Fig. 4. Effects of serotonin treatment on starving worms. N2 adults were starved on an agar plate with or without serotonin for 6-7 h at 20°C and were then tested for distribution on a temperature gradient. Blue, without serotonin; black, with 8 mmol l^-1 serotonin; red, 8 mmol l^-1 serotonin with 5 mmol l^-1 serotonin added 1 h before the test. Results of two experiments were combined.