14.4.1. Classification by function of defensive chemicals
Amongst the diverse range of defensive chemicals produced by insects, two classes of compounds can be distinguished by their effects on a predator. Class I defensive chemicals are noxious because they irritate, hurt, poison, or drug individual predators. Class II chemicals are innocuous, being essentially antifeedant chemicals that merely stimulate the olfactory and gustatory receptors, or aposematic indicator odors. Many insects use mixtures of the two classes of chemicals and, furthermore, Class I chemicals in low concentrations may give Class II effects. Contact by a predator with Class I compounds results in repulsion through, for example, emetic (sickening) properties or induction of pain, and if this unpleasant experience is accompanied by odorous Class II compounds, predators learn to associate the odor with the encounter. This conditioning results in the predator learning to avoid the defended insect at a distance, without the dangers (to both predator and prey) of having to feel or taste it.
Class I chemicals include both immediate-acting substances, which the predator experiences through handling the prey insect (which may survive the attack), and chemicals with delayed, often systemic, effects including vomiting or blistering. In contrast to immediate-effect chemicals sited in particular organs and applied topically (externally), delayed-effect chemicals are distributed more generally within the insect’s tissues and hemolymph, and are tolerated systemically. Whereas a predator evidently learns rapidly to associate immediate distastefulness with particular prey (especially if it is aposematic), it is unclear how a predator identifies the cause of nausea some time after the predator has killed and eaten the toxic culprit, and what benefits this action brings to the victim. Experimental evidence from birds shows that at least these predators are able to associate a particular food item with a delayed effect, perhaps through taste when regurgitating the item. Too little is known of feeding in insects to understand if this applies similarly to predatory insects. Perhaps a delayed poison that fails to protect an individual from being eaten evolved through the education of a predator by a sacrifice, thereby allowing differential survival of relatives (section 14.6).