Fig. 1. Determination of critical oxygen partial pressure (P_c). This is the raw data file from an individual grasshopper, showing CO₂ emission (solid line) and atmospheric O₂ (broken line). As oxygen partial pressure (P_O2) drops incrementally, CO₂ transiently increases. Briefly, we compared 95% confidence intervals constructed around the mean CO₂ p.p.m. in each box. Box A is the P_O2 where CO₂ was significantly less than at the next higher P_O2 (box B) and less than the average CO₂ at all higher P_O2s (box C).