Quick question about evolution

[Yoo]

Hi everyone! This is my first thread on this site, and I'm starting small.
I just had a thought today, thinking about venomous animals.

Why is the venom of some animals, like certain kinds of spiders and scorpions, strong enough to kill even a human? 

Surely they don't need venom that strong for their prey, which is not much bigger than mice I'd say, so that can't be the reason. I find it hard to believe that it's for self defence, because the venom doesn't kill or weaken the threatening predator fast enough for it not to be able to kill its prey.

Maybe these sorts of questions shouldn't be on here, because they're too specific, but I'm curious and maybe some of you find this interesting as well.
(English isn't my first language so don't kill me for getting words wrong 'n stuff)

Edit: The question has been answered, but the discussion is now about how an evolutionary arms race could have started.

Another question!
You know those flashy colored frogs? Poisonous animals often have very distinct colors, this has been interpreted as a warning to predators. There are even 'fake' ones, that aren't even poisonous, but do have the flashy colors to put off predators.
The first poisonous animals probably were poisonous out of self protection, like those frogs we see today. Now let's focus on the moment where those animals got flashy colors. I can't really think of how a poisonous animal would benefit from showing its poisonness to predators. Your first reaction might be: That way predators know it's poisonous, so they won't eat it.
I have a problem with that simple explanation, because: the first poisonous animal that showed its poisonness to predators was, obviously, the first one. The predators had no built-in mechanism for avoiding that particular 'warning sign', so they wouldn't refrain from eating it. You might say that the color might have just put the predators off, but simply being brightly colored doesn't usually put predators off. Then we would probably see a whole lot more brightly colored animals, like bright-green rabbits or something. And of course bright colors in animals are usually accompanied by some kind of venom. So why did the bright colors evolve?
I also don't get why being poisonous for self defence can you anywhere at all as an insect, because who cares if the predator dies or not AFTER eating you?

[Alex K]

I think some specific prey animals had evolved increasing immunity to the venom, which led to venom arms races causing some snakes to be incredibly poisonous to other animals that aren't normally prey

[ScienceAf]

Hi everyone! This is my first thread on this site, and I'm starting small.
I just had a thought today, thinking about venomous animals.

Why is the venom of some animals, like certain kinds of spiders and scorpions, strong enough to kill even a human? 

Surely they don't need venom that strong for their prey, which is not much bigger than mice I'd say, so that can't be the reason. I find it hard to believe that it's for self defence, because the venom doesn't kill or weaken the threatening predator fast enough for it not to be able to kill its prey.

Maybe these sorts of questions shouldn't be on here, because they're too specific, but I'm curious and maybe some of you find this interesting as well.
(English isn't my first language so don't kill me for getting words wrong 'n stuff)

Yep, the venoms arms race boi.

You should make an introduction thread. Or post your conversion story.

Not you Alex K...
Enough with the customary system. It's all metric.

[Alex K]

google yields an example

http://www.iflscience.com/plants-and-ani...squirrels/

[Aoi Magi]

Venom isn't solely used to catch prey but for self defense as well, and when every other predator or threat around you is venomous, one needs to evolve their potency to stay safe. Also in same environment, potential preys would've evolved immunity to venom as well, thus triggering the need for more potent venom.

[Yoo]

I think some specific prey animals had evolved increasing immunity to the venom, which led to venom arms races causing some snakes to be incredibly poisonous to other animals that aren't normally prey

I've read about arms races, didn't think of that though, you're porbably right. 
Some thoughts: 

Because the prey is usually very small, the amount of venom they get is a lot relative to their size, so even though they are better adapted to handle it, they react just as strongly to the venom as we do...?

What does a poisonous predator have to gain from very weakly poisoning a prey, like the earliest spiders probably did? Is that really a strong enough pressure for the first incidental venomous spider to have a real advantage?

Btw, will make an intro!

[The Grand Nudger]

What does a poisonous predator have to gain from very weakly poisoning a prey, like the earliest spiders probably did? Is that really a strong enough pressure for the first incidental venomous spider to have a real advantage?

Injury, incapacitation. Hurting or disorienting something is halfway to putting it on the plate, at least. In defense..if your bite just hurts..and mine makes you sick......I'm gonna win that fight.

[Cyberman]

What does a poisonous predator have to gain from very weakly poisoning a prey, like the earliest spiders probably did? Is that really a strong enough pressure for the first incidental venomous spider to have a real advantage?

Hunting prey is only one side of the coin. Defending yourself against a larger predator is another. Survival pressure would thus favour higher toxicity of venom.

[Gawdzilla Sama]

A strong poison is a "don't fuck with me" system. Mammals and reptiles take note.

[robvalue]

Hey there! Interesting question.