82 
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. Not 
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 MacrosipJium granariaBuckt.,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 we 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 Natural History and Antiquities of Selborne. By the Rev. Gilbert "White, M. A., London ,1836, pp. 
36S-366. 
