THE SOCIAL INSECTS 



immediate reaction of this type which leads a grasshopper 

 to sit on the sunny side of a mound and to place itself so 

 that the largest possible area of its body is exposed to the 

 sun. Similarly the fully grown maggots of blowflies crawl 

 away from the light, and in nature this leads them away from 

 their food to pupate in the soil. In an often-repeated experi- 

 ment, the maggots are placed on a board which is lighted 

 from two sides by beams of unequal intensity. The track of 

 the maggot then divides the angle between its original posi- 

 tion and the lights in proportion to their brightness. 



Another simple type of behaviour is the reaction to internal 

 messages, such as those from the stomach or alimentary canal 

 which lead an insect to search for food, or from the ripening 

 ovaries which lead to a search for an egg-laying site. A 

 hungry insect may become restless and merely wander about, 

 more or less at random, until it finds food. External stimuli 

 such as light, shade, and wind, will partly determine its path. 

 If normal spider beetles are put in a cage of which one end is 

 kept moist, the other dry, most of them will collect in the 

 dry end; but if they have been kept very dry for some days 

 previously, they collect at the damp end. Their behaviour 

 is a resultant between an external gradient and their internal 

 condition. Behaviour of this sort, where the pattern is simple, 

 is not usually called instinctive. Where the behaviour is 

 more elaborate, as when the red admiral seeks out a nettle 

 and lays eggs in a characteristic way on a special part of the 

 plant, the pattern is said to form an instinct. Instinctive 

 behaviour has a complex, inherited pattern. The performance 

 is normally perfect on the first occasion, so that learning is 

 not involved. The sequence of acts requires a particular 

 internal condition of the animal, but it can be carried to com- 

 pletion only in a situation which also provides certain specific 

 external stimuli. 



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