Although possibly apocryphal, one John BS Haldane is claimed to have mused that the Creator had an inordinate fondness for beetles. With all due respect, though, I note that one might also make this claim for mites. Because of their small size and insinuatory nature, mites readily associate with many different kinds of organisms both externally and internally. These various symbiotic (=living together) relationships may be good for the mites and bad for the host (parasitism); good for the mites and good for the hosts (mutualism); good for the mites and of little importance to the host (commensalism); or mysterious (i.e. unstudied and unknown, but still living together). Now there are some insects that actively seek out mites with dinner in mind. The Microscydmus beetle above, about 0.63 mm in length, was a bit of a bully and attacked only mites half its size (Mollerman & Walter 2001 Acarology: Proceedings of the 10th International Congress). But in general, it is the mites that do the seeking, and beetles are often the habitat of choice.
Although mites like to hang out on beetles, not all beetles are equally attractive. Beetles that live in discrete habitats seem to have the most mites. Ground beetles (Carabidae) that like to hide under logs or rocks, for example, are a prime mite habitat. Many beetles have a peculiar fascination for dung and most of these have similarly prurient mites hanging on to them, at least to and fro the dung pat. Hitching a ride on a winged animal (phoresy – ‘to be carried’) is the norm in mites that live in patchy habitats such as flowers, tree holes, beach wrack, compost piles, carrion, and dung. Some dung mites like to hang onto dung flies, but by far the majority prefer to use beetles. Indeed, the fascination that mites have for beetles does seem to go well beyond the ordinary and well beyond the dung pat. Take almost any beetle that lives in wood and you may find them covered with mites. For example, the Mountain Pine Beetle (MPB Dendroctonus ponderosae) that is slaying its way through Canada’s lodgepole pines has at least a dozen different kinds of mites that hitch a ride from dead to dying tree on the beetles in Alberta. The better known and just as pine-unfriendly Southern Pine Beetle (SPB D. frontalis) has more than 100 species of mites associated with it over its broad range. These mites do just about anything one could imagine, from feeding on the beetle larvae, to feeding on potential beetle antagonists in the galleries, to feeding on fungi, to transmitting their own preferred fungi that may out compete the fungus the beetle grubs need to survive (and thus bring an outbreak to a halt).
One could go on and on about mites’ fondness for beetles, and I’m just the kind of person who might want to. However, it is Valentine’s Day and I’m making dinner for my wife, so I’ll end on an exceptional note. Even among the beetles, some beetles are more mitey than others. I think the pre-eminent position must be held by the beetles of the family Passalidae – Betsy Beetles. More than two dozen different families of mites make their homes, or at least their transportation, on Betsy Beetles. Many of these are only known from these beetles – although little is known about the interactions. In many cases, as with the extremely large (up to 5 mm in length) mites in the genus Megisthanus, only adults are found on the beetles. Although these mites have fearsome-looking chelicerae, they seem to do the beetles no harm and when living off the beetles in their galleries in logs, feed on other small invertebrates and possibly help keep beetle pests down. What seems most likely is that the adult male mites use the beetles as a place to find mates (usually only one male is found on a beetle, although several females are often present) and the mated females disperse to new logs on the beetles.
As I warned you, I could go on and on and on about beetle mites, but I think I’ll end here for the moment with one of the better known associations –
sextant sexton beetles (Nicrophorus spp.) and Poecilochirus mites (Parasitidae). The exceptional research by David Sloan Wilson and others has demonstrated that this classic mutualism between mites and beetles is actually a bit more complicated – it can vary from mutually beneficial to parasitism depending on circustances. And that is probably true of most of these associations. Although we like nice clear answers, simple dichotomies, real life is just one complication after another.