The hygric hypothesis does not hold water: abolition of discontinuous gas exchange cycles does not affect water loss in the ant Camponotus vicinus.

The discontinuous gas exchange cycle (DGC) of insects and other tracheate arthropods temporally decouples oxygen uptake and carbon dioxide emission and generates powerful concentration gradients for both gas species between the outside world and the tracheal system. Although the DGC is considered an adaptation to reduce respiratory water loss (RWL) - the "hygric hypothesis" - it is absent from many taxa, including xeric ones. The "chthonic hypothesis" states that the DGC originated as an adaptation to gas exchange in hypoxic and hypercapnic, i.e. underground, environments. If that is the case then the DGC is not the ancestral condition, and its expression is not necessarily a requirement for reducing RWL. Here we report a study of water loss rate in the ant Camponotus vicinus, measured while its DGC was slowly eliminated by gradual hypoxia (hypoxic ramp de-DGCing). Metabolic rate remained constant. The DGC ceased at a mean P(O2) of 8.4 kPa. RWL in the absence of DGCs was not affected until P(O2) declined below 3.9 kPa. Below that value, non-DGC spiracular regulation failed, accompanied by a large increase in RWL. Thus, the spiracular control strategy of the DGC is not required for low RWL, even in animals that normally express the DGC.

Thermolimit respirometry: an objective assessment of critical thermal maxima in two sympatric desert harvester ants, Pogonomyrmex rugosus and P. californicus.

The critical thermal maxima (CT(max)) of two sympatric, diurnal, thermophilic harvester ants from the Mojave Desert, USA (Pogonomyrmex rugosus and P. californicus) were measured by ramping their temperature upwards at a rate of 0.25 degrees C min(-1) during flow-through respirometry with optical activity detection. Rates of CO(2) emission ((CO(2))) increased exponentially to plateau values that were twice as high in P. californicus as P. rugosus on a mass-specific basis. (CO(2)) then fell sharply, during which gross motor activity (measured optically) and spiracular control (measured from (CO(2)) variation) abruptly ceased, yielding two independent measures of CT(max). As determined by loss of muscular coordination, the CT(max) of Pogonomyrmex rugosus was 51.57+/-0.38 degrees C (mean +/- S.D., while that of Pogonomyrmex californicus was 51.74+/-0.25 degrees C. As determined by loss of spiracular control, the CT(max) of Pogonomyrmex rugosus was 51.59+/-0.35 degrees C, while that of Pogonomyrmex californicus was 51.78+/-0.37 degrees C. In each species a pronounced post-mortal peak of (CO(2)) was observed. The major ecological and behavioral differences of the two species are not reflected in their CT(max) values, which do not differ significantly. 'Thermolimit respirometry' allows CT(max) to be estimated objectively with coefficients of variation (S.D./mean) <1%, lending confidence to comparisons between species or treatment groups.