It's a threat of paralysis IRL...
Agree with Bumber. Some of the most potent real life venoms and toxins work by preventing or forcing muscle contraction, eventually leading to suffocation.
Let me reiterate:
- ...because it seems like overkill, potentially making it a matter of game balance.
- ...if the game always ignored the lungs for paralysis syndromes... is still a prone target... still has a lot of potential for Fun.
Often, when it comes to games, I'm an advocate for realism. But even I recognize that, sometimes, game balance is more important.
First, consider that spider bites and snake bites are only
very rarely fatal to humans. Fear of snakes and spiders is way out of proportion to the actual risk they pose.
In the US, annually, fewer than one in 37,500 (7-8,000 bites per year) will be bitten by venomous snakes and only one in 50 million people will die (5-6 fatalities). That's a fatality rate of about 0.07%. (Granted, a lot of that is due to modern medicine and anti-venom treatments.) For perspective: A person is nine times more likely to die from being struck by lightning than to die of a venomous snakebite. (
Citation]
Spider bites are even less dangerous.
Worldwide, only three deaths have been associated with widow spiders. Fatalities due to recluse and other dangerous species are also very rare. (
Citation)
If we
must be sticklers for realism, consider how silly it sounds for a creature to bite through plate armor made of steel, iron, bronze or even copper.
Do you believe that predators like wolves can? Granted, predators like lions, tigers and bears have a bite pressure in the hundreds of PSI. And, at the extreme, crocodiles can bite for thousands of PSI.
But, bite pressure is not the whole picture. Can you bite through copper or iron? I do
not recommend trying as you would likely chip or break a tooth. People have been known to chip or break teeth on ice or hard candy, which I think we can agree are not as hard as metals like bronze or iron.
Some people may point out that enamel has a higher Mohs hardness than copper. But a tooth is more than just the enamel on the surface. The tooth dentin
underneath is not nearly as strong and it can crack or break. It's more than just hardness, too. There are other stresses and measures to consider, such as
Young's Modulus. By that measure, tooth enamel is not as strong as copper, even.
Now, consider that the fangs of a venomous snake are
hollow,
narrow teeth. Should they be able to penetrate copper / bronze / iron / steel armor without breaking? Consider, too, that the fangs of the
giant, savage versions of DF critters are proportionally longer, making the stresses involved even greater.
Now, let's consider how giant spiders and giant insects are quite unrealistic:
- Arthropods have a special respiratory system called a book lung, which is very unlike eukaryotes and mammals. It would not scale up well - they wouldn't be able to get enough oxygen.
- Arthropods don't have bones, but rely on an exoskeleton and have hollow tubes for legs. Scaled up to a dingo or larger, they would likely be crushed under their own weight. And consider the forces and stresses required for very long, extremely skinny legs. (I took a Statistical Engineering class.)
- Arthropods go through moulting, where they shed their old cuticle (skin). It splits along predefined weaknesses where it is thinnest. During this process, they are extremely vulnerable to predators. "Moulting may be responsible for 80 to 90% of all arthropod deaths."
Finally: The reason venomous critters
have venom is to prey upon and/or defend themselves from creatures which they would otherwise be at a disadvantage against. Such creatures are usually
small and/or
fragile.
DF has spiders (
GCS) the size of a freak'n
grizzly bear. Even if that was possible, they should be
quite fragile. As long as they don't inject their venom, one could cripple them with ease with a sword, axe, hammer or similar. They'd more vulnerable than
vertebrates, with a vertebral column and a bone skeleton. (But, in-game, giant arthropods are
quite tough and still quite deadly, even without their venom!)
Anyway, there are valid reasons why the lethality of their venom would not necessarily scale with body size. For one, it's
much easier to produce a volume of venom that
debilitates with pain compared to one designed to stop the heart or suffocate.
A couple snippets from
an article on arthropod venom:
...So, a lot of fairly fragile hunters (like spiders, scorpions, centipedes, predatory "true bugs", and parasitic wasps) have venoms specifically to help give them an advantage over prey that they otherwise wouldn’t be able to tackle.
...It is hard to produce enough toxin to actually kill an animal that is, say, 10,000 times your size. But pain! Ah, pain we can provide! It takes much less toxin to kick the old pain sensors into overdrive than to, say, stop their heart.
So, it looks like the big reason why venom is so popular with arthropods is that they are so small, and the need for dealing with things that are bigger comes up a lot for them. While the two uses for venom are superficially similar (and it is possible to use hunting venoms for defense, and vice versa), you do tend to end up with significantly different venom characteristics depending on who it is you intend to use it on most of the time.