8o INSECTS 



It also happens sometimes that a normal constituent 

 of plant tissue, like tannin, is greatly intensified in the 

 gall structure which may contain many times more 

 tannic acid than any other part of the tree. The galls, 

 therefore, become of commercial value and in some 

 localities the oak forests yield a considerable revenue 

 from this source. 



Few Cynipid galls are really injurious to the plants 

 on which they appear. Those on the leaves are never 

 so; those on twigs are rarely so; those on the larger 

 branches sometimes cause a choking and sometimes 

 a weakening that results in a fracture under strain; 

 but frequently a badly galled branch will in its de- 

 velopment split off and shed the old galls. Root galls 

 sometimes threaten the life of a plant; but most of 

 the fatal galls found on trees, shrubs, and vines are due 

 to other causes. 



In the Diptera we have plant feeders in great variety , 

 and many of them endanger the life of the plants at- 

 tacked; but they have a decided preference for soft 

 tissues and there are no borers in solid, living wood. 



Among the long-legged, mosquito-like species with 

 long antennae, the crane-flies have larvae that live 

 underground and feed on the plant roots. They are 

 sometimes called "wire-worms" because of their long 

 slender form, and "leather-jackets" because of their 

 texture. They differ from the beetle larvae that are 

 also called wire- worms, by the more cylindric shape, 

 the more pointed head, and absence of legs. Their 

 feeding on roots is sometimes extensive enough to 

 cause the death of the plant attacked. The crane- 

 flies are the largest of the long-legged forms, some of 

 them measuring with legs extended nearly three inches. 



The gall-midges are at the other extreme and in- 

 clude very small species with very long, often prettily 



