Most arthropods have segmentally arranged sclerotized plates separated by soft, flexible cuticle. Clearly defined segmental plates visible on the underside of the abdomen are a clear indication that an arthropod is not a mite.
Misinterpretations allowed (you can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options): Ricinuleids, some primitive oribatid mites, and some prostigmatans have series of plates that could be mistaken for segments (see Feature 4c).
Many insects (e.g., beetles), a variety of armored mites (especially Oribatida, Uropodidae, and Labidostommatidae), and some opilionids and spiders are well protected against predation by thick sclerotization that obscures any segmentation that may be present dorsally.
Misinterpretations allowed: Ball-rolling millipedes and isopods have well-defined segments, and so do not meet this feature state, but are allowed as misinterpretations. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)
The bivalved carpace of terrestrial seed shrimp (Ostracoda) is distinctive.
Misinterpretations allowed: Though distinctive, the ptychoid body shape of box mites (see Feature 5d) or some ball-rolling beetles may seem similar and are allowed here as misinterpretations. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)
Ptychoidy or jacknifing has evolved in several groups of oribatid mites, most notably in the box mites. When attacked or otherwise disturbed, these mites are able to retract their legs (as a penknife is folded away) and close down the front end of the body against the rear portion and present a smooth, featureless surface to the attacker. Some insects and mites are similarly able to retract their legs.
Misinterpretations allowed: Seed shrimp (Ostracoda) and water fleas (Cladocera) have a carapace (hinged in ostracods) into which the body is withdrawn that could be confused with ptychoidy, and are so included here as a misinterpretation. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)
Ball rollers are strongly segmented and capable of rolling into a ball that presents the dorsal surfaces of the sclerotized segments to an attacker. This is a common ability in isopods (pill bugs) and occurs in some millipedes (pill millipedes) and insects.
Misinterpretations allowed. Seed shrimp (Ostracoda) have a bivalved carapace into which the body is withdrawn that could be confused with ball rolling and some mites are able to protect their bodies with ptychoidy or movable pteromorphs, and so they are included here as misinterpretations. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)
Some groups of oribatid mites retain the notogasteral portion of each molt and carry them stacked on their backs.
A few groups of oribatid mites carry the notogasteral portion of one or more immature molts on their backs. These can be raised in a pagoda-like series or only the final (tritonymphal) molt may be carried.
Some insects and mites produce waxy growths from their cuticles.
Some oribatid, uropodine and microgyniine mites produce a thick, skin-like cerotegument that encloses their entire body and often bears reticulate or papillate ornamentation.
Any arthropod may have adhering soil or debris, but some oribatid and mesostigmatic mites and a few opilionids coat their bodies in a layer of soil, presumably as tactile and/or visual camouflage. Setae may project through this layer, but often the only parts of the body surface visible are the joints of the legs, junctures between sclerotized plates, and areas capable of retraction, such as the legs and margins of the aspidosoma in the box mites below. The body surface can be seen by peeling away the soil layer, as in the box mite in the center below.
Other mites (e.g., some Tarsonemidae (Fungitarsonemus), some Uropodina), and some insects collect pieces of debris, often including balls of algae, lichens, or pollen, and pile them up in mounds or layers on their backs (presumably using a glue or wax to attach the debris).
Many mites are protected by a pincushion of long, spine-like setae. Tufts of similar setae also protect polyxenid millipedes and some insects.
A few very strange deep-soil mites (Nematalycidae), gall and rust mites (Eriophyoidea), and Onychophora have annulate bodies that appear worm-like. Some immature insects are also worm-like.
Author: David E. Walter
Content last updated Sept. 2006, unless indicated otherwise
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