Doubtless since his early beginnings man has been aware of the fact that various arthropods produce substances poisonous to him, on his coming in contact with them. A literature that goes back three centuries indicates that these properties have not been without interest to the scientist and other observers.
We can divide irritating insects into three groups:
Insects that cause a reaction through the injection of salivary secretion, or some constituent of the alimentary canal, when their mouth parts pierce the integument. Practically all such species suck blood. Among these may be enumerated mosquitoes, gnats, horseflies, bedbugs, fleas and lice.
Insects that cause a decided reaction through the introduction of the products of glands situated in the caudal portion of the body. Bees and wasps come in this category.
The preceding groups involve mature forms. Tubercles on the dorsal and lateral areas of a number of caterpillars bear a