Wasserthal, L. T. (2015): Flight-motor-driven respiratory airflow increases tracheal oxygen to nearly atmospheric level in blowflies, Calliphora vicina. J. Exp. Biol. 218, 2201-2210.

It is widely accepted that an efficient oxygen supply and removal of CO
2 in small flying insects is sufficiently performed by diffusion with open spiracles. This paper shows that in the tethered flying blowfly, gas exchange occurs by autoventilation and unidirectional airflow. The air is inspired through the mesothoracic spiracles (Sp1) during the down-stroke of the wings and is expired through the metathoracic spiracles (Sp2) during the up-stroke. This directed airflow through the thoracic tracheal system was documented by pre-atrial pressure measurements at the Sp1 and Sp2, revealing a sub-atmospheric mean pressure at the Sp1 and an over-atmospheric mean pressure at the Sp2. In the mesothoracic air sacs, the mean pressure is sub-atmospheric, conditioned by the only slightly opened spiracles. In a split flow-through chamber experiment, the CO2 released through the Sp2 confirms this uni-directional respiratory gas flow, implicating an inner tracheal valve. In the thoracic tracheal system, the PO2 during flight exceeds the high resting PO2 by 1-2 kPa, reaching nearly atmospheric values. In the abdominal large air sacs, the PO2 drops during flight, probably due to the accumulation of CO2. Periodic heartbeat reversals continue during flight, with a higher period frequency than at rest, supporting the CO2 transport via haemolymph towards the metathoracic tracheae and abdominal air sacs.

Cal F2-11

Tethered blowfly ready for monitoring air sac pressure and heartbeat during flight. From "Inside JEB"

Calli flight 2015 3468

Calliphora exhibits full wingbeat amplitude when tethered elastically at the Mesonotum.


Fig. 2. Pre-atrial pressure differences between Sp1 and Sp2 during two wing-beat cycles. (male15*/2000) (A) At both tubed right spiracles, the pressure increases during up-stroke and decreases during down-stroke of the wings. The mean pressure at Sp1 (green dashed line) is negative, whereas the mean pressure at Sp2 is positive (red dashed line). 0 = atmospheric pressure. The wing-beat (black curve) is recorded by its shadow that is cast on a silicium photo diode. Sampling rate: 40 kHz, *captured wild specimen.


Fig. 4. PO2 and relative intratracheal pressure in the scutellar air sac during rest and tethered flight. (A) During flight, the mean pressure (green curve) decreases to sub-atmospheric and the PO2 increases to almost atmospheric (female14/2009, 38 days old). (B) After typical resting gas exchange characterised by periodic heartbeat reversals, the PO2 exceeds the resting value of 20 kPa by 1 kPa during flight. After flight, the PO2 decreases (female17*/2009 ). (C) Flight period between phases of rest with heartbeat reversals. Before or at the onset of the flight, the PO2 decreases rapidly and then rises 1 kPa over the resting values. After the flight stops, the PO2 decreases to the previous resting values (female 2/2011, 137 days old). Black bars, backward periods of heartbeat, visualised by periodic pressure changes, *captured wild specimen.


Fig. 8. CO2 emissions in a flying blowfly with tubed Sp2 in a split-chamber flow-through experiment. (female12*/2000) (A) The insect compartment with the first thoracic and all abdominal spiracles is connected directly to the IRGA, while the tubed posterior thoracic spiracles (Sp2) open into the empty compartment, which is connected to the CO2 scrubber. Only a small amount of CO2 is emitted during each flight phase from the Sp1 + abdominal spiracles. (B) In the crosscheck, only the Sp2 tubes are connected to the IRGA via the empty compartment. The bulk of CO2 is emitted from the Sp2, *captured wild specimen.

Reprint available under: