Fig. 4. Response of a male N. succinea to a moderate concentration of CSSG. Images show the swim track, plotted every 0.2 s. The concentration of CSSG in the syringe was 10^–6 mol l^–1, yielding a trail concentration <10^–7 mol l^–1. The drawings show the position of the original CSSG trail and do not show the dispersion of the chemical as the worm swims through it (to observe dispersion of the trail, see Movie 1 in the supplementary material). Swim tracks are sequential recordings of 5 s (A–E) and 4 s (F). (A) Typical `wall-swimming' behavior prior to encountering CSSG. (B,C) CSSG stimulates circling behavior. (D–F) Larger circles result as the worm encounters more dilute dispersed CSSG.