It is clear from these results that although G. californicus does employ a DGC, it is a marginal one, lacking stringent spiracular closure. In fact the C phase contributed up to 45% of overall CO2 release at high temperatures, whereas the DGC of G. californicus took the form of small peaks of CO2 release "riding" on a large interburst baseline. Although solphugids, a clade of closely related tracheate arthropods (Shultz, 1990, Wheeler and Hayashi, 1998), have independently evolved a stringent, three-phase DGC that closely resembles that of insects (Lighton and Fielden, 1996), pseudoscorpions have apparently developed only a modest degree of spiracular control.
Some of this difference may be explained by total-animal CO2 buffering capacity. Pseudoscorpions emit only about a tenth of the volume of CO2 per unit mass in the O phase that hexapods and other tracheate arthropods do (see, e.g., Lighton et al., 1993; Quinlan and Lighton, 1999). For example, the fairly closely related clade of the solphugids release 20 µl mg-1 (Lighton and Fielden 1996), approximately six to eightfold more. The ability of pseudoscorpions to regulate CO2 release is therefore minimal compared with animals that employ the "classic" DGC. This is reflected in their high DGC frequency, about double to eight-fold that of similar sized insects (Lighton, 1991b, 1996). Investigation of CO2 buffering capacity in pseudoscorpions would be worthwhile; it is worth noting, in this context, that the primary trigger for the burst phase in pseudoscorpions has recently been shown to be hypoxia, rather than the more conventional hypercapnia (Lighton and Joos, 2002).
The temperature dependence of the pseudoscorpions' DGC revealed no surprises. While overall metabolic Q10 was 2.16, the Q10 of DGC frequency was 1.82. Thus DGC frequency increased with temperature more slowly than rate of CO2 emission, indicating that pseudoscorpions modulate both frequency and volume of gas exchange to accommodate changes in metabolic flux rates. The increase of CO2 volume per DGC with increasing metabolic flux rates was mostly confined to the interburst phase. Although the O phase VCO2 increased with temperature with a Q10 1.97, O phase volume decreased because of a steep decrease in O phase duration. The majority of insects studied so far modulate DGC frequency, rather than volume, to accommodate changes in metabolic flux rates (see Lighton 1998 and 1996 for discussions).
It is certainly in the magnitude of interburst CO2 emission that the DGC of G. californicus differs most radically from other tracheate arthropods that employ a DGC. The fact that interburst VCO2 increases rapidly with temperature demonstrates that G. californicus is unable to sustain sufficient oxygen flux rates without losing a large amount of CO2 and thus water vapor at the same time. This in turn suggests that the trans-spiracular gradient of oxygen partial pressure is minimal in this species. The likely presence of a minimal trans-spiracular oxygen partial pressure gradient, and an accompanying high rate of interburst CO2 release, suggests from respiratory water loss considerations that the O phase trigger in G. californicus may be predominantly hypoxic rather than hypercapnic. Examining the effects of ambient oxygen partial pressure on the DGC of this species can test this hypothesis. This work has now been performed and a hypoxic O phase trigger in G. californicus has been confirmed (Lighton and Joos, 2002).
If G. californicus is unable to maintain a protracted and steep oxygen partial pressure gradient across its spiracles, then its rate of respiratory water loss in its interburst phase will be higher than in comparable animals that employ a "classic" DGC with a fully constricted C phase and a long F phase marked by low VCO2 and endotracheal oxygen partial pressures as low as 4 kPa (see Lighton, 1998 and references therein). This is amenable to experimental testing. It is perhaps significant that pseudoscorpions are limited to marginal habitats in areas where water vapor pressure is likely to be high, such as in soil litter and under stones. Of course, the link (if any) between the DGC and respiratory water loss is somewhat controversial (see Lighton, 1998 for a discussion). Notwithstanding the above, the niche range of pseudoscorpions appears to be wider than that of another tracheate arthropod, the harvestperson (opiliones), in which spiracular control is minimal, and no cyclicity, let alone discontinuity, of external CO2 emission is evident (Lighton 2002).
We thank the National Science Foundation (IBN 9306537 and 9603873 to JRBL) and the Packard Foundation (fellowship to JRBL) for financial support, and Sable Systems International for the loan of instruments. We thank Anke Schmitz for helpful discussions, Michael Quinlan for technical assistance at an early stage of this study, and Robbin Turner for assistance in manuscript preparation. JRBL was also supported by personal funds.