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poisons, or those substances which kill by being eaten. It 
must be evident to all that these can only do harm to insects 
that devour the tissue of the plant, nearly always the leaves. 
Those insects that feed by inserting a sharp beak and suck- 
ing the sap of the plant can not be successfully combatted 
by the use of food poisons. Against them we must, as a 
rule, use some substance that will kill by external contact. 
There are, of course, a great number of preparations that 
would kill in this way but we have to use something that 
will destroy the insect without serious injury to the plant on 
which it is feeding. 
It is not necessary that one be versed in the science of 
entomology in order to be able to determine into which of 
these two classes an insect that is doing damage to his 
plants belongs. If the foliage of the plant is being removed, 
or if the leaves are full of holes or ragged as the result of 
the insect attack, it is reasonably certain that the marauder 
has a good pair of jaws and is devouring the solid parts of 
the leaves; but, if the leaves only turn pale or brown and 
curl, and are not eaten into, it is then quite certain that the 
insect is doing the damage by extracting the juices of the 
plant. Examples of such insects are Plant Lice, Scale Lice, 
the Squash Bug, Chinch Bug, etc. Examples of the former 
class are Grasshoppers, Potato Beetles, Leaf Rollers, the 
Codling Moth, etc. 
Then there are insects in both of the above classes that 
are best destroyed by the use of certain volatile substances 
which kill by being inhaled. As examples I might mention 
certain grain-eating weevils, ants, root-infesting lice, vermin 
in houses as Bed Bugs, and Clothes Moths, etc. 
In Its broadest sense the term insecticide is also made to 
include certain substances which are used only as repellants. 
These do not kill and are of much less value than either of 
the other classes. Napthaline, camphor and carbolic acid 
are examples of such substances. 
I’REPARA'I'ION AND USE. 
It will be understood that almost any insecticide may 
need to be used in different strengths under varying condi- 
tions. In the following formulae I give the ordinary prepa- 
tions. 
