Circular 21 - Insecticides and Subsidiary Materials 
the poison is taken into the insect's stomach 
with the food. Some examples of stomach poisons 
are Paris ci^^^n, calcium arsenate, lead arsenate, 
and cryolite. Contact insecticides are developed 
primarily to kill insects that obtain their food 
by sucking or extracting the juices from plants 
or the blood of animals upon which they feed. 
Such insects are not affected by stomach poisons 
and must be combatted with contact insecticides. 
These materials kill the insects by their caustic 
action, by suffocation, or corrosion of the in- 
sect's body. Certain species of chewing insects 
may be killed by contact insecticides, whether 
applied directly to their bodies or indirectly 
through the material which they are daiaaging. 
Some examples of contact poisons are pyrethrum, 
nicotine, and derris. 
Insecticides are applied in several ways: 
as liquid sprays or dips, dusts, or fumigants, 
depending on the insect problem. In the case 
of v/ood products certain chemicals of a preser- 
vative nature may be applied by impregnation 
rather than by superficial treatment as just 
mentioned. 
Insecticidal sprays consist of a liquid 
such as water or oil to v.'hicli is added one or 
more active insecticidal ingredients in defi- 
nite quantities \mere they remain in suspension 
or solution so that they can be applied to 
plant, man, or animal, or their habitations to 
destroy insects thereon or therein. Such mix- 
tures are usually applied with a mechanical 
device called sprayer or atomizer (oee Circular 
20) and under considerable pressure or mixed 
with air so that they can be broken into very 
fine particles or droplets. 
