256 



NATURAL HISTORY. 



Fig. 245. — Hessian-fly with a bit of wheat 

 straw, showing the place occupied by 

 the ' flaxseed ' stage of the insect ; a 

 and b represent the larvae and pupa: 

 all enlarged. 



undergo a complete metamorphosis in passing into the pupal and adult 



conditions. 



In number of species, the flies stand second only to the beetles ; and 

 although only about twenty-five thousand species have been described from 

 the whole world, the probabilities are that the total number of species 

 is between seventy-five and a hundred thousand. In number of individ- 

 uals, the flies surpass probably every other group of insects. While 

 beetles, butterflies, and bugs usually occur in solitary individuals, one has 



but to recall the immense numbers of some 

 species of flies to see how greatly they out- 

 number all the rest. 



Some of the flies, like some of the Hy- 

 menoptera already mentioned, produce galls 

 on plants, and one of these species, the cele- 

 brated "'Hessian-fly,' has acquired a great 

 economic importance. The name Hessian 

 was applied to it under the belief that they 

 were introduced with straw by the Hessian 

 mercenaries during the Revolutionary War. 

 This view is probably erroneous. No one has 

 yet found in Germany any fly like our pest, and besides, there is good 

 reason to believe that the Hessian-ily committed damage in this country 

 before the Revolution. It is far more probable that the insect was a 

 native American species which suddenly changed its food plant as so 

 many other forms have done within more recent times. 



The earliest recorded ravages of the Hessian-fly were in the neighbor- 

 hood of New York, and from there they have spread over the whole 

 country. The fly is but about an eighth of an inch in length, and yet 

 in certain years their numbers are so enormous that the damage they 

 cause extends up into the millions of dollars. The fly lays its eggs on the 

 young shoots of the wheat, and the larva5 on hatching out bore their way 

 into the centre of the stalk, where they feed on the sap of the growing 

 plant. In this way they rob the kernels of part of their food supply, 

 causing them to fill imperfectly ; and they also weaken the stalk, so that 

 it is easily beaten down by the wind and rain. The first brood of eggs 

 hatches out in August, from the spring wheat ; and then the females 

 lay their eggs in the winter wheat, and these in turn hatch out in the 

 spring. 



Other allied species feed upon the cranberry and the heads of the wheat ; 

 but none of them have acquired anything like the prominence of the 

 Hessian-fly. 



