SOCHA, J.J.**; HARRISON, J.F.; LEE, W.-K.; WESTNEAT, M.W.; Argonne National Laboratory; Arizona State University; Argonne National Laboratory; Field Museum of Natural History: Tubes squeeze and the air flows out: Correlated patterns of CO₂ emission and tracheal compression in the beetle, Platynus decentis

Active tracheal compression is a prominent feature of internal dynamics in multiple species of insects. Although the kinematics for one beetle species have been described, the physiological function of tracheal compression in gas exchange has not been determined. Possible roles of this mechanism include to convectively transport air or to increase the local pressure within the tracheae; alternatively, compressions may be a secondary consequence of other physiological processes. To determine if compressions are associated with excurrent gas exchange in the ground beetle Platynus decentis, we used simultaneous flow-through respirometry and phase enhanced x-ray imaging at the Advanced Photon Source (ANL) to synchronously record CO₂ emission and observe changes in diameter of the major thoracic tracheae. Each observed tracheal compression (which occurred at an average frequency and duration of 15 per min and 2 s, respectively) was associated with a local peak in CO₂ emission rate, with the start of each compression occurring simultaneously with the start of the local rise in CO₂ emission. No such peaks were observed during inter-compression periods. Furthermore, these peaks were observed both prior and post x-ray beam exposure, indicating that active tracheal compression is a natural physiological process and not an artifact of x-ray irradiation. In one specimen, the volume of CO₂ emitted per peak was 0.0032 � 0.0006 mm³, representing up to 30% of the emission volume at mid-peak. Although not unexpected, these data confirm that tracheal compression is indeed a gas exchange mechanism, and strongly suggest that internal convection is a major component of ventilation for Platynus decentis and other insects that employ this mechanism. Use of the APS was supported by the U.S. D.O.E., O.S., B.E.S., under Contract No. W-31-109-Eng-38.