FOREST LIFE. 211 



In the center of an oak apple there is a little cell, 

 within which a larva lives till it gets its growth. 

 This larva is hatched from an egg laid in the tissue 

 of the leaf by a small, four-winged insect, called a 

 gall-fiy. When the young larva began to feed on 

 the leaf, the leaf began to grow around it in a won- 

 derful way ; so that very soon the larva was sur- 

 rounded by a large ball of plant growth, which 

 served as a home and furnished food for the larva. 



Why the plant grew in this way no one knows. As 

 a rule, when a leaf-eating larva feeds on the tissue of 

 a leaf there is no extra growth ; but when the larva 

 of a gall-fiy begins to feed, an abnormal growth of 

 the plant commences. More than this, this growth 

 is of a definite form which is different for the differ- 

 ent species of gall-flies. Hence, when an entomolo- 

 gist who has studied these insects sees a familiar gall, 

 he knows at once what species of insect produced it. 

 It is natural to suppose that the larva excretes a 

 poison, which acts on the plant in such a way as to 

 produce this remarkable result. There are certain 

 other gall-producing insects which belong to a differ- 

 ent order than those that produce the oak-apples, 

 the galls of which begin to grow before the larvae 

 hatch. In these cases it is supposed that a drop of 

 poison is deposited with the egg by the parent insect. 



Many species of gall-flies undergo their transfor- 

 mations within their galls, while in other species the 

 full-grown larva leaves the gall and enters the ground 

 to transform. 



The gall represented by Fig. 178 is produced by 

 a single larva. But certain species of gall-flies lay 

 many eggs together, and there results the growth of 



