hegner] GALL INSECTS AND INSECT GALLS 209 



leave the galls and pupate in the ground. The more complex galls 

 have a wall of several layers and a central cavity, called the ker- 

 nel, in which the larvae pupate. There may be several kernels 

 in a single gall. The adult gall-flies that develop in this type of 

 gall have powerful jaws by means of which they gnaw their way 

 out. 



Many of the galls produced by mites are of the simplest type. 

 As a rule the gall is on the upper surface of a leaf with an open- 

 ing on the lower surface. The very simplest type is merely an ab- 

 normal growth of hairs on the surface of the plant, among which 

 the mites live. The most common type of mite gall is formed 

 by an infolding of the leaf caused by an unequal growth of the 

 tissues, thus forming a cavity, simple or branched, which is lined 

 with numerous hairs. In most cases several zones can be recog- 

 nized in a gall; an inner nutritive zone rich in protoplasm and an 

 outer protective zone which is thick-walled and contains less 

 protoplasm, but is provided with a considerable amount of tannin. 



The galls produced by the true bugs (Hemiptera) range from 

 a very simple leaf curl to structures that are very complex. The 

 galls caused by certain plant lice are especially interesting 

 because the tissues of the affected region grow around and enclose 

 the insect. The insect establishes itself on a leaf or stem and 

 draws nourishment from the plant, causing the cells in this region 

 to shrink somewhat, while the adjacent cells are stimulated to 

 excessive growth. Thus the tissues gradually bend over the insect 

 and almost completely enclose it. 



For accounts of the stucture and formation of the more complex 

 types of galls the reader must be referred to the detailed des- 

 criptions given in the books listed at the end of this article. 



The Protection of the Larva 



One of the principal benefits derived by the gall insect from 

 the gall is the protection of its young, the larvae and pupae. Among 

 the agents from which gall insects need protection are weather 

 conditions, insectivorous birds and mammals, and parasitic in- 

 sects. The outer covering of the gall, which is of several layers, 

 forms a protection from mechanical injury, and this is sometimes 

 added to by the formation of an inner shell. Some galls are cov- 

 ered by imbricated scales, as in the willow-cone gall, thus pro- 



