Flight-motor-driven respiratory airflow increases tracheal oxygen to nearly atmospheric level in blowflies (Calliphora vicina)

Abstract

It is widely accepted that an efficient oxygen supply and removal of CO 2 in small flying insects are 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 downstroke of the wings and is expired through the metathoracic spiracles (Sp2) during the upstroke. 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 subatmospheric, conditioned by the only slightly open spiracles. In a split flow-through chamber experiment, the CO 2 released through the Sp2 confirmed this unidirectional respiratory gas flow, implicating an inner tracheal valve. In the thoracic tracheal system, the P O2 during flight exceeds the high resting P O2 by 1-2 kPa, reaching nearly atmospheric values. In the abdominal large air sacs, the P O2 drops during flight, probably due to the accumulation of CO 2 . Periodic heartbeat reversals continue during flight, with a higher period frequency than at rest, supporting the transport of CO 2 via the haemolymph towards the metathoracic tracheae and abdominal air sacs.

Structure of the thoracic spiracular valves and their contribution to the unidirectional gas exchange in flying blowfliesCalliphora vicina

“…It was hypothesised that the pressure difference at the spiracles can only be attained by the involvement of some valve (Wasserthal, 2015). The aim of the present paper was to analyse the structural preconditions and activity of the valves at the thoracic spiracles during rest and locomotion, especially during flight, to test the hypothesis that valves contribute to unidirectional airflow as postulated previously (Wasserthal, 2015).…”

“…For all experiments, the flies were tethered (using Pattex, Henkel, Düsseldorf, Germany; Fixogum Marabu, Tamm, Germany) as described previously (Wasserthal, 2015). The action of the interior valve lids was observed after carefully removing the filter trichomes.…”

“…He concluded that the spiracle opening matches the metabolic need by diffusion. In flying Caliphora vicina at Sp1 and the metathoracic spiracle (Sp2), the tracheal pressure, CO 2 and H 2 O emission were recorded (Wasserthal, 2015); during flight, a unidirectional respiratory gas flow through the anterior body was measured with an inflow of fresh air through the anterior spiracles (Sp1) and an outflow of CO 2 -loaded air through the posterior spiracles (Sp2+Sp abdomen). The directed airflow is dependent on volume changes of the tracheal system due to deformations of the flight apparatus.…”

“…The directed airflow is dependent on volume changes of the tracheal system due to deformations of the flight apparatus. This auto-ventilation leads to a sub-atmospheric pressure in the thoracic tracheal system with a mean subatmospheric pressure at Sp1 and a mean over-atmospheric pressure at Sp2, resulting in an increase of the tracheal P O2 (Wasserthal, 2015). It was hypothesised that the pressure difference at the spiracles can only be attained by the involvement of some valve (Wasserthal, 2015).…”

“…This auto-ventilation leads to a sub-atmospheric pressure in the thoracic tracheal system with a mean subatmospheric pressure at Sp1 and a mean over-atmospheric pressure at Sp2, resulting in an increase of the tracheal P O2 (Wasserthal, 2015). It was hypothesised that the pressure difference at the spiracles can only be attained by the involvement of some valve (Wasserthal, 2015). The aim of the present paper was to analyse the structural preconditions and activity of the valves at the thoracic spiracles during rest and locomotion, especially during flight, to test the hypothesis that valves contribute to unidirectional airflow as postulated previously (Wasserthal, 2015).…”