The Activity of Slugs I. the Induction of Activity by Changing Temperatures

Slug activity is associated with conditions of high humidity, and this association has, not unnaturally, led to the hitherto untested assumption that it was in some way induced by the necessarily damp conditions. The experimental examination of this phenomenon, described in this paper, has shown that certain changes in temperature, and not conditions of high humidity, induce activity. Such temperature changes occur in the situations inhabited by slugs on just those occasions when slug activity is observed in the field. It seems desirable to define what is implied by the term 'activity'. The natural historian, when describing an fmimai as noctumally active, means that it can be found at night moving about, presumably in search of food and mate, whilst during the day it is found at rest. Where this behaviour has been investigated in the laboratory (Szymanski, 1920; Gunn, 1940; Bentley, Gunn & Ewer, 1941), activity has usually been taken to mean locomotor activity because it is possible to obtain a measure of this, and it necessarily accompanies the acquisition of food and mate. In the case of slugs, where food and mate may be present in the environment during the day as well as during the night, feeding and mating occur normally only at night and follow the onset of nocturnal locomotor activity. Consequently the investigations have been directed to the study of this aspect of activity, which is regarded as the usual precursor of other activities. Unless otherwise stated, activity means locomotor activity. This usage of the term has the additional advantage of making the work directly comparable with that of other authors on different animals (Szymanski, 1920; Gunn, 1937, 1940; Bentley et al. 1941). Most of the work has been done on the grey field slug Agriolimax reticulatus (incorrectly named A. agrestis by some authors), which is a pest of root crops, cereals and garden vegetables in this country. This species was chosen because it was conveniently abundant. Other species have been used occasionally, in which case this is stated in the text.