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Chemical defence

Spraying, bleeding, oozing and stinging are four different solutions. Only the ones that go through a wound are venom.

Spraying, oozing, frothing, bleeding and stinging are separate solutions that get lumped together. Only the ones that put something through a wound are venom. And most defensive chemistry works by being intolerable rather than by being lethal.

Three things vary independently in chemical defence, and keeping them apart makes the whole subject tractable. There is the compound — what the chemical is and what it does to tissue. There is the delivery — whether it is secreted onto the skin, sprayed, regurgitated, released by deliberately rupturing the animal’s own body, or injected. And there is the origin — whether the animal makes it, takes it from food, or hosts microbes that make it. Any combination occurs, which is why a single label like "poisonous" or "venomous" so often fails to describe a real animal. Delivery is the axis that carries the venom distinction. A wasp injects; a bombardier beetle sprays; a poison frog simply has it in the skin; a ladybird bleeds a droplet of alkaloid-bearing fluid from its leg joints. Only the first of those is envenomation, and the difference is not chemical fussiness — it predicts how the defence is used, how much it costs to deploy, and whether the animal can meter it. Origin is the axis most often overlooked, and the poison frogs are the clearest case: their alkaloids come from mites and ants, so the defence disappears in captivity. The same is true of many chemically defended insects, and an increasing number of cases turn out to involve symbiotic microbes making the compound rather than the animal. It is worth being clear about what most of these chemicals actually do. A minority are lethal. The majority are deterrents — they hurt, taste appalling, irritate mucous membranes, or foul the predator so thoroughly it stops. Deterrence and lethality are different design targets, and something built to make an animal let go quickly does not need to be dangerous at all.

Early coverage · 49% complete · reviewed 2026-09-03

What this page covers

Chemical defence occurs across insects, arachnids, molluscs, amphibians, reptiles, some mammals, plants, fungi and bacteria. The best-characterised systems are arthropod, largely because they can be provoked in a laboratory.

Often confused with: Venom, which is the subset delivered through a wound; A single strategy, when compound, delivery and origin vary independently; Toxicity, when many defences work by being unpleasant rather than by being poisonous

Quick facts

Three independent axes
The compound, the delivery, and where the compound came from
Only injection is venom
Spraying, oozing and reflex bleeding are not, however unpleasant
Often borrowed
Many compounds come from diet or from symbiotic microbes
Mostly deterrent
Built to make an attacker stop, not usually to kill it

Compound, delivery, origin

They vary independently, which is why one-word labels keep failing.

Spraying, bleeding, frothing, oozing and stinging are not the same thing. Only the ones that put a substance through a wound are venom, and most chemical defences do not.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

Chemical defences differ independently in compound, delivery route and origin. Delivery modes include glandular secretion, spraying, reflex bleeding, regurgitation and injection, and only injection through a wound constitutes envenomation. Compounds may be synthesised or sequestered from diet or symbionts.

Who this applies to
Applies across chemically defended animals and plants.
Studied in
Arthropoda, Animalia, Plantae
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

A descriptive and definitional claim resting on a very large body of chemical-ecology work in which the three axes plainly vary independently.

How far it can be extended

The independence of compound, delivery and origin has been documented across many arthropod and vertebrate lineages.

Caveats

  • A spitting cobra directing venom at an attacker’s eyes fits neither category cleanly, which is why a third term has been proposed.
  • Defensive and prey-subduing chemistry can share compounds in the same animal while being deployed quite differently.

Still unanswered

  • How often defensive chemistry in animals is produced by symbiotic microbes rather than by the animal, which is being revised upwards as sequencing improves.

Last reviewed 2026-09-03

The evidence (3 studies)
The same question asked three ways
AnimalDeliveryWhere the compound comes from
WaspInjected through a stingMade by the animal
Bombardier beetleSprayed, hot, in pulsesMade from stored reactants at the moment of use
Poison frogPresent in the skin; acts on contactSequestered from mites and ants
LadybirdReflex bleeding from leg jointsMade by the animal
SkunkSprayed, aimed, with a limited supplyMade by the animal

The middle column is the one that decides whether a word applies. Only the wasp is envenomating anything. The rest are chemically defended by other routes, and calling them all venomous loses the distinction that predicts how each defence behaves — whether it can be metered, whether it runs out, and whether the animal has to make contact to use it.

Made, eaten, or grown by somebody else

The origin of a defensive compound predicts more than its chemistry does.

Poison frogs do not make their poison. They take alkaloids from the mites and ants they eat and store them in the skin — so a frog raised on other food is not poisonous, and wild populations differ with their prey.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

Dendrobatid skin alkaloids are sequestered from dietary arthropods, principally oribatid mites and ants, rather than synthesised. Frogs raised on alkaloid-free diets lack skin alkaloids, and alkaloid profiles vary geographically with the composition of the local arthropod fauna.

Who this applies to
Established for dendrobatid poison frogs; comparable dietary sequestration occurs in several other groups.
Studied in
Dendrobatidae
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

Demonstrated experimentally by manipulating diet and confirmed in the field by matching alkaloid profiles between frogs and their prey.

How far it can be extended

Dietary origin has been demonstrated across multiple dendrobatid species and confirmed by matching compounds between prey and frog in the field.

Caveats

  • Sequestration is not passive: the frogs have mechanisms for taking up and storing these compounds without harming themselves, and in some cases modifying them.
  • Species and populations differ substantially in which alkaloids they carry and in how much, so "poison frog" covers a wide range of actual toxicity.

Still unanswered

  • How the frogs tolerate compounds that are toxic to other animals, which is only partly understood.

Last reviewed 2026-09-03

The evidence (2 studies)
Three origins, and what each implies
OriginExampleWhat follows
Synthesised by the animalBombardier beetle reactants; skunk sprayCostly to make; supply is finite and replenished slowly
Sequestered from foodPoison frog alkaloids; many defended insectsVaries with diet, geography and season; lost in captivity
Made by symbiotic microbesAn increasing number of documented casesDepends on retaining the partner, and can be lost with it

The third row is the one currently moving. As sequencing has improved, defences long assumed to be the animal’s own chemistry have repeatedly turned out to be microbial in origin, and the count keeps rising. That does not diminish the animal — hosting, feeding and containing a chemical factory is real work — but it changes what the defence depends on, and an animal that loses its symbionts loses its chemistry.

Most of it is meant to be intolerable, not fatal

Which is a different design target and produces different chemistry.

A defensive chemical has one job: make the attacker stop, now. Killing it later is no use to an animal already in a mouth, and killing it at all is arguably counterproductive, since a predator that survives an unpleasant experience is the one that learns to leave your species alone. That is why so many defensive compounds are irritants, foul-tasting or painful rather than dangerous, and why the animals whose chemistry hurts humans most are frequently not the ones whose chemistry is most toxic.

  • Speed matters more than potency: a deterrent that acts in ten minutes deters nothing.
  • A supply is finite. Skunks carry a limited quantity and take time to replenish it, which is why the warning display comes first — spraying is a last resort because it is expensive.
  • Warning first is the general pattern. Chemical defence very often pairs with a conspicuous signal, because the best outcome is not having to use it.
  • Some defences are only released by damage to the animal itself, which limits how often they can be used.

Related

The research behind this page

9 studies, newest first. Each one has a page explaining what it found and what it could not show.

2015Science

Mechanistic origins of bombardier beetle (Brachinini) explosion-induced defensive spray pulsation

The spray is emitted in pulses of a few hundred to a thousand per second.

2013Trends in Ecology & Evolution

Complex cocktails: the evolutionary novelty of venoms

Venoms are complex mixtures of many proteins, most recruited from existing physiological functions, and their composition varies substantially within species — between populations, between adults and juveniles, and with the prey being taken.

2007Proceedings of the National Academy of Sciences

Oribatid mites as a major dietary source for alkaloids in poison frogs

Oribatid mites contained a wide range of the alkaloids found in poison frog skin, identifying them as a major dietary source alongside ants.

2005Harvard University Press

Secret Weapons: Defenses of Insects, Spiders, Scorpions, and Other Many-Legged Creatures

Arthropod chemical defences are enormously varied in chemistry and delivery — sprays, froths, regurgitation, bleeding, glandular secretion — and many are acquired from diet or from symbionts rather than synthesised.

1999Proceedings of the National Academy of Sciences

Spray aiming in the bombardier beetle: photographic evidence

Beetles directed the discharge accurately towards the point being attacked, including over the back and to either side, by aiming the abdominal tip and using reflecting structures where the tip could not point directly.

1997Proceedings of the National Academy of Sciences

Firefly “femmes fatales” acquire defensive steroids (lucibufagins) from their firefly prey

Photuris females lack the defensive steroids that Photinus fireflies carry, and acquire them by eating Photinus.

1994Journal of Chemical Ecology

Dietary source for skin alkaloids of poison frogs (Dendrobatidae)

Frogs raised without their natural prey lacked skin alkaloids almost entirely.

1969Science

Biochemistry at 100 °C: explosive secretory discharge of bombardier beetles (Brachinus)

The discharge leaves the beetle at approximately 100 °C.

1965Science

Aggressive mimicry in Photuris: firefly femmes fatales

Photuris females answered the flash codes of Photinus males with the response pattern of a Photinus female, drawing the males close, and then captured and ate them.

Where to go from here

Each of these follows from something on this page — a relationship in the evidence, a claim people ask about, or the next mechanism along.

How complete this page is, and what it is still missing

NatureHQ publishes its own gaps. This page is at 49% completeness against what we would call a finished subject, and was last reviewed on 2026-09-03. It carries 4 claims and answers 0 mapped search questions.

  • no research from the last few years is attached — check for newer work
  • no popular claim about this subject has been checked yet
  • Skunks are used as an example and do not have a page of their own.
  • Marine chemical defences, which are enormously diverse, are barely represented.
  • How often defensive compounds are made by symbiotic microbes rather than by the animal is being revised upwards and is not settled.