Many large arachnids (e.g., spiders, scorpions, whip-scorpions) have one or more pairs of slit-like openings to internal book lungs. Mites never have book lungs and exchange gases either directly through the cuticle or via tracheal spiracles—often with an associated peritreme.
Spiracles and tracheae have evolved independently in many groups of terrestrial arthropods. Hexapods and myriapods with tracheae typically have paired stigmatal openings on several thoracic and abdominal segments, and rarely just behind the head (some springtails) or at the base of the antennae (symphylans).
The spiracular openings of mites, when present, are usually above or behind the bases of legs III-IV (often appearing ventral on a slide-mounted specimen), on the leading edge of the idiosoma, or between the cheliceral bases.
Mesostigmatic mites have a single pair of stigmatal openings above the bases of legs III–IV (often appearing ventral on a slide-mounted specimen). Most of the time the stigmatal openings connect to a relatively long peritreme (shallow gutter) that runs along the bases of the legs, typically on a narrow plate. In some species, the peritreme is very short or absent.
Hard ticks have a single pair of stigmatal openings on sieve plates behind the bases of legs IV.
Many acariform mites (especially Oribatida) have a pair of well developed bothridial sensilla (see Character 26d) on the prodorsum. The cup-like bases (bothridia) of these structures resemble a stigmatal opening and were once thought to be respiratory organs, but apparently this is rarely true, and their primary function appears to be sensing air currents. The sensilla come in various shapes from setiform to pectinate to plumose to club-shaped. They are variously called pseudostigmatic organs, trichobothria, or bothridial sensilla.
Horn-like emergent peritremes occur in a few groups of Prostigmata.
Some Oribatida have 4 or more pairs of secretory structures called area porosae. This octotactic system is useful for identification of different groups, and is easily recognized in some taxa. Some of these porose areas do appear to have a respiratory function.
The Prostigmata is defined by it's prodorsal stigmatal openings, typically between the cheliceral bases or rarely on the 'shoulders' area (Heterostigmata). In some prostigmatans, a peritreme is formed on the edge of the prodorsum or on the cheliceral bases (or stylophore, if present).
Author: David E. Walter
Content last updated Sept. 2006, unless indicated otherwise
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