8 NEW YORK STATE MUSEUM 



and on rearing the gall wasps from a few twigs it was estimated that 

 over 500,000 insects were then living at the expense of the one 

 tree. Last summer the galls of Andricus gemmarius 

 were so numerous on a large pin oak near Newton, N. J., that the 

 sweet exudation from the deformations attracted hosts of bees 

 and flies. The unusual noise, humming or "roaring" of these 

 insects resulted in an examination which showed the large tree to be 

 generally infested. The twig illustrated in figure 59 gives an idea 

 of the abundance of the galls. Among gaU midges the weU-known 

 Hessian fly frequently becomes numerous enough to destroy extensive 

 areas of wheat; the Sorghiim midge and the clover midge make it 

 nearly impossible to grow the seed of these plants in certain sections, 

 while the rose midge, the violet midge and the chrysanthemum 

 midge are greenhouse species known only too well to the grower of 

 these highly cherished flowers. 



The collection and study of plant galls is a field open to many. 

 The specimens are to be found at all times of the year and those 

 with woody tissues require no special preparation if one desires to 

 make a collection. It is not difficult in numerous instances, especially 

 if collecting is deferred until the gall insects are nearly mature, to 

 rear the producers, though some care is necessary to distinguish 

 between the true gall makers and the very similar inquilines, or par- 

 ticularly in the case of gall midges, between gall makers and other 

 frequently associated enemies. Gall insects are small, fragile, and 

 can not be studied successfully without a good microscope, con- 

 siderable technical skill and a moderately good collection and 

 library. 



Insect galls may be defined as vegetable excrescences resulting 

 from insect activities and usually sheltering the immature stages 

 of the producers, though a wide acquaintance with these growths 

 ■ demonstrates the existence of innumerable gradations between the 

 apparently normal and the decidedly abnormal and, as a conse- 

 quence, it is difficult to establish a satisfactory distinction between 

 insect galls and deformations not worthy of classification under this 

 term. Some would include the mere curling of leaves and while 

 to a certain extent this is justified, in most cases unless the curling 

 is pronounced the deformation has not been considered as a gall 

 coming within the scope of this work. 



The two most important groups of gall insects — the gall 

 midges or the Itonididae, and the gah wasps or the Cynipidae — 

 each contain species living at the expense of plants and producing 

 little or no swelling of the plant tissues. In the former group there 



