Mites are notorious for not following the “rules.” For example, we generally teach that you can separate Insects from Arachnids by the number of legs, 3 pairs in insects, 4 pairs in arachnids. Simple, right? Mites are arachnids, and most have 4 pairs of legs, but not all
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- Larval parasitengone mite (Acariformes) with only 3 pairs of legs. The 4th pair develops in the embryo, but disappears before the larval stage.
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- The larva of Carios jerseyi, a fossil tick (Parasitiformes) with only 3 pairs of legs. Having a larva with only 3 pairs of legs is considered characteristic for all mites.
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- The larva of Diplothyrus lecorrei (Parasitiformes: Holothyrida). This larva is unusual because it actually retains (highly reduced) legs IV
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- Unidentified podapolipid female. Podapolipidae (Acariformes) are insect parasites. The number of legs in females in this family goes from 4 pairs to none at all. But …
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- … the males of podapolipid mites retain at least 3 pairs of legs, as in this Podapolipus berlesei (Acariformes)
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- Eriophyoidea, such as this Aceria anthocoptes (LT-SEM image; courtesy USDA-SEL) never have more than 2 pairs of legs, making them functionally tetrapods, just like lizards and mammals
And of course legs can get highly modified for various (known or unknown) purposes
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- Sometimes the modifications are minor, like the simplified legs IV in this phoretic nymph of Chaetodactylus (Acariformes: Astigmata)
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- A female of Podocinum pacificum (Parasitiformes), an example of using legs I as antennae, not as legs
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- Radfordia sp. (Acariformes: Myobiidae). In this group legs I (arrow) are modified to hold onto the hair of their hosts, small mammals
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- Dentocarpus taphozous (Acariformes: Chirodiscidae) hold on by clasping a hair of their bat hosts between legs I and II (arrows)
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- A male Pergamasus aequicornis (Parasitiformes, Parasitidae). In these mites males grasp females with legs II during mating. The pattern of spines on legs II is often species specific.
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- A male Edentalges quadrilobatus (Astigmata, Psoroptidae) from a giant ant eater. In this group males hold females using modified legs III.
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- Eatoniana plumipes (Parasitengona). Legs IV resemble pipe cleaners, function ????
About the Author: Dr. Hans Klompen is Professor in the Department of Evolution, Ecology and Organismal Biology & Director of the Ohio State University Acarology Collection.