THE TWO-WINGED FLIES 129 



These eggs very soon hatch into small maggots that bur- 

 row through the outer skin of the leaf and begin to de- 

 velop as miners of the interior tissue. They thus become 

 true leaf miners, and their presence is soon shown by the 

 difference in the color of the surface of the leaf. By 

 holding such a leaf up to the light, the maggot inside can 

 generally be seen. These larvae continue to develop 

 within the leaf for a few weeks before they become full 

 grown, and by this time a single larva will have injured 

 a large portion of a leaf. When full grown they burrow 

 through the outer skin and drop to the ground, where they 

 change to pupae slightly below the surface. A little later 

 they again change to adult flies. There appear to be sev- 

 eral broods each year. 



As is the case with the root maggots, these leaf-mining 

 species are by no means confined to cultivated crops. They 

 may very commonly be found at work upon wild plants, such 

 as the white pigweed or some of the commoner docks. 



OBSERVATIONS FOR PUPILS 

 HESSIAN FLY 



1. If you live in a wheat-growing region, learn what you can about 

 local damage by the Hessian Fly. If not now injurious, ask your farmer 

 friends if it has been in the past. 



2. Examine wheat fields to see if you find injured plants. If so, look 

 for the larvae or " flaxseeds " of this insect. 



3. If found, place the flaxseeds in a glass dish to see if you can rear 

 the flies. 



4. Read the account of the Hessian Fly in Circular 70, Bureau of 

 Entomology, U. S. Department of Agriculture. 



APPLE MAGGOT 



I. If you live in an apple region, determine whether this insect is pres- 

 ent. Examine windfalls of early varieties to see if the characteristic bur- 

 rows in the pulp are present. 



