Gas exchange in wind spiders (Arachnida, Solphugidae): Independent evolution of convergent control strategies in solphugids and insects

Solphugids are aberrant, archaic, tracheate arachnids that diverged from the other arthropod clades hundreds of millions of years ago. Their tracheal system is completely independently derived from that of insects. We investigated the dynamic gas exchange characteristics of two solphugid species ( Eremorhax titania and Eremobates sp.) from the Mojave Desert (Zzyzx, California). The solphugids utilized a discontinuous gas exchange cycle (DGC) almost identical to that of insects, comprising a closed-spiracle or C phase followed by a largely, or entirely, diffusive phase characterized by tissue-level O 2 uptake but very low CO 2 emission (functionally equivalent to the insect fluttering-spiracle or F phase) and finally an open-spiracle or O phase during which accumulated CO 2 escaped. O phase CO 2 emission volume was independent of temperature and metabolic rate (MR), comprising 20 μl g −1 body mass. DGC frequency increased with body temperature (and thus MR), and decreased with body mass, as predicted by the solphugids' sub-unity mass scaling exponent of MR on body mass. The two species did not differ significantly in any of the above relations. Both direct and indirect evidence suggest that the F phase hypoxic initiation and hypercapnic termination triggers are very similar in solphugids and insects. Highly convergent gas exchange strategies, together with associated control systems, have therefore evolved in two otherwise spectacularly divergent arthropod taxa.