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sary to use substances which will act externally on the bodies of these 
insects, either as a caustic or to smother or stifle them by closing 
their breathing pores, or to fill the air about them with poisonous 
fumes. Of value also as repellants are various deterrent or obnoxious 
substances. 
Wherever it is not desirable to use poisons for biting insects, some 
of the means just enumerated will often be available. 
GROUPS SUBJECT TO SPECIAL TREATMENT. 
The general grouping outlined above relates to the species which 
live and feed upon the exterior of plants for some portion or all of 
their lives, and includes the great majority of the injurious species. 
Certain insects, however, owing to peculiarities of habit, inaccessi- 
bility, or other causes, require special methods of treatment. Of 
these, two groups properly come within the scope of this bulletin: 
(1) Those working beneath the soil, or subterranean insects, such as 
the white grubs, root maggots, root-lice, etc., and (2) insects affecting 
stored products, as various grain and flour pests. 
Three other groups, which include species requiring very diverse 
methods of treatment, and therefore not coming within the limits of 
this bulletin, are (1) the internal feeders, such as wood, bark, and stem 
borers, leaf-miners, gall insects, and species living rue fruits; (2) 
peaeenold pests, and (3) animal parasites. 
The classification of insects outlined above, based on mode of nour- 
ishment and indicating groups amenable to similar remedial treat- 
ment, simply stated, is as follows: 
I. External feeders: 
(a) Biting insects. 
(b) Sucking insects. 
II. Internal feeders. 
III. Subterranean insects. 
IV. Insects affecting stored products. 
V. Household pests. 
VI. Animal parasites. 
INSECTICIDES FOR EXTERNAL BITING INSECTS (FOOD POISONS). 
THE ARSENICALS: PARIS GREEN, SCHEELE’S GREEN, ARSENATE 
OF LEAD, AND LONDON PURPLE. 
The arsenical compounds have supplanted, practically, all other sub- 
stances for the insects falling under this heading.’ The two arsenicals 
in most common use, and obtainable everywhere, are Paris green and 
London purple. The other two arsenicals mentioned,” viz, Se cheele’s ; 


1 Hellebore.—The powdered roots of the white heliebers ( Veratrum viride ) are often 
recommended and used as an insecticide, particularly as a substitute for the arsenites. 
This substance is useful when a few plants only are to be sprayed, as in yards and 
