Fig. 4. Temporal organization of the experiment. The animal was exposed to a controlled, gradual transition from normoxia to anoxia (A) while haemoglobin (Hb) oxygen-saturation (B), heart rate (C), appendage beating rate (D) and NADH fluorescence intensity (E) were measured continuously. At very low oxygen tensions, the thoracic appendages showed an irregular rather than a periodic temporal pattern of movement. Therefore, appendage beating rate was counted manually every 30 s (as indicated by the filled circles). NADH fluorescence always showed a linear decline in intensity which, at a certain low level of oxygen tension, was superimposed by the hypoxia-induced increase in mitochondrial NADH content. The data were therefore trend-corrected by calculating a linear regression line (dashed line) for the initial 12–18 min of the experiment during which Hb oxygen-saturation remained unchanged.