DIP TER A. 
445 
very serious injury; in this way the amount of a crop of 
grain is often greatly reduced. 
The larvae are small maggots, with nine pairs of spiracles. 
Many species are brightly colored, being red, pink, yellow, 
or orange. In almost every case a larva 
belonging to this family can be recog¬ 
nized as such by the presence of a horny 
piece on the lower side of the body, be¬ 
tween the second and third segments (Fig. Fig. 523. — Head-end of 
523). This piece is called the breast-bone. br 2 st-bone! wlng the 
Its homology and use have not been definitely determined. 
The different species vary as to the method of under¬ 
going their transformation ; in some the pupa is naked ; in 
others the pupa is enclosed in the dried skin of the larva; 
and in still others it is enclosed in a delicate cocoon. 
One of the most common and conspicuous of the galls 
made by gall-gnats is the Pine-cone Willow-gall (Fig. 524). 
This often occurs in 
great abundance on the 
tips of twigs of the 
Heart - leaved Willow 
(Salix cordatd). The 
gnat that causes the 
growth of this gall is 
Cecidomyia strobiloides 
(Cec-i-do-my'i-a strob-i- 
loi'des). The gall is a 
deformed and enlarged 
bud ; the lengthening of 
the stem is checked by 
the injury caused by 
the larva; but leaves 
Fig. 524.—The Pine-cone wiiiow-gaii. continue to be devel¬ 
oped which results in the cone-shaped growth. The larva 
remains in the heart of the gall throughout the summer and 
winter, changing to a pupa early in the spring. The adult 
