Respiration in insects is thought to be independent of the circulatory
system because insects typically lack respiratory pigments and
because oxygen transport occurs in the gaseous phase through a
ramified tracheal system by diffusion and convection directly to the
tissues. In the blowfly, as in other insects with periodic heartbeat
reversal, the haemolymph is periodically shifted between the anterior
body and abdomen, exerting alternating pressure changes on the
compliant tracheae in the thorax and in the abdomen. Simultaneous
pressure and O2 optode measurements show that, during negative
pressure periods, the tracheal partial pressure of oxygen (PO2)
increases by 0.5 kPa. In the quiescent fly, tracheal PO2 is rather high
(17.5.–18.9 kPa), although the thoracic spiracles remain constricted.
Microscopic video recordings and reflectance measurements
revealed that the dorsal soft edges of the valve lips of the second
spiracle leave a very small leak, which is passively widened during
backward pulses of the heart. Thus, negative pressure, combined
with increased leakage of the spiracle Sp2 valve enable inspiration
in the thorax. The positive pressure periods are correlated with a new
type of convective CO2 micro-bursts as shown in flow-through
measurements. The bulk of the CO2 is, however, released after
longer interbursts in macro-bursts with actively opening valves
reminiscent of the open phase in a cyclic gas exchange. When the
valves open, the PO2 in the thoracic air sacs unexpectedly drops by
a mean of 2.75±1.09 kPa, suggesting a displacement of O2 by the
transient accumulation of CO2 in the tracheal system before its
release.
Fig. 6. Influence of the heartbeat reversals on the
dorsal valve leakage of spiracle 2 in C. vicina (F7/11 at
22°C). (A) Upper trace: reflection of valve lips on Si
photocell. During widening of the leak the reflective
surface of the lips is reduced and the dark background
behind the leak increases. Lower trace: relative tracheal
pressure pulses. Negative pressure pulses are caused by
backward heartbeat (black bars) and positive pressure
pulses are caused by forward heartbeat (white bars).
(B) Detail of A. PPT, positive pressure pulse periods in the
thorax; NPT, negative pressure pulse periods in the
thorax.
Fig. 7. Oxygen uptake is concurrent with heartbeats. (A) PO2
in the scutellar air sac (upper traces) and simultaneous
myographic recording of heartbeat (lower trace) in male C.
vicina (M15/09 at 22°C). During backward pulse periods (black
bars), the PO2 increases. (B,C) Simultaneous recording of intratracheal
PO2 and pressure in the scutellar air sac (F6/08 at
22°C). (B) Survey of seven sequences. (C) Detail of B. The PO2
increases during the negative pressure periods (backward
pulse periods; black bars).