The daily cycles of cercarial emergence are recognized as an adaptive mechanism to enhance parasite transmission. Several hypotheses have been proposed to explain the functional significance of these daily cycles (Shostak and Esch, 1990). The most accepted hypothesis suggests that cercarial emergence is timed to coincide with the presence of the next host, particularly for those cercarial species in which the target hosts do not regularly cohabit with the molluscan host producing the cercariae (Combes et al., 1994).