ORDER DIPTERA 



253 



The adults emerge from their overwintering puparia very- 

 early in the spring. Hardly have the new leaves opened 

 than these flies can be seen flying about the elms. The eggs 

 are minute and difficult to find. They are inserted beneath 

 the upper epidermis usually at or close to the margin of the 

 leaf. The larva mines towards the center of the leaf making 

 a narrow linear track which is pale green in color. During 

 this time the larva moults twice leaving their mouth hooks 

 and larval skins behind to tell the tale. After the second 



Fig. 78. Mine of Agromyza ulmi on American elm. 



moult the mind is broadened to form a blotch about an inch 

 in length. The blotched portion of the mine is brown in 

 color and within can be seen the frass arranged in two more 

 or less regular rows of dots. The swaths made by the larva 

 in getting its food is feebly impressed on the parenchyma of 

 the leaf. When mature the larva escapes through a semi- 

 circular slit cut either in the upper or lower surface of the 

 mine and the larva falls to the ground where it forms its 

 puparium. 



There is apparently but one generation a year and this 



