THE SOCIAL INSECTS 



is called habituation. Most animals which avoid noise and 

 vibration can learn that some regularly recurring noise is 

 harmless to them, just as we acquire the power of sleeping 

 through the noise of a nearby railway. Thorpe showed that 

 the vinegar fly will not normally lay its eggs in an agar 

 medium flavoured with peppermint. But if eggs are put in 

 such a medium, the flies bred from them no longer avoid it 

 when they in turn are laying. In some insects, habituation 

 has also a positive side and they may learn to seek certain 

 stimuli, just as many people prefer their native cookery to 

 that of other lands. Several entomologists have shown that 

 bean weevils can be bred on beans of several kinds, such as 

 runner beans and cowpeas. The weevils bred from each kind 

 of bean have a marked tendency to return to it rather than to 

 one of the other kinds if they are offered simultaneously for 

 egg-laying. This sort of habituation may account for the 

 observation that some insect species are divided into strains 

 which are attached to different food-plants, and it may also 

 account for the observation that social insects will rarely 

 admit into their nests an individual of their own species from 

 another colony. The stranger evidently does not have the 

 right smell, though sometimes he may be able to acquire it. 

 A more complex type of learning which is also very widely 

 spread is known as trial and error learning. This is demon- 

 strated clearly when for example a cockroach is placed in a 

 maze containing food. The maze is a box divided by barriers 

 into a number of passages, some of which are blind alleys 

 whereas others eventually lead to the food-chamber. It is 

 arranged so that the correct turning is sometimes to the right, 

 sometimes to the left, and the time which it takes for the cock- 

 roach to find its way through the maze to the food can be 

 recorded. In these experiments, the insect, just as a man 

 might, takes at first many wrong turnings, but gradually the 



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