Sighting the mighty tiny mite

A couple of days ago, I mentioned oribatid mites, more-or-less in passing while looking at other moss, soil and decay fauna. As they are so important in the processes of decay and decomposition, feeding on all sorts of organic matter, I decided to return to my samples and see what else I could find. Unsurprisingly I found a few more but although I can't identify them to species (yet - I'm starting to have a look at Michael (1888) which you can download for free here), I thought it would be a good opportunity to give a bit of introductory information about this often unfamiliar group. So, what are oribatids?

Taxonomically, they form the order Oribatida within the Acari (mites) which are in turn arachnids. All are small (none more than 1.4mm in length) and those that I found were between 0.5 and 1.0mm. They are largely found in soils (especially in woodlands) and decaying matter (dead wood, leaf litter etc), and are important in soil processing and formation in much the same way as earthworms. Like other arachnids they have jaws known as chelicerae, and are eight-legged, but some other features may not be familiar.

Oribatid mite, approx 0.7mm long, ventral view

The pteromorph is a wing-or cloak-like extension of the carapace which wraps partly around the mite, presumably to armour it against damage from soil particles or predators. The genital shield is a round structure formed of two semi-circular halves which covers the reproductive structures - males have a penis-like structure called an aedeagus, much like that seen in beetles. Similarly, the anal shield is paired and covers the end of the gut.

Oribatid mite, approx 0.7mm long, ventral view

Unlike the more familar red spider mites (Trombidium spp.), these are not hairy/velvety but are smooth with only fine sculpturing and sometimes larger but fewer bristles. The legs do however have long bristles which perform a sensory function, and in fact the first pair of legs of some species are purely sensory (like antennae) and are no longer used for locomotion. There is a single median eye and with vision reduced (unsurprisingly as oribatids are mainly soil-dwelling), sensory bristles have become more important, not just on the legs, but also via the development of specialised bristles to form a 'pseudostigmatic organ' which can have a variety of shapes e.g. club or drumstick.

That is all the detail I can provide at present without simply summarising Michael (1888) and covering features not visible in my photographs. However, I am sure oribatids will show up on my invertebrate radar again, and I hope to be able to delve more deeply into their ecology and taxonomy.

Reference

Michael, A.D. (1888). British Oribatidae Vol.II. Ray Society, London.