30 
the somewhat peculiar local conditions required for their breeding 
are present. These conditions are, in the first place, running water 
continuous thru the breeding season, and, in the second place, either 
a rippling surface or a fairly rapid flow of the stream. It is also 
necessary that there should be solid objects in the water, not more 
than a few inches under the surface, upon which the eggs may be 
laid and to which the larvae may cling. The water must also, of 
course, contain a sufficient supply of the smaller plankton and other 
organic particles upon which the larvae feed. As they remain at¬ 
tached like plants, and can not search for food, they are dependent 
on whatever chance brings within the reach of the prehensile ap¬ 
paratus about the mouth. The species which breed in rivers find 
these conditions most general during high water, especially in 
spring. Then the current of the stream is comparatively swift and 
strong near the shores, and the marginal overflow reaches to 
trees and shrubs, stranded driftwood, and the like, which create 
the necessary surface disturbance and at the same time provide 
places of attachment for the eggs and larvae. In the smaller 
streams, on the other hand, times of flood are less favorable ex¬ 
cept where there is a rocky bed; but as the summer grasses grow, 
dipping into the stream, and marginal shrubs droop their twigs, 
loaded with leaves, into the water, and as the heavier objects on 
the bottom of the creeks and rivulets are brought near the surface 
by the shrinking of the stream, many suitable places may be 
found here and there for the black-fly to deposit its eggs and for 
the young to reach the pupation stage. 
Myriads of these insects are sacrificed, as our field notes show, 
when the waters fall, leaving the pupse exposed, and liable to dry 
out. Small fish, and certain carnivorous insects, especially case- 
worms, devour the larvre, and their numbers in summer and fall 
are rarely very great in our latitude. The bottom-lands of our 
principal rivers—the Illinois, the Mississippi, the Ohio, and the 
Wabash—from the middle of April to the middle of May, are al¬ 
most the only situations in which the black-flies may be called a 
plague. As the swarms of these insects are readily blown about 
by the wind, they are often carried to considerable distances from 
their place of origin; and cases are on record in which they must 
have been borne several miles m this way. The adults are not 
long-lived, and an outbreak does not ordinarilv continue annov- 
ing longer than ten davs. A storm of wind and rain mav, in fact, 
put an end to it in even less time. The black-fly plague is virtu¬ 
ally uncontrollable, except in a local way where it is due to a com¬ 
paratively small stream. Such a stream may he cleaned and cleared 
of breeding places, and may, if further measures are necessary, be 
treated with phinotas oil in a way to destroy all the black-fly 
