14.4.2. The chemical nature of defensive compounds
Class I compounds are much more specific and effective against vertebrate than arthropod predators. For example, birds are more sensitive than arthropods to toxins such as cyanides, cardenolides, and alkaloids. Cyanogenic glycosides are produced by zygaenid moths (Zygaenidae), Leptocoris bugs (Rhopalidae), and Acraea and Heliconius butterflies (Nymphalidae). Cardenolides are very prevalent, occurring notably in monarch or wanderer butterflies (Nymphalidae), certain cerambycid and chrysomelid beetles, lygaeid bugs, pyrgomorphid grasshoppers, and even an aphid. A variety of alkaloids similarly are acquired convergently in many coleopterans and lepidopterans.
Possession of Class I emetic or toxic chemicals is very often accompanied by aposematism, particularly coloration directed against visual-hunting diurnal predators. However, visible aposematism is of limited use at night, and the sounds emitted by nocturnal moths, such as certain Arctiidae when challenged by bats, may be aposematic, warning the predator of a distasteful meal. Furthermore, it seems likely that the bioluminescence emitted by certain larval beetles (Phengodidae, and Lampyridae and their relatives; section 4.4.5) is an aposematic warning of distastefulness.
Class II chemicals tend to be volatile and reactive organic compounds with low molecular weight, such as aromatic ketones, aldehydes, acids, and terpenes. Examples include the stink-gland products of Heteroptera and the many low molecular weight substances, such as formic acid, emitted by ants. Bitter-tasting but non-toxic compounds such as quinones are common Class II chemicals. Many defensive secretions are complex mixtures that can involve synergistic effects. Thus, the carabid beetle Heluomorphodes emits a Class II compound, formic acid, that is mixed with n-nonyl acetate, which enhances skin penetration of the acid giving a Class I painful effect.
The role of these Class II chemicals in aposematism, warning of the presence of Class I compounds, was considered above. In another role, these Class II chemicals may be used to deter predators such as ants that rely on chemical communication. For example, prey such as certain termites, when threatened by predatory ants, release mimetic ant alarm pheromones, thereby inducing inappropriate ant behaviors of panic and nest defense. In another case, ant-nest inquilines, which might provide prey to their host ants, are unrecognized as potential food because they produce chemicals that appease ants.
Class II compounds alone appear unable to deter many insectivorous birds. For example, blackbirds (Turdidae) will eat notodontid (Lepidoptera) caterpillars that secrete a 30% formic acid solution; many birds actually encourage ants to secrete formic acid into their plumage in an apparent attempt to remove ectoparasites (so-called “anting”).