58 



ZOOLOGY 



female ichneumon are, in some cases, used to drill holes into 

 trees occupied by insect burrows, so that her eggs can be laid 

 therein. These parasitic species are invaluable to agriculture 

 in keeping down injurious insects. 



The gall-wasps, popularly not distinguished from the strict 

 gall-flies, are familiar to us from their works. They lay eggs 



Fig. 04. 



-Larvii? of sa^Yfi\" on grape loaf Photo, b}' ^'. H. L. 



in various kinds of plants, especially in oaks and members 

 of the rose family. An excessive growth of the plant tissue, 

 called a gall, is caused either by a poison dropped into the plant 

 with the e.gg, or by the irritation of the developing emliryo.' 

 The galls of gall-wasps are often more or less spherical masses 

 which are closed, in consequence of which the confined insect 

 must bore its way out. The galls made by the same species 

 of insect on one kind of tree are quite similar, but if the same 

 insect stings another species of tree a different kind of gall is 

 produced. Also when different species of gall-wasps sting one 



1 Cf. Fig. 93, p. S4. 



