This document classifies insecticides into four main categories based on their mode of entry: 1) Stomach poisons which are ingested and act after being eaten, 2) Systemic insecticides which are absorbed by plants or animals and kill insects that feed on them, 3) Contact poisons which are absorbed through the insect's body wall, and 4) Fumigants which become gases and enter through the tracheal system. It also notes there are two additional classifications based on mode of action and toxicity.

2. Insecticides Classification
1-On the basis of mode of Entry.
Stomach Poisons
enter the insect body through the
gut and are fatal only after they are eaten. It is
mainly the oldest insecticides that fall within
this grouping, and few true stomach poison
are used today e.g.,boric acid (H3 BO4), used
against cockroaches and other crawling
insects in the household.

3. Systemic insecticides
are taken up and translocated
within plants and animals. Insects feeding on the
protected host contact the insecticide through
the gut, and susceptible individuals are killed.
Systemics in plants mostly kill piercing-sucking
insects; larger chewing insects usually are not
affected. Piercing-sucking pests are killed more
readily because they normally receive a greater
insecticide dose than chewing insects feeding on
the same plant. In livestock, systemics are often
used against internal parasites like cattle grubs,
Hypoderma species.

4. Contact poisons
are the major group of modern
insecticides. They usually enter the body when
the insect walks or crawls over a treated surface.
The insecticide is absorbed through the body
wall. If the treated surface is a food source like a
leaf or blossom, these poisons may also enter the
digestive tract and be absorbed through it.
Grouping insecticides by manner of entry begins
to break down when multiple entry sites are
encountered. Still, the primary entry site of
contact poison is from the environment and
through the body wall.s

5. Fumigants
are insecticides that become gases
at temperatures above 5°C. A fumigant’s
contact site is the tracheal system.These
insecticides are applied to enclosures and to
soil. Being volatile, they enter the tracheal
system, circulate, and subsequently are
absorbed by body tissues e.g.,Alominium
Phosphide

6. 2-On the basis of Mode of Action

7. 3-On the basis of Toxicity