218 



NA TURAL HIS TOE Y. 



Occasionally the shade-trees of our streets are seriously injured or even killed 

 by the ravages of some of these saw-flies. 



Fig. 21C — Male and female saw-flies (Sirex). 



Next in order come the gall-flies, mostly small forms whose products 

 are familiar to all. On the leaves of the oak one frequently finds large, 

 round, brown bodies, — oak-apples, they are called, — which are produced 



by one species of gall-fly; the peculiar swellings 

 on the stems of golden-rods are the product of 

 another. It would make a long list to enumer- 

 ate the various forms of galls, and the plants 

 on which they occur, and yet the experienced 

 entomologist on seeing one of these abnormal 

 formations, can say at once what species of 

 gall-fly was concerned in its production. 



Yet while naturalists know so much about 

 galls, there is another aspect of the subject on 

 which they are profoundly ignorant. The fe- 

 male gall-fly stings the leaf or plant, and in the 

 opening she makes she deposits an egg. The 

 injury this puncture produces is but very slight ; 

 and if we were to run a needle into the plant 

 in the same way no effects would follow. The 

 egg, however, makes a great change in the 

 plant, but in what way it produces this change 

 is a problem as yet unsolved. It may be that 

 some poison accompanies the egg, but this is 

 fig. 217.— Rose-gaii. and an enlarged certainly not the case in every instance. Now 



view of the insect (Iihodites rosse) , J 



which produces it. mark the result. The plant, as soon as the 



