Fig. 4. Pretreatment of flies (heat hardening) increases survivorship, measured by proportion of flies that are alive 24 h after thermal stress. HS=38.5°C heat shock for 1 h, with no pretreatment. Pre+HS=36°C heat hardening pretreatment for 1 h followed by a resting period (25°C for 1 h) prior to heat shock at 38.5°C for 1 h. Each data point represents the mean survivorship of 300 males or females (i.e. 75 flies from each of the four CN, LN or HN lines). Values are means ± s.e.m. Red lines depict change in survivorship in HN flies, with red filled triangles (pointing upward) for females; open triangles, males. Blue lines depict change in survivorship in LN flies, with blue filled triangles (pointing downward) for females; open triangles, males. Black lines depict changes in survivorship in the CN flies, with black filled circles for females; open circles, males.