DIP TERA. 



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 .-Headend of 



. . . . , , larva showing the 



523). lhis piece is called the breast-bone, breast-bone. 



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 cor data). The 

 gnat that causes the 

 growth of this gall is 

 Cecidomyia strobiloidcs 

 (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 

 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 



Fig. 524.— The Pine-cone Wil!ow-ga11. 



