VI. HOW INSECTS BEHAVE 



{Reflexes; instincts; swarming; migration; insect communities^ 

 social insects; signs of intelligence) 



We have seen how the various senses of insects tell them about their 

 surroundings, and how the insects themselves can move in various ways. 

 Behaviour is the way in which an insect reacts to a particular situatipn, 

 by receiving impressions from its various senses, and then responding 

 with an appropriate set of movements. 



Reflexes 

 The simplest item of behaviour is a reflex action, in which one sense 

 organ is linked by nerves directly to a muscle. Fig. 58 shows such a 

 reflex arc, passing through one of the nerve-jtmaions, or ganglia of the 

 central nervous system. An example of a reflex has been mentioned 

 earlier : when an insect's weight ceases to press on the tarsi of the legs, 

 the wings may be set in motion. 



Much of an insect's behaviour is made up of simple automatic 

 reflexes like this. So, indeed, is much of our own, as when we instandy 

 pull our finger away from a hot plate, or blink when something comes 

 near to our eye. 



The insect's nervous system is much less concentrated than ours, and 

 a reflex arc may be complete without going to the brain. Hence reflex 

 movements of the legs or wings can go on for a time even after an insect 

 has been decapitated. Nevertheless, under normal conditions, the re- 

 flexes are not entirely automatic, but may be modified or suppressed 

 altogether in certain circumstances. For example larvae often have a 

 reflex movement upwards and towards the light, which takes them up 

 into vegetation to feed; but when they are fully fed the reflexes are 

 reversed, so that they now move downwards and away from the light, 

 which takes them under cover on the ground, where they pupate. 



Instinctive Behaviour 

 A reflex is a single item of behaviour; a more complicated sequence of 

 acts that seem to follow automatically in certain circumstances is classed 

 as instinctive behaviour. 

 Certain species of horsefly of the genus Chrysops hve in the rain- 



86 



