SPIRACULAR CONTROL OF RESPIRATORY WATER-LOSS IN FEMALE ALATES OF THE HARVESTER ANT POGONOMYRMEX-RUGOSUS

It has been suggested that the discontinuous ventilation cycle (DVC) o bserved in many insects, including all ants described to date, is an a daptation to reduce respiratory water loss. To test this hypothesis, i t is necessary to measure respiratory water loss as a percentage of to tal water loss and to estimate what sustained rates of water loss woul d be in the absence of spiracular control. We used two independent tec hniques to measure real-time water loss rates in female alates of Pogo nomyrmex rugosus. The first measured water vapor emission and CO2 prod uction simultaneously using dual-wavelength infrared absorbance analys is (DWIRAA). The second measured water loss gravimetrically. Real-time measurement allowed the separation of cuticular water loss rates (int erburst) from water loss rates during the ventilation phase (burst) of the DVC. Cuticular permeability of P. rugosus female alates was only 27 ng h-1 CM-2 Pa-1, one-third of that reported for workers of the sam e species and the lowest yet reported for ants. Partly because of this low cuticular permeability, respiratory water loss represented a grea ter percentage of overall water loss (13 %) than has generally been re ported for other insects. The DWIRAA and gravimetric techniques gave e quivalent results. Peak rates of water loss during the burst phase wer e 2.8-fold higher than cuticular water loss rates alone (7.68 mg g-1 h -1 versus 2.77 mg g-1 h-1 at 25-degrees-C). This is a conservative est imate of water loss rates in the absence of spiracular control. Contra ry to findings in certain other insects that suggest a negligible role for respiratory water loss, we find that, in an insect that employs t he DVC and has low cuticular permeability, overall water loss rates ri se several-fold in the absence of direct spiracular control. Our findi ngs lend strong support to the water conservation hypothesis for the r ole of the DVC. In at least some insects, respiratory water loss rates can reach magnitudes significant enough, relative to other routes of water loss, for strong selective pressure to act on them.