Fig. 1. Drosophila flies are attracted to apple cider vinegar but avoid CO₂. Time spent by flies in each of the fields in a four-field olfactometer during a 600 s experiment. (A) Control situation when only air is delivered from the four corners (N=57). (B) Apple cider vinegar odor (N=39) is added to the air in one field (grey bar). (C) CO₂ of different concentrations (N_0.02%=38; N_0.1%=42; N_1%=35) is added to the air in one field (black bars). The orientation of the fields is indicated relative to the field laced with the test odor: L, left, O, opposite, R, right. Insets show examples of 10 min tracks of single flies for control, vinegar, and 1% CO₂ respectively. The broken line at 150 s indicates an equal amount of time in all fields. Deviations from equal distribution were tested with a Friedman-ANOVA (P<0.001; ns, no significant difference). Fields with different letters above the bars are significantly different from each other (Wilcoxon-Wilcox test; P<0.05 for 0.1% CO₂, P<0.001 for 1% CO₂ and vinegar). Values are means ± s.e.m.