368 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 Jose scale insect spreads far more 

 rapidly with the prevailing winds than against them, the wind 

 carrying the larvae 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, larvae 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 Jose 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. 



