The evolutionary history of oribatid mites (aka beetle, moss, and armoured mites) must have been too full of adventure, because these are now the most unadventurous of mites. Adults are slow-moving, deliberate, and heavily encumbered in armour. Although their subordinal name, Oribatida (aka Oribatei, Cryptostigmata), may be derived from the Greek for ‘mountain’ (ori) and ‘one who roams’ (bat), it is hard to imagine oribatids lumbering uphill for any great distance (which begs the question as to how they came to be found in soil almost everywhere, including mountain tops and rainforest canopies). Without a doubt, though, defence against predation is one of the dominant themes in the history of the Oribatida.
One remarkable type of defence that has evolved at least three times in the Oribatida is called ptychoidy (more Greek, ‘ptych’, a fold). Ptychoid mites are able to fold their legs into their bodies and close the anterior shell-like aspis over the legs, giving rise to a less English-tongue-twisting name, ‘box mites’.
Box mite seems an especially appropriate name for the members of the family Phthiracaridae that have large rectangular plates covering the genital and anal openings that rather resemble the leaves of a box. For those of you wondering, yes that is the same ‘phthir’ as the Greek for louse – why ‘louse mites’ I have no idea, and why a somewhat related family are called ‘good lice mites’, Euphthiracaridae, is equally mysterious to me.
Sometimes ptychoidy doesn’t seem to be enough protection. Here are couple of versions of an SEM (derived from single digital grab) of an Australian phthiracarid box mite that also encases itself with a layer of soil. Presumably this serves as a visual or, more likely, tactile camouflage that increases the chance a predator will move on (‘get along now, nothing but a bit of dirt, your dinner is elsewhere’).
Above is a grayscale image that has been mostly masked from the low contrast or messy background of the original SEM. The arrows point to a few areas in between the setae, claws, and legs where I had yet to clean out the background. Masking around the setae is the most tedious part of creating these images. The image below has been false coloured to show the sclerotized body as a reddish brown seen through the dirty tan of the soil layer, the legs a lighter colour, and the soft cuticle at the base of the legs (soft cuticle there is necessary for the mite to be able to withdraw its legs into the body) a sort of fleshy colour. In life, this soft cuticle was a semitranslucent white, but I find white the most difficult colour to recreate without losing all detail.