No. 430.] THE DIFFUSION OF INSECTS. 797 
this direction is offered by the spread of the San José scale in 
orchards, especially in the Middle West. If infested trees, 
brought from the nursery and transplanted to the orchard, 
happen to be set along the north or east margins of an orchard, 
the spread through that orchard is comparatively slow; but 
on the other hand, if the infested trees happen to be placed along 
the west or south margins, the progress is much more rapid. 
So also, where the introduction is into a section of country 
largely devoted to tree fruits, the spread over the country is 
much more rapid towards the northeast than in any other 
direction. This is for the reason that our prevailing winds, 
during the period when the young are carried from one place 
to another by these winds, blow from the southwest or west, 
more generally the former. In the case of the Hessian fly, 
Cecidomyia destructor, Y have noted that, where a field of wheat 
has been seriously attacked in the fall, and adjoining fields have 
escaped, some of these last may be seriously attacked the fol- 
lowing spring, while others seemingly equally exposed to attack 
almost entirely escape injury. A careful examination into the 
facts, however, will show that this is due to the direction of 
the prevailing winds at the time when the spring brood of flies 
were abroad, and the wind simply carried them toward those 
fields that were located in one direction and away from those 
located in the opposite direction, thereby to a certain extent 
protecting the one from attack and causing the destruction of 
the other. In the case of the San José scale, active only in the 
very young larva, I have noted that, where infested trees 
happen to be placed in a gully extending up a steep hill, the 
spread will be much more rapid upward than downward, as the 
air current is in that direction. So, then, prevailing winds 
have in some cases much to do with the spread of insects in 
certain directions. The influence of high winds on insects is 
illustrated in one way by the great number of butterflies that 
are sometimes encountered by ships at sea, long distances 
from land. Indeed, the entomological fauna of very many of 
the islands of the sea indicates very strongly that insects 
have become established on such islands by having been blown 
from the mainland, or from other islands located at considerable 
