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Peritremes and Stigmata
The respiratory systems in the Acari are the basis for their higher classification. In the Acariformes, spiracular openings (stigmata) are primitively absent. Hence, sarcoptiform mites were once divided into Cryptostigmata ('hidden stigmata'), Astigmata ('without stigmata'), and Endeostigmata ('internal stigmata'). The Prostigmata have uniquely derived respiratory openings on the prodorsum, typically between the chelicerae or on its lateral margins. Many oribatid mites have uniquely derived but obscure respiratory systems; they also have secretory organs called area porosae that were once thought to be respiratory.
In the Parasitiformes, spiracular openings and a tracheal system were primitively present as four pairs of dorso-lateral stigmata in the Opilioacarida that are added sequentially during development. In Holothyrida, ticks (Ixodida), and Mesostigmata these are reduced to a single pair of spiracular openings in the region of legs IV. In soft ticks (Argasidae) the stigmatal opening is lateral, but in hard ticks it is present on a distinctive sieve plate behind legs IV. In Mesostigmata, the stigmata are lateral and usually associated with peritremes, microtuberculate and slit tubes that runs anteriorly from the stigma (although these are sometimes regressed). Peritremes may run to the level of legs I or beyond (usually as a straight line but sometimes contorted) or be variously reduced or vestigial.
Peritremes as such are never present in the Sarcoptiformes, but analogous structures are found in some Prostigmata, either on the prodorsum near the cheliceral bases, or as horn-like emergent peritremes, or as a line or network dorsally on a gnathosomal capsule.