Questioning paradigms: caste-specific ventilation in harvester ants, Messor pergandei and M. julianus (Hymenoptera: Formicidae)

Do developmental constraints in ant colonies limit gas exchange strategies to those displayed by female alates (presumptive queens)? In the xeric harvester ant genus Messor, we found that M. pergandei and M. julianus female alates ventilated highly discontinuously, as predicted, but M. julianus workers ventilated less discontinuously and M. pergandei workers (which occur in more xeric habitats) ventilated continuously. We present the salient characteristics of the discontinuous ventilation cycles of the species and the manner in which they are modulated by CO2 emission rates at a single temperature (24 °C). We demonstrate that, in M. julianus workers, open-spiracle phase CO2 emission rate only slightly exceeds overall CO2 emission rate, making discontinuous ventilation marginal, a state extrapolated in M. pergandei to continuous ventilation. However, workers are plainly capable of far greater rates of CO2 emission than when inactive at 24 °C, so the lack of discontinuous ventilation in M. pergandei under normoxic conditions is not likely to be imposed by physiological constraints and may, in fact, be a response to its xeric environment. We hypothesize ­ aside from phylogenetic effects ­ that discontinuous ventilation occurs primarily in insects that may experience hypoxic and hypercapnic conditions, 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 that it will not necessarily occur in castes or species routinely exposed to xeric but normoxic conditions.