PARASITIC ENEMIES OF TREES AND PLANTS 



491 



IS to be sometimes observed 

 among them ; that a particular 

 part of the plant attacked is 

 always affected by the same 

 species of fly : and that "the 

 parasitic gall-flies live as true 

 internal parasites in other in- 

 sects, mainly plant lice and the 

 larvae of dipterous insects" 

 together with many other 

 facts. His "Life History of 

 a Gall-Gnat" is brimful of in- 

 terest, and a well illustrated 

 contribution to the subject, 

 most worthy of careful study. 

 In it he makes the statement 

 that "the most famous mem- 

 ber of this group is the so- 

 called Hessian Fly {Cecido- 

 myia destructor Say.). This 

 species lives, in the larval 

 state, in stems of wheat, and 

 annually damages the wheat 

 crop of the United States to 

 the extent of many millions of 

 dollars. It is supposed to have 

 been introduced into the 

 United States in the straw 

 brought over for bedding by 

 the Hessian troops during the 

 War of the Revolution. Hence 

 the popular name." 



We have often observed 

 those peculiar little whitish 

 galls on the willow trees ; they 

 are terminal on the twigs and 

 shaped like small cones in- 

 deed they are known as wil- 

 low cone-galls. It is said that 

 a minutq gnat deposits her 

 eggs on the very tip of the 

 twig she selects for the pur- 

 pose. Very soon it hatches, 

 and the gnat at once com- 

 mences to feed upon the ma- 

 terial at hand. This causes 

 the growth-arrest of the twig 



and the stunting of the leaves involved, which latter shrink 

 up to small, scale-like affairs that overlap each other. 

 The grub lives within this snug habitation, occupying it 

 all winter. In the spring it emerges as a minute two- 

 winged fly, a very beautiful little creature when seen 

 through the aid of a high-power microscope. Other in- 

 sects of small size resort to this gall of the willow to 

 live in it ; but they do not seem to interfere with its 

 rightful occupant. 



There is a gall which I noticed only on garden and 



DESTRUCTIVE G.\LL FOUND ON OAK LEAVES 



Fig. 9 This is possibly the gall known as Callirhytis capulus. and is here shown natural 

 size. The specimen was collected by the author in the District of Columbia, within the 

 city limits of Washington; it is not of frequent occurrence. The insert cut is of a drawing 

 made by the author of an "oak apple" that shows the internal radiating fibrous structure. 

 Compare this with Figure lo on the next page. 



wild rose bushes ; it appears to be a compound affair 

 that is, the larvae occupying it are living in community- 

 style. These galls possess a mossy appearance externally, 

 and are known to the student of them as mossy rose- 

 galls. Some curious ones are also found on hazel bushes 

 and they are well worth attention and study. 



During the summer of 1922, while collecting flowers 

 and insects in the woods and fields in the District of 

 Columbia, 1 ran into many specimens of galls on dil- 



