THE HESSIAN FLY. 73 



in different years will be attained, but rather to direct attention 

 to a class of extremely interesting natural phenomena which can- 

 not fail to become of value as they accumulate. 



129. (1) It has long been known in Germany that the race of 

 pine beetles increased most in warm dry summers, followed by cold 

 dry winters. 'Hot weather shortens the period of transformation, 

 and thus affording time for the maturation of the several broods, 

 causes a superabundant number of insects to be found. 'W 



(2.) The oak trees in Devonshire have suddenly appeared stud- 

 ded with gallnuts during the last three or four years, and in 

 numbers so abundant as nearly to equal the leaves. < 2 > The Hes- 

 sian fly and wheat midge are true gall flies, and the sudden in- 

 crease of one of their kindred giving rise to the common gall- 

 nut in countless multidudes, shows how universally the capability 

 of rapid and unexpected increase is shared by different species 

 of this allied generation. 



(3.) See paragraph 160. 



130. In the communication to the writer, (before referred to) 

 dated Feb. 2nd, 1857, Dr. Fitch says : " It has long been my 

 opinion that the great multiplication of the insect depredators 

 on wheat, and of insects generally, which takes place in particu- 

 lar years, is caused in part, at least, by certain peculiarities of the 

 atmosphere of that and of the preceding year. This subject is 

 alluded to under several of the species in my reports. What 

 those atmospherical peculiarities are, in the case of any particular 

 insect, is yet unknown to us. One of the general laws relating 

 to this matter, I think, will be found to be this that whatever 

 peculiarities of the season occasion a luxuriant growth of a par- 

 ticular plant, will also favour the multiplication of the insects 

 feeding upon that plant. But we are here treading upon slippery 



(1) David Gorrie, Esq., ; Farmers' Note Book ; Highland Agr. Soc. Trans. 



(2) Illustrated London News, March, 1857. 



