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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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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It’s completely a coincidental side effect. Almost all spiders are venomous to some degree for the purposes of immobilizing prey, but it’s just that most of those venoms don’t affect human. A few do and it’s just simply a quirk of evolution.

Bigger animals may still live for hours or even days when stung. If they had weaker poison their prey might be able to escape still, or at least destroy the whole web before being taken down. The poison has to act extremely quickly to stop the prey from moving, so it has to be strong.

Larger animals prey on them, these large animals learnt the color and shape of these deadly creatures and will avoid eating them. Many snakes and insects will copy the looks of the venomous once to try and avoid being eaten.

Some mechanisms are designed to kill things you eat. Some mechanisms are designed to kill things that eat you.

The poisons are evolved for the prey they attack… the affect on humans and others is coincidental.

For example, black widows. They use a alpha-latrotoxin to disable their prey. Since the spiders are small, and they sometimes eat small lizards, mammals and birds… they need to disable these animals almost instantly. Any delay means the animal can hurt or kill the spider, or just walk/run away. So they evolved an insanely potent poison that kills the small animals instantly.

So while their bites can kill larger animals, its just a byproduct. It will kill a larger animal slowly.