Fig. 1. The relationship between the number of fertile (open circles) and non-fertile (filled circles) sperm present in the female's sperm storage organ and the tendency to remate in the butterfly Pieris napi. Females that did not remate within 10 days following their initial copulation stored significantly more non-fertile sperm compared to remating females, whereas there is no difference in the number of fertile sperm stored. Values are means ± S.E.M. (redrawn after Cook and Wedell, 1999).