Why are snake and spider venoms so powerful when their normal victims are so small?
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The most dangerous venomous and poisonous creatures use natural neurotoxins that attack the nervous system and brain. This requires very small amounts of material to be life-threatening, even for humans, because anything that can adversely affect your brain or nervous tissue might also shut down your lungs or heart. Naturally, most of these animals are at least somewhat immune to their own venom, so it doesn’t matter if there’s a small amount in their prey creatures.
In evolutionary terms, a powerful venom helps the species by developing avoidance behaviors in anything that might want to eat it.
Plenty of snakes and spiders are venomous while not being especially dangerous to humans, because their venom just isn’t powerful enough to affect a creature hundreds or thousands of times its size. For example, out of thousands of spider species in North America, only the black widow and brown recluse are dangerous enough to be truly life-threatening for people. Tarantulas are gigantic in spider terms, and their venom can kill rodents with a single bite, but biting humans will only cause something like a big wasp sting.
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