THE TWO-WINGED FLIES 123 



the Pear Midge, the Clover-seed Midge and certain 

 other pests. 



Gall Gnats 



The Hessian Fly belongs to the family of Gall Gnats 

 (Cecidomyiidae). These are mosquito-like flies which live 

 in the larval state upon the growing tissues of plants, 

 commonly causing an abnormal swelling, which is called a 

 gall. In the case of the Hessian Fly, the adult insect is a 

 small creature with smoky brown wings, which appears ia 

 the wheat fields in autumn soon after 

 the young plants are up. It deposits 

 eggs upon the leaves, generally a little 

 above the joints. These eggs shortly 

 hatch into very small maggots that 

 work their way down to the joints and 

 into the space between the sheath and 

 the main stalk. Here they remain 

 and gradually absorb the sap of the 

 plant. They grow rather slowly, and hessian fly 



their presence causes a slight swelling Magnified 



of the surrounding tissues and prevents 

 the normal growth of the young wheat plant. After some 

 weeks, they become full grown in this larval stage, and now 

 change to what is called the flaxseed condition. To bring 

 about this change the outer skin of the larva separates from 

 the skin beneath and gradually hardens into a brown pro- 

 tective covering. This covering, which looks like a flax- 

 seed, is commonly said to be the puparium. It corresponds 

 in a way to the cocoon of a moth in that it serves as a pro- 

 tection to the insect inside, although its method of formation 

 is entirely distinct from that by which a cocoon is formed. 



The larvae remain within these flaxseeds throughout 



