THE INSECT WORLD 



Two allied families of beetles, the bark beetles and the 

 ambrosia beetles, though very different from the kinds which 

 feed on dung or carrion, have developed essentially similar 

 habits. The females, in some species assisted by the males, 

 excavate a tunnel in solid wood. A special fungus grows in 

 the tunnel, its spores being carried there either in a hollow 

 in the head of the female or in her alimentary canal, an 

 interesting analogy with what happens in the fungus-culti- 

 vating ants and termites, described later. The female lays her 

 eggs in side chambers and the young larvae may be fed by 

 the female on pieces of the fungus. In some species, the male 

 may guard the main entrance, perhaps chiefly to keep out 

 other males. These are remarkable societies while they last, 

 but they are never prolonged beyond the one generation. 



FULLY SOCIAL BEHAVIOUR 



It appears that to obtain a larger and more elaborate social 

 organisation the female must live long enough to overlap 

 with several generations of her young. Some solitary insects 

 do live long enough for this to be possible, though in most 

 of them there is very little overlap between the lives of the 

 adults of successive generations. An even more important 

 step is for more than one female to co-operate in looking 

 after the young. This happens in none of the elementary 

 social groups ; it provides the best distinction between social 

 and sub-social insects. A true social insect may be defined 

 as one in which the female tends or helps to construct a 

 brood-chamber for an egg (or larva) laid by another female. 

 This condition is realised only in the ants, bees, and wasps, 

 belonging to the order Hymenoptera, and in the termites; 

 the latter are unlike the Hymenoptera in that the males play 

 as big a part in the colony as the females. Only a very few 

 insect species have been able to develop fully social habits. 



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