The Environment. 



43 



The isolated galls were small, monothalamous, and in the shape of a truncated 

 cone, usually on the upper surface of the leaves and standing in an oblique position 

 The opening of the larval chamber was through the top and was guarded by hair- 

 like growths or trichomes, which pointed inward. This would indicate a cecidomyid 

 gall, but certain preparations showed the opening closed by a thin membrane. 

 Whether this latter condition was real, therefore proving the presence of two species 

 of galls, or only apparent, was difficult to determine, owing to a tendency of the galls 

 to coalesce, forming irregular masses. 



HISTOLOGY. 



The gall in its earliest state shows the reduction of the palisade into cells of 

 the mesophyll type. This condition is characteristic of the origin of all leaf galls. 

 As the gall develops, the cells, which constitute the lining of the larval chamber, are 

 rich in protoplasmic content, which decreases from inner to outer surface. This is 

 indicated very readily by the stains and is characteristic of the more highly devel- 

 oped galls and usually designated as the nutritive zone. A little later certain galls 

 showed a reduction of the nutritive zone and the formation of a protective zone of 

 sclerenchyma cells just outside the nutritive zone. The presence of this protective 

 zone is characteristic of the galls produced by cynipidous insects, and the writer has 

 never found them in galls caused by cecidomyid insects. 



From the above facts, it appears that we may have two species of galls, one pro- 

 duced by a cynipidous insect and the other by a cecidomyid, or a single gall which 

 has been parasitized. 



Fig. 6. The guayule barkbeetle {Pilyophthonis nigricans Bland), (a) Work of beetles and larvae in 

 bark and wood, (b) Adult beetle, greatly enlarged. Small figure at right shows natural size, (c) Egg- 

 galleries of parent beetles, with intervening larval mines, all grooved on surface of wood. (From 

 illustrations loaned by the Bureau of Entomology. U. S. Dep. Agric.) 



