“Some people use the words interchangeably because once in the body, the chemicals do similar damage, attacking the heart, brain or other vital targets. But the terms do mean very different things. Traditionally, venomous creatures bite, sting or stab you to do their damage, while you have bite or touch poisonous critters to feel their effects.
[V]enomous organisms need a way in, like fangs or teeth. All octopuses are venomous, along with some squid, plenty of snakes, spiders, and scorpions, a few lizards, vampire bats and even the slow loris Nycticebus kayan. Some fish, including lionfish, use spines to sting attackers with venom. The Brazilian frogs aren’t even the only venomous amphibians. When attacked, Iberian ribbed newts push out their own ribs so that spikes on the ends jab a predator with toxin.
Poisonous organisms take a more passive approach, often lining the skin or other surfaces with toxic chemicals. Poisons can either be brewed from scratch inside the animal or acquired through diet. Cane toads naturally secrete poison they make in glands behind their ears. Meanwhile, poison dart frogs generate a highly poisonous alkaloid skin coating they derive from munching on ants. Mama frogs pass the chemical on to tadpoles via egg sacs, so if you take a young poison dart frog out of its natural habitat, it will actually lose toxicity.
Because the toxins get delivered in different ways, venoms tend to be larger compounds that must be injected to break through skin, while poisons are usually smaller chemicals that can be absorbed.
[B]oth venom and poison can kill you in truly horrifying and painful ways. Field biologists, beware.”
The full article is at smithsonianmag.com.