134 NEW ZEALAND NATURE-STUDY BOOK 
the egg so firmly to the hair that it is almost impossible 
to remove it. ‘The eggs, it will be noticed, are always laid 
in such a position as to be within reach of the horse’s 
tongue, ¢.g., on the flank, the shoulders, or the inside of the 
knees. ‘The larva remains in the egg, and does not hatch 
out until the moment arrives for entering the horse. 
Warmed and moistened by the horse’s tongue, it then 
Fig. 91.—Hessian Fly. (1) Straw, with fly in chrysalis stage, showing 
damage to straw. This prevents the head from filling and causes the straw 
tofall, (2-3) Maleand female flies. (4) Parasite, which lays eggs in chrysalis 
of Hessian fly, and so helps the farmer to fight the pest. 
escapes from the egg, enters the horse’s mouth, and 
passes into the stomach. There, by means of a pair of 
strong hooks projecting from its mouth, it attaches itself 
to the mucous lining and feeds at the horse’s expense until 
it is ready to pupate. When full-fed, it loosens its hold 
and passing down the intestine reaches the ground in 
which it enters on the pupa stage. There is no method 
