Physiological and Behavioural Aspects of Water Balance and Respiratory Function in the Terrestrial Slug, Umaxmaximus

1. A method is described for establishing a stable relationship between body weight and hydration (i.e. 100% hydrated) in terrestrial slugs. This allows the use of '% initial body weight' as an indicator of relative body hydration. 2. During progressive dehydration in the slug, Umax maximus (L.), the haemolymph osmolality increases exponentially. This, together with measurements of wet weight/dry weight ratios of various tissues, indicates that the water loss during the initial stages of dehydration is from the haemolymph. 3. Although long-term (days) cyclical changes in body hydration were not observed, a daily cycle is described with highs at 12.00 midnight and lows at 12.00 noon. 4. It is shown that the huddling behaviour of slugs can reduce evaporative water loss from individual slugs. 5. Dehydration of a slug to about 90 % of its initial body weight initiates a rhythmic cycle of pneumostome closures. Further dehydration results in an increase in the duration of the closures and reduction of the diameter of the pneumostome when open. 6. The absolute and relative O2 uptake by the integument and lung are presented. In quiet slugs, the lung is responsible for about 20 % of the total O2 uptake, while in active slugs it is responsible for 50 % of the total. During activity there is a greater increase of O2 uptake across the lung than across the integument.