The Activity of Slugs

1. The effect of moisture and temperature on the activity of slugs has been investigated in the laboratory and in the field. 2. Atmospheric moisture has no direct effect on activity, though it may limit the duration of activity by influencing the water content of the body. Animals of high water content are not inherently active but they respond more readily to other stimuli than animals of low water content. 3. Between 4 and 20° C. activity is induced by falling temperatures and suppressed by rising temperatures. Temperature changes as slight as 0.1° C. per hour are perceived. 4. The daily rhythm of activity and rest thus follows the normal diurnal rhythm of falling temperature by night and rising temperature by day, except when daytime mists and showers superimpose a minor fluctuation and result in daytime activity. No activity is observed on continuously wet days which are normally without such fluctuations. 5. Between 20 and 30° C. activity is induced by rising temperatures and suppressed by falling temperatures. The ecological significance of this is discussed. 6. When maintained at constant temperature slugs show a persistent but immediately and steadily deteriorating diurnal rhythm of activity and rest which could not of itself account for the regular appearance of nocturnal activity in the field. 7. In a temperature gradient slugs aggregate about a preferred temperature. The ecological significance of this is discussed. 8. The difficulty of describing these reactions in the terminology put forward by Fraenkel & Gunn (1940) is considered.