Analysis of contact-rehydration in terrestrial gastropods: osmotic control of drinking behaviour.

Contact-rehydration in slugs is mediated by a specific behavioural pattern in which dehydrated slugs move onto a moist surface, assume a flattened posture while water is absorbed through the surface of the foot and move off once they are rehydrated. 'Drinking behaviour' is initiated when slugs have been dehydrated to the threshold level of 60-70% of initial body weight (IBW). Drinking behaviour is terminated once slugs have rehydrated to their individual rehydration set-points. The mean 'rehydration set-point' for Limax is 93.6 +/- 12.2% IBW (+/- S.D.). Slugs can achieve their individual set-point regardless of the extent of initial dehydration. Drinking behaviour can be initiated by injections of hyperosmotic mannitol solution and terminated by injections of dilute saline. This indicates that variation in the osmolality of the haemolymph is involved in the control of the behavioural sequence.