Fig. 1. Spontaneous swimming and responses of male N. succinea to low concentrations of CSSG. Points show animal positions at 0.2 s intervals. (A) No CSSG present. (B) CSSG present in a remote part of the tank (yellow strip). The image shows three different paths swum down the left side of the internal apex and one `cross-tank' path. (C) Lack of response to low concentration of CSSG. The trail was created with 0.2 ml of 10^–8 mol l^–1 CSSG and therefore has an estimated trail concentration of <10^–9 mol l^–1. (D) Responses to a trail made with 10^–7 mol l^–1 CSSG (estimated trail concentration of <10^–8 mol l^–1 CSSG).