583 
them in check. In fact, were it not for these natural checks to aid 
man in preventing the destruction of his plants, all his efforts would 
prove unavailing. Among the most active in this warfare are the 
parasitic flies, which are mostly very small and with four trans- 
parent wings. They lay a single egg on a louse, which hatches 
and produces a maggot in the interior of the host, and which feeds 
on the juice of the louse until it changes to a pupa, from which it 
emerges through a small hole cut in the body of the louse. A close 
inspection of a family of plant lice will disclose many with this hole 
in them. A parasitised louse may be told, even before its death, 
by the swollen abdomen, which will distinguish it from its non- 
parasitised mates. 
One of the most numerous of the predaceous insects is the 
Syrphus or flower flies, which may b seen hovering around flowers 
remaining for a short time stationary in the air. They bear a 
Syrphus fly larve in the act of sucking out an aphis. 
resemblance to some species of wasps, with broad yellow band 
across the abdomen. The fly in its matured state is of no econo 
mic importance. The larve, 
however, are very destruc- 
tive to plant lice. The 
parent fly lays a single 
egg among a Cluster of lice, 
ss : 
this hatches in a few days, 
and immediately com- 
mences its work of destruc- 
Syrphus fly. Syrphus fly larve suck- 
ing out the vital fluids tion. It seizes a louse, and 
of an aphis. holding it aloft while it kicks 
and struggles, sucks the juice 
from its body, and then casts the empty skin away. These maggots 
are blind and have no distinct feet, but they have no trouble in 
