QUESTIONING PARADIGMS - CASTE-SPECIFIC VENTILATION IN HARVESTER ANTS,MESSOR PERGANDEI AND M-JULIANUS (HYMENOPTERA, FORMICIDAE)

Do developmental constraints in ant colonies limit gas exchange strate gies to those displayed by female alates (presumptive queens)? In the xeric harvester ant genus Messor, we found that M. pergandei and M. ju lianus female alates ventilated highly discontinuously, as predicted, but M. julianus workers ventilated less discontinuously and M. pergand ei workers (which occur in more xeric habitats) ventilated continuousl y. We present the salient characteristics of the discontinuous ventila tion cycles of the species and the manner in which they are modulated by CO2 emission rates at a single temperature (24 degrees C). We demon strate that, in M. julianus workers, open-spiracle phase CO2 emission rate only slightly exceeds overall CO2 emission rate, making discontin uous ventilation marginal, a state extrapolated in M. pergandei to con tinuous ventilation. However, workers are plainly capable of far great er rates of CO2 emission than when inactive at 24 degrees C, so the la ck of discontinuous ventilation in M. pergandei under normoxic conditi ons is not likely to be imposed by physiological constraints and may, in fact, be a response to its xeric environment. We hypothesize - asid e from phylogenetic effects - that discontinuous ventilation occurs pr imarily in insects that may experience hypoxic and hypercapnic conditi ons, such as ant queens during claustral colony foundation and perhaps workers, within the nest environment; that discontinuous ventilation is not necessarily essential to reduce respiratory water loss; and tha t it will not necessarily occur in castes or species routinely exposed to xeric but normoxic conditions.