Chemical ecology of the Insecta comprises an invisible environment where semiochemicals are the principal factors regulating the mating and host-selection of the one million or more species. Biologically effective concentrations of these semiochemicals range from 10−10 to 10−3 μg at the insect antennal receptors. These levels are virtually undetectable by conventional microchemistry, and can be measured only by behavioral bioassay and by electrophysiology which are about 10,000 and 100-fold more sensitive than gas chromatography.