodius. The habits of these various small families are little-known, but Pilorhagidia and Pentapalpus are probably predatory, and the large hairy Eriorhynchus may be herbivorous. Similarly uncertain in habits are the sclerotised mites of the Penthalodidae, whose two genera Penthalodes and Stereotydeus may be quite colourful and intricately ornamented.
The Penthaleidae is a family of plant-feeding mites that is most notorious for including some significant agricultural pests. These include the winter grain mite Penthaleus major and the red-legged earth mite Halotydeus destructor (it's all in the name, really). Both these species feed on a range of crops. They both have dark central bodies with bright red legs, but the winter grain mite also has a reddish patch on its back marking the dorsal position of the anus (where the drop of liquid is being extruded in the photo above). Red-legged earth mites lack such a patch, and their anus is terminal in position.
The remaining two families are the most diverse. The Eupodidae often have the femora of the hind legs larger than the other legs, allowing them to rapidly jump backwards when threatened (such enlarged femora are also present in the Cocceupodidae). A number of Eupodes species feed on micro-algae. One eupodid, Benoinyssus najae was originally thought to be an animal symbiont as it was collected from the nasal fossae of a cobra. This species has since been found in soil and leaf litter, and it is thought that its original collection site was accidental.
The final family is the Rhagidiidae, a cosmopolitan group of carnivorous mites. Rhagidiids feed on other small arthropods, and are found in a wide range of habitats, including some that are restricted to caves. Members of the Rhagidiidae spin silk from glands near the chelicerae that they use for protection. Nymphs of Rhagidia species spin a web around themselves before they moult. In at least one species, Rhagidia longisensilla, the web is also used to capture prey: springtails that run into the web become entangled in it, and the mite is thus able to capture springtails twice its own size (Ehrnsberger 1979). I am not aware of any other animal outside spiders that use silk in this way, and I'd be interested if anyone else is.
Ehrnsberger, R. 1979. Spinnvermögen bei Rhagidiidae (Acari, Prostigmata). Osnabrücker naturw. Mitt. 6: 45-72.
Jesionowska, K. 2010. Cocceupodidae, a new family of eupodoid mites, with description of a new genus and two new species from Poland. Part I. (Acari: Prostigmata: Eupodoidea). Genus 21 (4): 637-658.
Olivier, P. A. S. 2008. Dendrochaetidae, a new family of mites (Acari: Prostigmata), with descriptions of a new genus and species from South Africa. African Zoology 43 (1): 16-24.
Walter, D. E., E. E. Lindquist, I. M. Smith, D. R. Cook & G. W. Krantz. 2009. Order Trombidiformes. In: Krantz, G. W., & D. E. Walter (eds) A Manual of Acarology, 3rd ed., pp. 233-420. Texas Tech University Press.