Meeting Abstract
Several adaptive and non-adaptive hypotheses for the evolution and expression of discontinuous gas exchange (DGE) in insects have been proposed. Our recent work on orthopterans joins a large body of evidence, which questions the generality of the available hypotheses. Results from interspecific and intraspecific comparisons and an experimental evolution study are not consistent with predictions of adaptive and non-adaptive explanations. Still, they provide insights into the mechanistic basis of DGE in actively ventilating insects. Simultaneous electromyogram and respirometry reveal two distinct forms of ventilation in intricate interplay with three possible spiracular states. During the burst of gas exchange, alternating closure of abdominal and thoracic spiracles is coupled with activity of abdominal ventilatory muscles to achieve fast unidirectional ventilation through major tracheal trunks. Ventilation frequency during this phase gradually decreases in response to changing tracheal gas composition. During the interburst, when there is negligible gas exchange with the environment, locusts ventilate their tracheal content in order to facilitate diffusion through the fine tracheoles. This is achieved through slow ventilation activity, coupled with increased activity of spiracular closer muscles, and is triggered by decreasing tissue O2 levels. A classic “flutter phase”, characterized by short bursts of CO2 emission, is missing in locusts in normoxia, but is evident under hypoxic conditions. These short events of CO2 release result from fast ventilation events and transient relaxation of spiracle closure muscles. A model consisting of two separate pattern generators with converging output could explain the observed motor activity throughout the DGE cycles.
