Oribotritia: Some Mites Just Want to be Left Alone

I'm aware that a lot of people would not automatically think of mites when categorising cuteness, but I dare you to look at the animal in the photo above (taken by Tom Murray) and tell me it's not adorable. This is a box mite of the genus Oribotritia, a cosmopolitan genus found on all the continents except Australia. More than 80 species of Oribotritia have been described so far, and there's probably more to come. Box mites are armoured mites with a body form known as 'ptychoid'; as described in an earlier post, this means that the legs can be drawn back close to the body, and the prodorsum (the articulated shield at the front of the body covering the mouthparts folded over to cover the soft parts. The figure below from Schmelzle et al. (2009) shows how it works. The mite on the left is Oribotritia banksi, the one on the right is a different ptychoid, Rhysotritia ardua:

This figure also shows some of the distinguishing features of Oribotritia. There are a number of families of ptychoid mites; interestingly, indications are that not all ptychoids are directly related to each other. Instead, the ptychoid morphology has evolved a number of times. Oribotritia belongs to the 'true' box mites, in which the notogaster (the main 'body' of the mite) is a single undivided dorsal plate, while the genital and anal plates are long and together occupy the entire length of the underside. The three main families of true box mites are the Phthiracaridae, Oribotritiidae and Euphthiracaridae. Phthiracarids have the ventral plates broad and the venter overall more or less U-shaped; the other two families have the venter narrow and triangular. In oribotritiids like Oribotritia, the genital and anal plates have another elongate pair of plates running outside them, but in euphthiracarids (like Rhysotritia ardua in the figure above), all the ventral plates have become fused into a single plate pair.

When fully withdrawn into its sclerotised shell, the ptychoid mite is pretty effectively sealed away from would-be predators. As well as this mechanical defense, glandular openings on the side of the notogaster secrete defensive oils to repel predators chemically. The overall aim is that the mite should simply be left unmolested to pursue its own interests: feeding on decaying vegetation.

REFERENCES

Balogh, J., & P. Balogh. 1992. The Oribatid Mites Genera of the World. Hungarian Natural History Museum: Budapest.

Schmelzle, S., L. Helfen, R. A. Norton & M. Heethoff. 2009. The ptychoid defensive mechanism in Euphthiracaroidea (Acari: Oribatida): a comparison of muscular elements with functional considerations. Arthropod Structure and Development 38: 461-472.