Fig. 4. Correlation of tracheal compressions with respiratory pulses of CO₂ in Pterostichus stygicus. Data are from one trial (mass 280.0 mg) showing representative CO₂ emission before x-ray exposure (A), during x-ray exposure (B), and after x-ray exposure (C). Green vertical lines indicate the start of a tracheal compression, identified from x-ray movie records (digitized block between pair of arrows, t=3.8 min). The inset box in C depicts a simulated CO₂ trace using a 1/10 slower flow speed: 150 ml min^–1 (purple) vs 1.5 l min^–1 (gray). To simulate the slower speed, CO₂ data were smoothed using a running average of 15 points (4.5 s); this interval was chosen to approximate the transit time of gas through the infrared detector at this flow speed (see Gray and Bradley, 2006). Simulated low flow speeds obscure the prominent high-frequency component of the signal.