Fig. 3. Comparison of metabolic rate of D. melanogaster, as measured by CO₂ production, for control flies at 23°C and 16°C, and rapidly cold-hardened (RCH) flies at 16°C after a brief exposure (5 min) to 23°C. Error bars represent ± S.E.M. Bars with different letters are significantly different (Bonferroni multiple comparisons test, P<0.0001).