308 ENTOMOLOGY 
winds, and oceanic islands have undoubtedly been colonized in 
this way. On land, Webster has found that the direction in 
which the Hessian fly spreads is determined largely by the 
prevailing winds at the time when these delicate insects are on 
the wing, and that the San José scale insect spreads far more 
rapidly with the prevailing winds than against them, the wind 
carrying the larve as if they were so many particles of dust. 
The pernicious buffalo-gnat of the South emerges from the 
waters of the bayous and may be carried on a strong wind to 
appear suddenly in enormous numbers twenty miles distant 
from its breeding place. Mosquitoes are distributed locally by 
light breezes, but cling to the herbage during strong winds. 
Ocean currents may carry eggs, larvee or adults on vegetable 
drift to new places thousands of miles away. Thus the Gulf 
Stream annually transports thousands of tropical insects to the 
shores of Great Britain, where they do not survive, however. 
Fresh-water streams convey incalculable numbers of insects 
in all stages; and insects as a whole are very tenacious of life, 
being able to withstand prolonged immersion in water, and 
even freezing, in many instances, while they can live for a long 
time without food. 
The universal process of soil-denudation must aid the dif- 
fusion of insects, slowly but constantly. 
Birds and mammals disseminate various insects in one way 
or another, while the agency of man is, of course, highly im- 
portant. Intentionally, he has spread such useful species as 
the honey bee, the silkworm and certain useful parasites; inci- 
dentally he has distributed the San José scale, Colorado potato 
beetle, gypsy moth and many other pests. 
Barriers.—The most important of the mechanical barriers 
which limit the spread of terrestrial species is evidently the sea. 
Mountain ranges retard distribution more or less successfully, 
though a species may spread along one side of a range and 
sooner or later pass through a break or else around one end. 
Mountain chains act as barriers, however, chiefly because they 
present unendurable conditions of climate and vegetation. 
