Mites are pretty remarkable creatures. I don't know if any other group of animals can rival mites for ecological diversity. There are mites burrowing in the leaf litter of forests, there are mites living in the sediment at the bottom of the sea, there are mites living off the secretions in your hair follicles, there are are mites that live as parasites of other animals. Whatever freakish thing you can think of an animal doing, odds are that there is a mite doing it right now. Mites also include one of the few groups of terrestrial arthropods to have developed a mineralised exoskeleton - the oribatids or beetle mites. Our newest Taxon of the Week is one of the subgroups of oribatids, the Holonota.
The Holonota include the most heavily armoured of the oribatids, with most of the body encased in hardened plates. Morphologically, Holonota are distinguished from other oribatids by having the entire body covered by two dorsal plates, with the division between the two plates between the positions of the second and third pairs of legs. Technically speaking, various Holonota may be dichoid, holoid or ptychoid (Norton, 2001). In dichoid forms, a non-hardened zone runs around the body between the second and third pairs of legs, allowing the body to bend at that position. Holoid forms only have this articulation dorsally, with the ventral surface fused to a solid plate. Ptychoid forms, on the other hand, have reduced the ventral hardening and are actually able to withdraw the legs and close the anterior dorsal plate over them - the mites' answer to ostracods.
Norton (2001) suggests that the development of heavy armour in the oribatids may be related to their lifestyle. Most oribatids are long-lived (at least for mites) and have low reproductive rates, a situation that may result from their usual diet of low-nutrient decaying vegetation and fungi. Slow growth and replacement rates may result in the selective favouring of features that extend the life expectancies of individuals.
Morphologically, Holonota have been divided between three groups, the Mixonomata, Desmonomata and Circumdehiscentiae (Maraun et al., 2004). The Mixonomata includes dichoid and ptychoid forms, and has been suggested to be paraphyletic to a holoid clade formed by the other two taxa. In turn, the Desmonomata are probably paraphyletic with regards to the Circumdehiscentiae. The holoid Circumdehiscentiae are one of most speciose groups of oribatids, and show the highest degree of plate fusion, with the armour of almost the entire underside fused with the anterior dorsal plate (Norton, 2001). Molecular analyses, in contrast, have been divided in their support for this arrangement. Maraun et al. (2004) failed to support the morphological view, and did not even recover monophyly for the Holonota as a whole or for the Circumdehiscentiae. More recently, however, the morphological phylogeny with monophyletic Holonota and serially paraphyletic Mixonomata and Desmonomata was supported by the results of Domes et al. (2007).
In light of the repeated evolution of ever-greater degrees of sclerotisation within the Holonota, it might seem surprising if one lineage was to do a complete volte-face and lose all trace of armour, but exactly this possibility has been suggested (Norton, 2001). The Astigmata are a lineage of mites related to the Oribatida, but ecologically distinct. While oribatids are armoured, slow-living, litter feeders, astigmatans are unarmoured, fast-breeding and mostly live in close association with other animals, often as parasites. Astigmata include such luminaries as Sarcoptes scabiei, the skin-burrowing monstrosity that causes scabies*. Despite these differences, it has been suggested that Astigmata are actually derived from oribatids through paedomorphosis (retention of juvenile characters as adults) - and specifically from Desmonomata, one of the most heavily armoured groups of oribatids. However, support for Astigmata as derived desmonomates remains equivocal. While supported by some morphological characters and gland chemistry, the suggestion has not garnered molecular support. Mauran et al. (2004) supported an oribatid ancestry for Astigmata, though not necessarily from Desmonomata (they also only included a single species of Astigmata in their analysis). Domes et al. (2007), analysing a larger selection of astigmates, rejected a position for Astigmata within Oribatida.
*Offhand, if you had to invent a name for a revolting skin condition, could you ever come up with a more appropriate-sounding term than 'scabies'?
Domes, K., M. Althammer, R. A. Norton, S. Scheu & M. Maraun. 2007. The phylogenetic relationship between Astigmata and Oribatida (Acari) as indicated by molecular markers. Experimental and Applied Acarology 42 (3): 159-171.
Maraun, M., M. Heethoff, K. Schneider, S. Scheu, G. Weigmann, J. Cianciolo, R. H. Thomas & R. A. Norton. 2004. Molecular phylogeny of oribatid mites (Oribatida, Acari): evidence for multiple radiations of parthenogenetic lineages. Experimental and Applied Acarology 33 (3): 183-201.
Norton, R. A. 2001. Systematic relationships of Nothrolohmanniidae, and the evolutionary plasticity of body form in Enarthronota (Acari: Oribatida). In Acarology: Proceedings of the 10th International Congress (R. B. Halliday, D. E. Walter, H. C. Proctor, R. A. Norton & M. J. Colloff, eds.) pp. 58-75. CSIRO Publishing: Melbourne.