Fig. 2. Hovering kinematics in hypoxic air. As ambient air was replaced with pure nitrogen, the partial pressure of oxygen declined but the air density varied only slightly. Otherwise, conditions were identical to that of the first experiment described in Fig. 1. (A) The wingbeat frequency decreased slightly but significantly as oxygen partial pressure decreased (see text). (B) Stroke amplitude varied considerably but exhibited no clear pattern with changing partial pressure of oxygen. Values are means ± S.E.M. All symbols as in Fig. 1. 1 mmHg=133.3 Pa.