I i _,«___, t,-,-,^ J J 



82 THE SPEIXG GRAIX-APHIS OK GEEEX BUG. 



the presence, during each extended invasion, of some important influ- 

 ence that shapes, to a marked degree, the course of these invasions 

 across the country northward and northeastward from the point of 

 their origin in the South. Probably this is due primarily to the direc- 

 tion of the winds during the months between January and June. 



The degree of influence exerted by the winds in the diffusion of 

 Toxoptera is, however, dependent upon several other factors. In the 

 first place, with wingless individuals alone present, it is clear that no 

 amount of wind of whatever velocity would distribute the species to 

 any considerable degree. Therefore, it is necessary to understand the 

 vital forces that regulate the abundance of winged individuals, which, 

 at the critical period, would probably be almost without exception 

 viviparous females. Field observations have shown, not only among 

 this but among other species of aphidids, that a curtailing of the food 

 supply is a most potent influence in producing the aerial form. Xot 

 only has it been observed with Toxoptera 'that as the food plants lose 

 their vigor, affording less nutrition, the winged individuals become 

 more and more abundant in the fields, but both Mr. Phillips and Mr. 

 Urbahns have been able, by regulating the food supply, to produce 

 these winged individuals, artificially at will, in their rearing cages. In 

 the case of Macrosiphum granarial$uckt.,it has always been noticed 

 that though the heads of wheat be literally swarming with wingless 

 females and young, these young do not perish as the food supply 

 becomes exhausted on account of the ripening of the grain, but 

 develop into winged adults which fly away, leaving only the cast larval 

 and pupal skins on the ripening wheat heads. Therefore, so long as 

 there is an abundant supply of vigorous young grain the percentage of 

 winged adults appearing will be comparatively few. The condition of 

 the food supply, then, is a prime factor in the diffusion of Toxoptera, 

 except when greatly decimated in numbers from excessive parasitism. 



If the temperature be below the point of activity for the species, it 

 is very clear that the velocity of the wind would have no effect what- 

 ever upon the diffusion of the insect. The conditions necessary, then, 

 for the wind to exert its greatest influence will be a decreasing food 

 supply for the insect under a temperature considerably above that 

 actually necessary for its activity, with numbers not seriously reduced 

 by parasites: under these conditions, many species of aphidids are 

 known to be carried about in immense numbers by the winds. 



White, in his Natural History of Selborne ■ has this reference to 

 a migration of small aphidids. 



As vre have remarked above that insects are often conveyed from one country to 

 another in a very unaccountable manner, I shall here mention an emigration of small 

 Aphides, vhich was observed in the village of Selborne no longer ago than August 1, 

 1785. 



1 Xatur?.l History and Antiquities of Selborne. By the Rev. Gilbert White, M. A., London, 1836, pp. 

 365-366. 



