The phylogeny of the arthropods is interesting in many respects, but the independent evolution of tracheation in several distinct clades is particularly noteworthy. Hexapods are the best known tracheate arthropods, and have developed a fairly complex spiracular control strategy for managing CO2 and O2 gas exchange in their tracheal system (see reviews by Kestler, 1985, Sláma, 1994, Lighton, 1994, 1996, 1998). This system is often referred to as the discontinuous gas exchange cycle, or DGC (sensu Lighton, 1996).
Recent evidence suggests that the DGC evolved independently in several non-hexapod clades of tracheate arthropods, including the ticks (Lighton et al., 1993a; Lighton and Fielden, 1995), solphugids (Lighton and Fielden, 1996) and diplopods (Klok et al., 2002). Karel Sláma has described respiratory cyclicity in a pseudoscorpion (Sláma, 1995); however, analysis of the DGC requires recordings of rates of CO2 emission (VCO2), which his microrespirometric system does not provide. The analysis of VCO2 has been shown by direct spiracular observation to be an excellent indicator of spiracular activity (Lighton et al., 1993b).
We here quantify the DGC of a pseudoscorpion, Garypus californicus, a native of seashore habitats on the western coast of the United States, as a function of temperature and thus metabolic flux rate. Pseudoscorpions possess two pairs of spiracles that are located laterally on the posterior margins of abdominal sternites 3 and 4. The spiracles open into four short tracheal trunks from which smaller branches ramify to supply the body. Pseudoscorpions are of particular interest because a clade of closely related tracheate arthropods, the solphugids (Shulz, 1990, Wheeler and Hayashi, 1998), have evolved a DGC that closely resembles that of insects (Lighton and Fielden, 1996).