In a previous post, I introduced you all to the oribatid mites. Oribatids come in a wide range of varieties, and the animal in the figures above is a member of the oribatid family Galumnellidae. Galumnellids belong to a group of oribatids, the galumnoids, marked by their well-developed pteromorphs: the roughly triangular structures at either side of the front of the body. Many oribatids have pteromorphs developed to a greater or lesser degree, but the pteromorphs of galumnoids are particularly noteworthy for their size and for the development of a hinge between the pteromorph and the main body, so that the pteromorph can be folded down to cover the legs for protection (other species have the pteromorphs as fixed outgrowths of the body). The name 'pteromorph', of course, means 'wing-shaped', and you can readily find cases where galumnoids have been referred to as 'winged mites' (especially in older publications). Woodring (1962) even suggested that galumnoids might provide a useful analogy for the evolution of wings in insects. However, pteromorphs are not actually wings like those of insects, being used only for protection, not flight. In animals as small as oribatids, the relative viscosity of the air becomes very high, not to mention the relative force of small air movements. Vary small arthropods that move aerially either develop long hairs or similar structures so that they can be passively lifted and carried by the breeze (like the line of silk produced by ballooning spiders) or have reduced wings with long fringes of hairs to maintain wing surface area while minimising air resistance (such as mymarid wasps, thrips or ptiliid beetles). A solid plate like the galumnoid pteromorph would be to difficult to move*.
*Similar issues affect suggestions that the absent fossil record of the earliest winged insects may indicate that flight evolved at small sizes. It seems almost certain that the first flying insects were relatively large.
The Galumnellidae can be distinguished from other galumnoid mites by the lack of protruding lamellae on the prodorsum (the top of the 'head'), the pointed rather than rounded rostrum, and the shape of their chelicerae. The chelicerae of galumnellids are long and slender, compared to the shorter, stronger chelicerae of their relatives in the Galumnidae. Galumnella has been shown in the laboratory to be panphytophagous (Badejo & Akinwole 2007)—that is, it will accept any type of plant or algal food, both living and dead.
Badejo, M. A., & P. O. Akinwole. 2007. Preliminary study of the feeding habits of seven species of oribatid mites from Nigeria. Systematic and Applied Acarology 12: 121-125.
Balogh, J., & P. Balogh. 1992. The Oribatid Mites Genera of the World, 2 vols. Hungarian Natural History Museum: Budapest.
Woodring, J. P. 1962. Oribatid (Acari) pteromorphs, pterogasterine phylogeny, and evolution of wings. Annals of the Entomological Society of America 55 (4): 394-403.