80 INSECTS AND MAN 



to most members of the family, and is probably an adapta- 

 tion to varying climatic conditions. No nourishment is 

 taken during this stage, but moisture is absolutely necessary. 

 Within the third-stage sheath or puparium, the larva turns 

 into a pupa (fig. 18, c), and remains as such for a variable 

 time, depending on the temperature. When the adult is 

 about to emerge, the end of the puparium is forced off, and 

 the pupa, by active wriggling, not only frees itself from 

 its covering, but also from the leaf sheath, and almost 

 immediately the pupa case splits and the fly appears. The 

 length of life of the adult stage is one of hours rather than 

 days, lasting only so long as is necessary for mating and 

 the deposition of eggs. 



The accurate study of the life-history of the Hessian 

 fly presents many difficulties few insects show such 

 variations during the life-cycle; and this is not to be 

 wondered at, for it is rare to find an insect so well able 

 to adapt itself to varying climatic conditions. Its annual 

 broods vary from one to six, depending on the latitude, 

 and, in addition, it possesses the power of acceleration or 

 retardation, according to the conditions of the year, some- 

 times appearing early, sometimes late. 



The Hessian fly is extensively parasitised, a fact which 

 of course tends to keep it within reasonable bounds, in 

 most seasons. Curiously enough, its most deadly enemy, 

 a minute Hymenopteron called Merisus destructor, occurs 

 naturally, both in Europe and in America, that is to say, it 

 is not a species that has been introduced with the object of 

 controlling the fly, and it is usually when the Hessian fly 

 reaches a new wheat-growing district ahead of its parasites, 

 so to speak, that the greatest damage occurs. 



MEDITERRANEAN FRUIT FLY 



Why are so many of the insects, described in this chapter, 

 either native or introduced American species ? The question 

 is sure to be asked, so let us answer it now. The reason 



