poisonous snakes again

[comloulou]

As we've established how many snakes there are can anyone tell me why poisonous snakes, jellyfish and other nasties have poison so disproportionately toxic? Surely the amount and strength needed to overpower prey is considerably smaller than they carry.

[tief]

It seems to be generally agreed that venom glands evolved from digestive glands and thus actually form part of the digestive process - in fact the very beginning of it. For instance, snake venom is a salivary mixture that can contain up to 130 different components. Some of them help break down muscle and tissue from the inside out, thus kick starting the digestive process before the snake even begins to swallow its victim.
It's worth noting here that 'venomous' creatures 'actively inject' their toxins whereas 'poisonous' creatures 'passively contain' their toxins (i.e.. their toxins generally only come into effect if they are touched, bitten or swallowed). For example, you would describe snakes, scorpions, spiders, or wasps as venomous, but caterpillars, frogs, or beetles, as poisonous.
With snakes it takes time to make venom and they're not inclined to waste it. (This is why many snake bites are 'dry bites' - i.e.. they haven't injected any poison.) Herpetologists are reticent to give a definite answer as to why snake venom is so disproportionately lethal. They seem to agree that nature developed a system that will kill prey efficiently and since it continues to work, it continues to be selected for, and there just hasn't been any reason for nature to apply any brakes to the process.
With regard to the other nasties, it seems to be much the same thing. A spider's venom, for instance, helps break down the inside of its prey into a mushy goo that it can suck up. So it needs a certain amount of venom for the kill and the rest for the digestive function. As in snakes, it takes time to replenish venom supplies. Spiders use a minimum amount on prey that doesn't struggle too much, but will use much more for prey that doesn't behave itself. Some prey have defensive behaviours that endanger the spider, so a quick kill is essential.