Tolerance to severe acid stress. Influence of ambient pH on in vivo hemolymph pH (A), heart rate (B) and appendage beating rate (C) of animals acclimated to pH 7.8 (open symbols), pH 6.0 (grey-filled symbols), and pH 5.5 (filled symbols). Data are given as means ± S.E. (N = 3–4). Dotted lines indicate a period of irregular limb beating activity. Each acclimation group was exposed to its acclimation pH during the initial and final phases of the experiment.