Fig. 1. Fluid secretion rates of Tenebrio Malpighian tubules in response to different bath K⁺ concentrations. Increasing the K⁺ concentration to 140 mmol l^-1 K⁺ stimulated fluid secretion rates significantly (P<0.02). A 10-fold decrease in the K⁺ concentration irreversibly decreased fluid secretion rates (P=0.01). Data are presented as means ± S.E.M. (n=8 for both secretion assays).