Eli5: Why do some spiders have poison that can kill mammals several hundred times it’s size , if they only eat other insects/arachnids, ect
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Others have answered and got partial answers, but the truth is pretty simple.
A spider has a very limited window in which to bite and immobilise prey, or bite and hurt/kill a predator or larger animal that could kill it by simply stepping on it.
If your prey gets away and dies much later, you probably don’t get to eat it, so you wasted energy on creating venom and hunting. Do that often enough and you die of starvation. Packing a bigger punch in the venom means the prey is more likely to die or be immobilised quickly enough to secure the food, and live a little longer.
Equally, having weak venom means that the predator, or simply larger creature that doesn’t notice you, is more likely to kill you before you escape.
Having very effective and powerful venom (not poison – venom is injected and poison is ingested) gives you a better chance to survive both eating and being eaten, especially in small, fragile creatures like spiders.
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