1. Slugs are normally active at night, but this is not a response to darkness. The onset of nocturnal activity does not necessarily coincide, either in the field or in the laboratory, with the onset of darkness. On the contrary, dark-adapted, inactive animals become active on illumination.
2. The activity which follows illumination is short-lived, adaptation being complete within an hour.
3. Dark-adaptation is slower than light-adaptation and takes between 1 and 2 hr.
4. The response occurs whether illumination is constant or intermittent and in white or red light. Once adapted to red light the slugs do not respond to illumination by white light.
5. This effect of light, in initially activating the slugs, clearly cannot account for the occasional daytime activity observed in the field, when the slugs emerge from dark resting places.
6. Once the slugs are light-adapted, the activity during the daytime remains at a very low level even under continued illumination. This low level is probably the same as occurs in the dark under similar conditions.
7. Air currents played on the body or tail of the slug increase the speed of locomotion or induce activity if the animal is at rest.
8. Air currents played on the head or tentacles results in a klinotactic response whereby the animal turns from the region of draught or moves down stream.
9. These reactions must supplement the reactions in a temperature gradient which lead the slugs at the close of activity to sheltered resting places.
- Copyright © 1954 The Company of Biologists Ltd.