Patterns of Hemolymph Pressure Related to Tracheal Tube Collapse in the Beetle Pterostichus commutabulis

Rhythmic collapse and reinflation of tracheal tubes is a form of active ventilation that augments convective gas exchange in multiple orders of insects. The underlying mechanism driving this phenomenon is not known. Among other things, tracheal tube collapse could be caused by either direct impingement of trachea or by a difference of pressure gradients between the intra-tracheal air and the surrounding hemolymph. To determine the relationship between hemolymph pressure and tracheal tube collapse in the ground beetle (Pterostichus commutabulis), we performed direct measurements of hemolymph pressure inside the beetle's prothorax while simultaneously using synchrotron phase contrast imaging to observe morphological changes in the trachea. We observed that a pressure pulse co-occurred with every tube compression observed throughout the body, suggesting that pulses in hemolymph pressure are responsible for tracheal collapse. To assess the effects of the experimental x-ray conditions imposed on the subjects during imaging, hemolymph pressure was also directly measured in the prothorax of beetles less restricted in non-x-ray trials. To compare the pressure patterns in the two experiments, a novel method of identifying and analyzing pressure pulses was developed and applied to the data sets. The comparison provides the first quantitative characterization of a directly measured hemolymph pressure environment, and demonstrates strong similarities in the pressure patterns recorded in both tests. However, pulses occurring during the x-ray experiments exhibited larger average magnitudes. Further video analysis however shows that collapse of the primary tracheal tubes was observed to occur even in the presence of the smallest simultaneously measured pressure pulse (1.01 kPa), suggesting that collapse of the primary tracheal tubes.