122 A BOOK OF INSECTS 



hand recoils from heated metal — they do not alone suffice 

 for the requirements of creatures so highly organised as 

 insects. All those who have watched living insects are 

 aware that they perform more or less complex actions, 

 either for their own immediate good or for the benefit of 

 their offspring; and that these actions, while they are 

 often so remarkably successful as to appear rational, are in 

 reality quite independent of consciousness, experience, or 

 tuition. This kind of behaviour is called instinctive. A 

 given stimulus sets going a connected series of responses, 

 each of which inevitably calls forth its successor. Thus 

 the female brimstone butterfly seeks a buckthorn leaf, to 

 which she fixes her egg. She cannot taste the leaf nor 

 can she have any foreknowledge of the needs of an off- 

 spring which she will never see. Her action is purely 

 instinctive, evoked by some kind of stimulus — probably 

 the distinctive odour of the requisite plant. Again, many 

 moths, which spin their cocoons within a leaf, first securely 

 fastens the leaf to the twig with a wrapping of silk ; yet 

 they can have no conception of the risk of falling that 

 neglect of this precaution would involve. Still more 

 remarkable is the behaviour of a small weevil known as 

 Rhyiicliitcs bctidcr, which rolls up birch leaves in order to 

 provide food and shelter for its young. Beginning at the 

 edge of the leaf, not far from the stalk, the insect makes 

 a long s-shaped cut to the midrib. She then ascends the 

 leaf for a short distance, and makes a similar cut from the 

 other side of the midrib to the opposite edge. These cuts 

 have been examined by mathematicians, who (to quote 

 Dr. David Sharp) have extolled them as being conducted 

 on highly satisfactory principles. In a word, they are 

 exactly those which are needed to overcome the leafs 

 tendency to spring back, and thus to render the rolling 

 most easy of accomplishment. Subsequently, the insect 



