THE INSECT WORLD 



HOW SOCIAL BEHAVIOUR DEVELOPS 



We may now consider what we mean by the term " social 

 insect ". Many solitary insects are gregarious, that is they 

 share certain common needs or react in the same way to 

 certain external stimuli so that dense populations assemble 

 locally. Many unrelated species of caterpillar may be found 

 feeding on one plant, and large numbers of assorted insects 

 may be trapped at a street-lamp. A suitable earthbank may 

 harbour the nests of hundreds of solitary bees or wasps, but 

 they never help one another, and indeed frequently fight, like 

 two families trying to share one kitchen. Gregariousness has 

 never led to social life in insects, though in the locusts, where 

 dense populations of nymphs aggregate into marching bands 

 and where the adults may fly in enormous swarms, there is 

 at least a rudimentary organisation of the behaviour of the 

 group. 



All animals must behave so that their young will find the 

 conditions necessary for survival and growth. In the swift 

 moth this instinctive maternal care amounts to no more than 

 scattering eggs broadcast over a field, where there is a good 

 chance that the young caterpillars will be able to burrow into 

 the soil and to find the roots on which they feed. In this 

 moth, as in most other insects, the female also requires the 

 co-operation of a male for a brief period, to fertilise her eggs. 

 The female of the large cabbage white butterfly lays her 

 eggs in a group on a cabbage, the food-plant of the cater- 

 pillar. These when young sit side by side on the leaf, and 

 it is thought that this gives them some mutual protection 

 owing to their unpleasant smell and conspicuous colour. 

 The same sort of behaviour is seen in the cinnabar caterpillar 

 on ragwort. Other caterpillars, like those of the small 

 ermine moth on the garden euonymus, spin a joint web in 



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