Friday, October 29, 2010
Halloween is the time of year for creepy things. It’s also a time for wearing something you might not wear on any other day. On both counts, it’s a good time for a story about a beetle wearing a multi-layered coat of mites. Just such a beetle turned up on my doorstep a little ahead of Halloween on October 8.
What are mites? Mites are arachnids (Class Arachnida) like spiders and daddy longlegs. They are members of the order Acari, a word that is derived from the Greek “akares” meaning tiny. Not surprisingly, they are very small. There are around 40,000 species known so far and most experts think that represents only around 5% of all that there are. Ticks and chiggers are in the same order as mites. Like other arachnids, mites have eight legs. Immature stages (i.e. mite larvae or nymphs) may have only six legs. They are slow moving wingless creatures.
How small are they? Most mites are smaller than 1 millimeter long. At the very tiny end of the scale there are those that live inside a bee’s trachea. There are also those that live within a human hair follicle – not only live there but raise a family. Is this creepy enough?
Lifestyle: There are several suborders of mites. One of the big groups is the Orabatid mites which live in leaf litter and in the soil. These little creatures are scavengers and play an important role in breaking down plant matter and increasing soil fertility. Another large group is the Parasitiformes which as the name suggests, includes a lot of parasites like the mites that cause mange and scabies. But this group also includes many scavengers and predators. The mites on the beetle are in the genus Poecilochirus. They are predators and are among the larger of the mites but they have an interesting set of problems. They don’t have a taste for beetle; they eat fly eggs and fly larvae; but they are slow moving, wingless, and need to travel relatively far to find their prey. For some mites, the answer to the transportation problem is blowing in the wind. For others like these beetle-riding mites, the answer is hitchhiking.
Commensalism: Commensalism is a relationship where one party benefits at no cost to the other. An example is insect-eating birds that associate with horses and cattle which flush insects from the grass with their hooves. (So there is such a thing as a free lunch.) There is another type of commensalism known as “phoresy” in which one animal uses the other only for transportation. This is the case with the mites on the beetle and it may stretch the definition of commensalism in a curious way. The beetle derives benefit from its payload of mites.
What kind of beetle? Any beetle won’t do. The mite needs to choose a beetle that is travelling to a restaurant that serves fly eggs and fly larvae. Carrion beetles and burying beetles lay their eggs on the same small dead animals to which flies are attracted for the same reason. The beetle larvae and fly larvae both feed on carrion. The beetle in this photo is a burying beetle (Nicrophorus) and it is acting like the kind of bus you sometimes see in third world countries – it’s a tad overloaded. The mites are good at holding on. When I picked this beetle up to photograph it, none of the mites came off. After sitting on my hand for a minute or two, the beetle flew off. Its flight was compromised and it looked like there was going to be many stops on the way to the restaurant. I was surprised that it was able to fly at all. Usually, when I find a burying beetle, it has only three or four mites.
Payback for the ride: The mites are not free-loaders. By eating the fly eggs and larvae in the carrion, they are eliminating the competition for the beetle larvae. Fewer fly maggots, more food for the beetle offspring. But it’s not all paradise in a piece of carrion. If there are too many mites and not enough flies have visited and laid eggs, the mites may have to eat beetle eggs. If you are pondering deep questions like “how many spiders can dance on the roof of a general store” or “how many mites can ride on a burying beetle,” consider that evolution has probably defined the “right” amount of mites as the most the beetle can accommodate while still being able to fly. Not sure about spiders and the store.
Sources: Borror and Delong’s Introduction to the Study of Insects, Charles A. Triplehorn and Norman F. Johnson; Insects: Their Natural History and Diversity, Stephen A. Marshall
Submissions: Please feel free to claim this space and write the Biodiversity Corner on any species that occurs in the wild and you have found in Carlisle. ∆
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