Injected directly into the flesh by inch-long fangs or needle-sharp harpoons, venom is weaponised chemistry.
In any list of the world's most venomous animals, snakes often get top billing. Not all snakes are venomous, but certain groups have evolved to channel toxins through grooves or tubes in their teeth.
According to Dr Bryan Fry of the University of Queensland in Brisbane, Australia, one of his local species produces more venom than any other.
"The mulga snake has exceeded 1.3g of dry venom from a single milking," says Fry, whose passion for his subject has earned him the nickname "Venom Doc".
The deathstalker's venom is delivered in short bursts
Also known as the king brown snake, this species is common across Australia. It shelters under timber and rubbish piles. Fortunately, despite the seeming abundance of venomous snakes, snake bites are rare in the country.
Fry says several large snakes can produce a similar yield of venom, including the king cobra from India, the Gaboon viper of sub-Saharan Africa and the eastern diamondback rattlesnake from the US.
However, yield is not the only way to measure how venomous a species is. That is highlighted by the species often described as the world's most venomous scorpion: the dramatically-named deathstalker.
This pale yellow arachnid lives in deserts in the Middle East. It hunts at night for worms, centipedes and other invertebrates.
The more targeted the venom, the more damage it can do
Scorpions paralyse their prey with their notorious sting. Known as the telson, this last segment of their tail contains venom sacs and a barb that injects the toxins.
At the opposite end of the scale to the mulga snake, the deathstalker's venom is delivered in short bursts to hit its small prey. Yet despite never growing longer than 11cm, the deathstalker packs a serious punch. Tests suggest that 0.25mg of deathstalker venom would be enough to kill 1kg-worth of mice.
That is a pretty potent venom. But even this seemingly precise measure is tricky to interpret, because not all venom is equal.
Venom can be used as a defence against predators or to attack prey. The more targeted the venom, the more damage it can do. So it is not easy to find the most venomous animal on Earth, since they have evolved to target different species.
Lacking enthusiastic human volunteers for their trials, scientists have tended to test venoms on mice. These studies reveal the "median lethal dose", meaning how much venom it takes to kill 50% of the mice being tested.
This isn't a perfect measure, because many animals are quite unlike mice. But it is the best we have, and the current chart-toppers might surprise you: marine snails.
Cone snails are carnivores. Being slow-moving, they have shells that act like suits of armour, but they can also deploy venom when threatened. Some species eat worms while others pick off fish. It is the latter that pack the biggest venomous punch.
"For species of cone snail that hunt fish it is important to have a very fast-acting and powerful venom, because otherwise fish can easily escape from such a slow moving predator," says Dr Ronald Jenner of the Natural History Museum in London, UK.
Its venom can wreak devastating effects on the human nervous system, blood, muscles and organs
To take down their targets, cone snails have modified teeth called "radulae". They are sharp, hooked and hollow, like a cross between a harpoon and a hypodermic needle. The snail launches one at an unsuspecting fish, whereupon it delivers a cocktail of toxins that target the nervous system. Once the fish is paralysed, the cone snail can devour it.
Cone snails constantly produce new radulae so they are rarely without their killer cutlery. The teeth are sharp enough to pierce wetsuit fabric and pose a problem for divers. The geography cone snail has a beautiful mottled shell that often attracts people, but it is also one of the most venomous species.
The human lethal dose for its venom has been estimated at just 0.029-0.038mg for every kg of body mass. 65% of human stinging cases are fatal without medical attention – although only 36 such fatalities have been recorded since 1670.
That brings us to the final factor influencing venomous animals: how likely they are to pose a threat.
The inland taipan, which also goes by the moniker "fierce snake", is commonly accepted as the world's most venomous snake based on the potency of its venom. Its median lethal dose for mice has been calculated at 0.025mg/kg. It only hunts mammals, so its venom can wreak devastating effects on the human nervous system, blood, muscles and organs.
The black mamba is actually olive-coloured
But the inland taipan lives in the remote deserts of central Australia and biologists describe it as reclusive. Its coastal cousin has a weaker venom, but it is considered by many to be more dangerous. It lives in dunes and woodlands along the tropical coast of Australia, where people are likely to come into contact with it.
As with most snakes, aggression is a last resort for the coastal taipan. But when it is threatened it can bite several times in quick succession, repeatedly delivering a full load of venom.
This same behaviour explains why the black mamba has such a vicious reputation.
Native to sub-Saharan Africa, the black mamba is actually olive-coloured: it is named for the colour of the inside of its mouth. When cornered it displays its open mouth and hisses to warn off the threat. It will only sink its venomous fangs in when provoked further.
However, humans are not staying away. Growing populations mean that settlements are encroaching on snakes' territories in many parts of the world. That means life-threatening snakebites are a growing problem.
"Scorpions sting 500,000 people a year in Mexico alone, with 150,000 of these requiring anti-venom," says Fry. "Snakes, however, bite 1 million people in India alone, with 50,000 of these dying and about half the survivors having some sort of permanent injury."
Venoms may be complicated, but whether you are a diving tourist or a subsistence farmer, it is wise to know them.
"Collectively, envenomation is the most neglected of all tropical diseases," says Fry.