THE SOCIAL INSECTS 



with wooden posts. In many tropical countries the wooden 

 parts of houses have to be protected, especially where they 

 come in contact with the earth. Even the use of concrete 

 pillars to support the ground floor is not infallible, since the 

 termites will build covered runways up the concrete. The 

 damage is particularly insidious since it is entirely internal : 

 the first sign of trouble is apt to be the collapse of a beam. 

 During the Second World War crates of military stores which 

 had to be piled on the ground in such countries as New 

 Guinea were often attacked. A few weeks later, when the 

 crates were moved, they fell to pieces. 



During his travels at the end of the eighteenth century, 

 Humboldt noted how rare it was to find old books in South 

 America, and this was attributed to the attacks of termites. 

 In such countries, to-day, all books are treated with a solution 

 of a poison such as corrosive sublimate, to protect them 

 against termites and other insects. The protection of houses 

 is more difficult, but special modes of construction, employ- 

 ing concrete, have been devised, and there are expensive ways 

 of impregnating wood with preservatives. 



A curious but less important source of trouble occurs when 

 runways are constructed for African aerodromes. It may be 

 very expensive to level out the hard, projecting termites' 

 nests and the strips may require repeated treatment because 

 the nests are continually being reconstructed. 



THE IMPORTANCE OF TERMITES TO TROPICAL PLANT-LIFE 



Termites play an important part in the economy of tropical 

 nature in other ways which impinge on human interests, 

 though less directly. Trapnell has shown that where they 

 are really numerous, as in parts of Rhodesia, they greatly 

 influence the type of vegetation. Just as in England the 

 vegetation of chalk downs differs from that of sand or clay, 



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