BEHAVIOUR Or 



the creature retains its responses to dampness, and slows down or 

 stops when it comes to a damp place. Once it gets over the effects 

 of desiccation it becomes photo-negative again and moves away 

 from the light. 



These simple responses are closely linked with a woodlouse's way 

 of life. It spends the day hiding in dark, damp places, but at night 

 it comes out and walks about. The negative reaction to light makes 

 sure that it gets under cover before daybreak, so avoiding possible 

 daytime predators. The positive reaction to light develops in emer- 

 gencies, when the daytime hiding-place dries up. The only thing 

 that these reactions do not explain is why the woodlouse walks 

 about at night; they do explain why it does not normally walk 

 about during the day. 



The nocturnal wanderings of woodlice are due to an innate 

 tendency to activity at night, which, once begun, is not prevented 

 by the creatures responses to humidity and light. The night is dark, 

 and usually much more humid than the day. Coupled with this it 

 has been found by experiment that the tendency to stay still in 

 damp places is not so strong after the animals have been in the 

 dark for some time. 



It would be a mistake to suppose that all woodlice have identical 

 reactions to humidity and light. The account given above applies to 

 Oniscus and Porcellio. The pill-woodlouse, Armadillidium vulgar e, 

 is rather more resistant to desiccation than most other woodlice, 

 and can often be seen walking about in bright sunlight, particularly 

 in the morning. 



It is probable that most other Crustacea have a similar series of 

 responses, though they are almost certainly related to other features 

 of their environments. They have not been analysed in the same 

 detail as the responses of woodlice, but they undoubtedly result in 

 the creatures being kept in the conditions which suit them best. 



The responses of woodlice to light and humidity are almost 

 mechanical, no element of learning is involved. By the very nature 

 of the dangers to which it is exposed a woodlouse cannot benefit 

 from learning that it will die if exposed to dry air for too long; 

 there can be no benefit from previous experience. But there are 

 situations which present crustaceans with the opportunity of learn- 

 ing, and some at least are capable of benefiting from it. 



The simplest form of learning is that known as habitation. This 

 process involves the dropping of responses to stimuli that experience 

 has shown to be harmless. If a jam-jar containing Daphnia is 



