Fig. 10. Knowing their dependencies on ambient oxygen partial pressure (P_O2amb) made it possible to correlate limb beating activity with haemolymph oxygen concentration ([O₂]). In haemoglobin (Hb)-rich animals, the physiological effect of a sevenfold higher Hb concentration manifested itself in the range of P_O2amb from 4.75 to 1.32 kPa, within which the extra Hb could still provide enough oxygen to sustain aerobic energy provision in the highly active limb muscles. Within almost the same range of P_O2amb, Hb-poor animals were also able to maintain a high limb beating activity, but energy provision was already supported by anaerobic mechanisms (see Fig. 8). Note that, in the two groups, anaerobic energy provision was initiated at different levels of [O₂] (horizontal arrows). The open circles indicate the P_O2amb and the corresponding [O₂] that caused an impairment of cardiac activity. The breadth of the graphs reflects the range of uncertainty in determining the proportion of [O₂] that is physically dissolved in the haemolymph. The red arrow indicates the P_O2amb at which Hb-rich animals were raised.