SKULKERS IN DRY FOOD MATERIALS * 439 



others bore into and are concealed within individual grains 

 of wheat. 



The Hessian Fly (Mayetiola (Cecidomyid) destructor], 

 a notorious destroyer of growing wheat, is an excellent 

 example of introduction through insignificant passivity. 

 The damage is caused by the Fly in its grub stage, the 

 maggots attacking the stems of growing corn at one of the 

 lower joints. or knots. Now this being so, it seemed likely 

 that the insect had been introduced in the maggot stage with 

 straw, but careful and prolonged examination of imported 

 straw revealed no trace of the foreign pest. The resting 

 period of the fly, which succeeds the maggot stage, is passed 

 in a dark brown oval pupa case, resembling, in both shape 



Fig. 70. Hessian Fly (Cecidomyia destructor] natural size and enlarged. The seed- 

 like puparia, in which form the Fly was probably introduced to Britain, are shown as 

 of natural size. 



and colour, a rather small flax-seed. There can be little 

 doubt that to this resemblance we owe the Hessian Fly that 

 it was first brought to Britain in cargoes of foul grain, where 

 the pupa cases of the insect passed unnoticed amongst the 

 real seeds of flax and of a dozen other weeds. 



Whence it reached Britain has never been satisfac- 

 torily determined, but the fact that the insect parasites bred 

 from the imported pupae were identical with those which 

 had been found in native Russian pupae, suggested very 

 strongly that the Hessian Fly came to Britain from Eastern 

 Europe. But wherever its original home may have been it 

 was not long in advertising its settlement in our Islands. 

 First discovered in 1886, it spread in the following year 



