Functioning of unidirectional ventilation in flying hawkmoths evaluated by pressure and oxygen measurements and X-ray video and tomography

Flying sphingids generate unidirectional ventilation with an inflow through the anterior thoracic spiracles and an outflow through the posterior thoracic spiracles. This phenomenon was documented by the CO2 emission and tracheal air pressure in split-chamber experiments in preceding studies. In the present study, we evaluated the function of the air pump mechanism by measuring the tracheal pressure and PO2 in the air sacs and monitoring the wing beat using photocells. Microelectrodes recorded the abdomen flexing muscles and abdominal transverse muscle septum. The crucial structure was the vertical mesophragma, with longitudinal flight muscles attached anteriorly and large fused metathoracic air sacs posteriorly, continuous to the first abdominal segment. Longitudinal flight muscles and abdomen lifting muscles contracted synchronously, producing positive pressure pulses within the mesothoracic air sacs. In the scutellar air sacs, the PO2 with starting full flight was elevated to 18-20 kPa, with a pressure increase of 35-50 Pa. In contrast, in the metathoracic air sacs, the O2 concentration during flight could rise to 10 kPa, then decline to 5±1 kPa. The metathoracic air sacs provided compliance for ventilation by the flight muscles. The initial rise and subsequent decrease of the PO2 in these posterior metathoracic air sacs indicated the unidirectional flow path of the air used. Serial X-ray frames of flying Acherontia atropos visualised the cyclic phragma movement and volume changes in the metathoracic air sacs. The results showed that the contracting dorsolongitudinal flight muscles expanded the metathoracic air sacs, acting as a suction pump.