Influence of periodic heartbeat reversal and abdominal movements on hemocoelic and tracheal pressure in resting blowflies Calliphora vicina.

In Calliphoridae and Drosophilidae, the dorsal vessel (heart and aorta with associated venous channels) is the only connection between the thorax and the abdomen. Hemolymph oscillates between the compartments by periodic heartbeat reversal, but both the mechanism and its influence on hemocoelic and tracheal pressure have remained unclear. The pumping direction of the heart regularly reverses, with a higher pulse rate during backward compared with forward pumping. A sequence of forward and backward pulse periods lasts approximately 34 s. Pulse rate, direction, velocity and the duration of heartbeat periods were determined by thermistor and electrophysiological measurements. For the first time, heartbeat-induced pressure changes were measured in the hemocoel and in the tracheal system of the thorax and the abdomen. The tracheal pressure changed from sub-atmospheric during backward heartbeat to supra-atmospheric during forward heartbeat in the thorax and inversely in the abdomen. The heartbeat reversals were coordinated with slow abdominal movements with a pumping stroke at the beginning of the forward pulse period. The pressure effect of the pumping stroke was visible only in the abdomen. Periodic hemolymph shift and abdominal movements resulted in pressure changes in the hemocoel and tracheal system alternating in the thorax and abdomen, suggesting an effect on respiratory gas exchange.