Body Articulations

In theory, the acarine body has only two divisions: the capitulum (gnathosoma) and the idiosoma.  In practice, however, several kinds of mites have more or less subdivided idiosomas.  Most mites are holoid, that is they do not have a flexible articulation of the idiosoma between legs II-III that extends dorsally through the sejugal furrow.  The coxae of legs II and III may be approximate or distant in a holoid mite, and the dorsal shield may be entire, divided, or fragmented: the critical feature is the band of flexible cuticle completely encircling the idiosoma. 

However, some Sarcoptiformes are dichoid, that is they have a band of flexible cuticle encircling the body and running between legs II-III, and a few are trichoid, i.e. they have a second articulation of the idiosoma behind legs IV, or are tripartite and have two flexible bands on the opisthosoma (some Endeostigmata and Prostigmata).  Additionally, some dichoid mites have the ability to fold in their legs and mouthparts and close-up to resemble a seed, an ability called ptychoidy or jacknifing.

Some mites have incomplete divisions of a sclerotized idiosoma, i.e. plates separated by flexible cuticle, but the articulations do not extend ventrally.  Such dorsal divisions of the plates are called scissures, and 1-2 are commonly present in certain kinds of oribatid mites (e.g. Enarthronotides); many Mesostigmata have divided dorsal shields.  If several separate plates are present, the mites may seem to be segmented; however, true segmental folds are present only in some Endeostigmata.  In contrast, the important plant-parasites in the Eriophyoidea have worm-like bodies with a series of annulations, a condition also found in some Endeostigmata.

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