80 
These lice increase in numbers very rapidly, as explained else- 
where, and cause considerable damage. They do not injure the 
head directly or to such a degree as the other insects, but by 
absorbing a large amount of sap, weaken the whole plant.. 
Moreover, a lousy head, even that of a cabbage, is notan appe- 
tizing thing and finds no ready sale. If it were not for the 
assistance of a number of beneficial insects, these leaf-lice 
would soon increase to such an extent that cabbage-growing 
would hardly bea paying business. Among these beneficial 
insects is a gaily-colored fly, about the size of our common 
house-fly, but striped with yellow and black. It lays its eggs 
near a colony of the appids or leaf-lice. From the eggs issue 
tiny. soft, yellow maggots, which attack the appids and suck 
the juice from their bodies. These maggots, atfirst very small, 
grow until about one-eight of an inch in length, when they set- 
tle in quiet places and change to puparia. Shortly after 
this, the adult flies (Syrphus-jlies), issue ready to repeat the 
process. 
Another very beneficial, though a much smaller insect, is a 
parasite which deposits an egg in the living louse. The grub 
or maggot which hatches from the egg, at first seems to eat 
the less vital parts of the host but afterwards the entire con- 
tents. It now lines the inside of the empty skin of the louse 
with a very thin layer of delicate white, silken threads, and in 
doing so, distends and rounds it. This done, a circular lid is 
cut out from the inside but not entirely separated. The adult 
wasp, an aphidiust, would otherwise remain a prisoner, but 
now all that it has to do to gain liberty is to press against the 
iid, from the inside of the cocoon, and the parasite is ready to 
start another cycle of usefulness. Such swollen brownish 
aphids are often seen among the living ones, and attract our 
attention by their roundness, or by the circular hole through 
which the parasite issued. 
When cabbage lice become very numerous, they should be 
destroyed by the use of kerosine emulsion; one part of the 
emulsion to fifteen or twenty parts of water is a good propor- 
tion. Since, however, the lice are found chiefly upon the 
under side of the leaves, and are, moreover, well protected by 
a powdery, waxy material which sheds water, this spraying is 
only successful if very thoroughly done and with a strong 
