Fig. 7. Detailed representation of the data of Fig. 6A–D. The appendage beating rate (fA) profiles (dotted lines) of haemoglobin (Hb)-poor and Hb-rich animals appeared to be aligned parallel to each other until the reduction in ambient oxygen partial pressure (P_O2amb) from 3.3 to 2.6 kPa (mean 2.95 kPa) induced significantly diverging changes in fA (Hb-poor, fA=+10.23±12.70 min^-1; Hb-rich, fA= -6.19±5.06 min^-1; unpaired two-tailed t-test: t=2.94, d.f.=10, P=0.01; means ± S.D., N=6). Similar diverging changes were initiated in NADH fluorescence intensity I_NADH (solid lines) when the P_O2amb was reduced from 5.3 to 4.2 kPa (mean 4.75 kPa) (Hb-poor I_NADH=+1.30±0.80 min^-1 versus Hb-rich I_NADH=+0.42±0.44 min^-1; unpaired two-tailed t-test: t=2.36, d.f.=10, P=0.04; means ± S.D., N=6). This divergence in I_NADH extended over the P_O2amb range from 4.75 to 1.32 kPa (shaded area), where I_NADH of Hb-rich animals appeared to stabilize at 103 %, whereas that of Hb-poor animals rose from 104 to 123 %. Note that, in Hb-poor animals, the increase in fA at 2 kPa coincided with the increase in I_NADH. Arrows indicate significantly diverging responses in the respective variables. The standard deviations of the curves have been omitted for clarity.