Terrestrial arthropods and colleagues investigating defensive spray in
bombardier beetles. The paper is specially treated to have a
color reaction with the spray, which is normally clear. There are many strategies terrestrial arthropods employ in terms of chemical defense. The first of these strategies include the direct use of secondary metabolites. Many insects are distasteful to predators and excrete irritants or secrete poisonous compounds that cause illness or death when ingested. Secondary metabolites obtained from plant food may also be sequestered by insects and used in the production of their own toxins. One of the more well-known examples of this is the
monarch butterfly, which sequesters poison obtained from the
milkweed plant. Among the most successful insect orders employing this strategy are beetles (
Coleoptera), grasshoppers (
Orthoptera), and moths and butterflies (
Lepidoptera). Insects also biosynthesize unique toxins, and while sequestration of toxins from food sources is claimed to be the energetically favorable strategy, this has been contested.
Passion-vine associated butterflies in the tribe
Heliconiini (sub-family
Heliconiinae) either sequester or synthesize
de novo defensive chemicals, but moths in the genus
Zygaena (family Zygaenidae) have evolved the ability to either synthesize or sequester their defensive chemicals through convergence. These defensive responses can include (but are not limited to) avoidance and escape responses, safeguarding offspring, aggressive behaviors, and applying "direct defenses" (i.e. toxins or defensive chemicals similar to the strategy of the monarch butterfly discussed above). Delaying oviposition can reduce the risk of predation and falls under the category of protecting offspring. Similarly, spider mites are also able to sense damaged body parts of individuals of the same species, or conspecifics, and present the same avoidance behavior as with predator cues. Furthermore, spider mites exhibit a similar behavior with egg-laying as the fruit fly and will elect to move to areas absent of predator cues before oviposition. Spider mites will not avoid areas with other, non-predator volatiles meaning these organisms are able to chemically distinguish threats from non-threats. Alternately, chemical detection of predators or threats can instigate aggressive behaviors in some terrestrial arthropods, rather than escape and avoidance behaviors. These wasps have evolved to detect pheromones in the venom of members of the same species. Identifying volatiles from the venom of conspecifics allows the vespid wasps to discern a nearby threat. When detected, these pheromones induce an attacking behavior within members of the same species. These wasps will then work together to defeat the threat. Similarly, honeybees (
Apis mellifera scutellata) release a warning pheromone when threatened. These pheromones intensify the honeybees' defenses by increasing the duration of the stinging behavior in all nearby honeybees. Aphids, small insects that can be found feeding on the sap of plants, exhibit many strategies in terms of chemical defense. Aphids have structures called
cornicles along the posterior side of their abdomen which are used to deliver secretions containing both volatile and nonvolatile compounds. The winged offspring are able to better avoid predation; however, winged individuals are less fertile. This trade-off between wings and fertility shows the success of this particular defensive strategy. This protects farther conspecifics from the alarm chemical so they do not experience any needless pause in feeding or respond unnecessarily. These factors are used to highlight the specificity of the chemical defense systems of aphids. As discussed above, waxy cornicle smears are typically used to physically defend an aphid from a predator. In this case, however, the chemical alarms in the wax are eliciting a behavioral change; therefore, this particular strategy can be considered chemical defense.
Chrysopa glossonae, a
lacewing, uses the wax of the woolly alder aphid to chemically disguise itself from formicine ants (of the sub-family
Formicinea) who have learned to avoid attacking the aphid. This means that nearby formicine ants will ignore the lacewing as it would the wooly alder aphid. This is another instance where waxy secretions are used for chemical defense rather than physical.
Marine invertebrates Marine invertebrates employ a diverse array of strategies in terms of chemical defense. Some of these strategies include: secondary metabolite production, storage and modification of another organism's secondary metabolites, chemical warnings, predator warnings, phagomimicry, and chemical "clothing." The success of these strategies is exemplified by the number of species who exhibit these chemical defenses.
Sea sponges, of the phylum Porifera, are just one example of marine invertebrates who benefit from the production of
secondary metabolites. Sponges have the ability to produce their own secondary metabolites rather than rely on the storage and modification of another organism's chemical defenses. Spicules are sharp, needle-like structures protruding from the sponge and are used as a form of physical defense. Secondary metabolites and spicules have an inverse relationship because, as the quantity of secondary metabolites increase, the number of spicules decrease. Sponges that exhibit a larger production of secondary metabolites experience less predation, aiding in the idea that secondary metabolites are used as a defensive mechanism. For instance, a sea sponge produces pigments which gives them their vibrant colors. The pigments in the sponges accumulate in the sea slugs as they feed, allowing the sea slug to be camouflaged within its environment. The color of the sea slug is dependent on which sponge they consume. For example, a sea slug that appears pink when found feeding on a pink sponge can turn green when migrating to a green sponge. This storage mechanism is advantageous because the defensive chemicals are located near the surface of the sea slug and are readily available for any mucus secretion. One of the most distinguishing factors between these two marine invertebrates is sea snails have a shell while sea slugs do not. This loss of shell provides insight to the success of the sea slug's chemical defensive strategies.
Clams, referring to many species of mollusks, feed by pumping. "Pumping" occurs when clams pull surrounding water in, feed on microorganisms present in the water, and release the newly filtered water. These crabs, when harmed, emanate a chemical warning that is species specific, meaning these chemical warnings are only detected by other blue shell crabs. These warnings can come from damaged whole crabs or body parts of the blue shell crabs. Phagomimicry, as the name suggests, is a type of chemical mimicry. Many organisms have evolved to use mimicry as it is a highly successful mechanism of chemical defense. Sea hares, when attacked, quickly release a fog of chemicals into the surrounding environment. The chemical cloud consists of two main parts: the ink and the opaline. In the field, invertebrates such as the Atlantic decorator crab (
Libinia dubia) experience significantly less predation when "clothed" in noxious seaweed than their unclothed conspecifics. are derived from invertebrate prey.
Bufadienolides, defensive chemicals produced by toads, have been found in glands of natricine snakes used for defense. The evolutionary advantage of producing such toxins is the deterrence of predators. There is evidence to suggest that the ability to produce toxins evolved along with
aposematic coloration, acting as a visual cue to predators to remember which species are not palatable. ) is among the species of poison frogs that have potential significance to medical research. Besides providing defense from predators, the toxins that poison frogs secrete interest medical researchers.
Poison dart frogs, of the
Dendrobatidae family, secrete
batrachotoxin. This toxin has the potential to act as a muscle relaxant, heart stimulant, or anesthetic. Multiple species of frogs secrete epibatidine, whose study has yielded several important results. It was discovered that the frogs resist poisoning themselves through a single amino acid replacement that desensitizes the targeted receptors to the toxin, but still maintains the function of the receptor. This finding gives insight to the roles of proteins, the nervous system, and the mechanics of chemical defense, all of which promote future biomedical research and innovation.
Mammals Some mammals can emit foul smelling liquids from
anal glands, such as the
pangolin and some members of families
Mephitidae and
Mustelidae including
skunks,
weasels, and
polecats.
Monotremes have venomous spurs used to avoid predation and
slow lorises (Primates: Nycticebus) produce venom which appears to be effective at deterring both predators and parasites. It has also been demonstrated that physical contact with a
slow loris (without being bitten) can cause a reaction in humans – acting as a contact poison. == See also ==