THE SOCIAL INSECTS 



distracted if they are not kept moist. The termites, because 

 of their soft external skeleton, are the most sensitive of all the 

 social insects to water-loss, and not only are their nests 

 waterproof but termites usually travel everywhere in tunnels 

 which they burrow beneath or construct above the ground. 



GROWTH 



The location of the insect skeleton on the outside of its 

 principal organs has two important consequences. To get 

 movement, there have to be many joints, and the character- 

 istic insect limb is one result. It is jointed in much the same 

 way as the armoured suit of a deep-sea diver. Again, growth 

 is possible only if the armour is taken off, and this is done 

 in the process of moulting. The external skeleton becomes 

 loosened from the tissues beneath and a new and larger one 

 is laid down beneath it. It is at first soft and wrinkled but 

 very soon after the moult it expands and hardens. In the 

 majority of insects, there is a fixed number of moults during 

 the growing period and they take place at fairly regular 

 intervals. Once the adult form is reached, no more moulting 

 occurs and there is no more growth. Thus one can roughly 

 estimate the age of a honey bee worker by the amount of 

 wear which its wings and skeleton show. 



The physiology of growth and of moulting in insects has 

 been very actively studied during the last twenty years. The 

 story is complicated and not yet fully understood, but it 

 seems that moulting and the changes of form to which it 

 gives rise are controlled by the brain and by certain ductless 

 glands, mostly associated closely with it. In the bug Rhodnius 

 Wiggles worth showed that the early stages do not moult if 

 decapitated during the first Hive days after the large meal of 

 blood which normally suffices for the whole period. Bugs 

 decapitated later moult normally. This suggested that a 



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