I was watching the Learning Channel tonight, and saw a show on venomous creatures. When they started talking about the brown recluse, I got to thinking… What evolutionary advantage could have caused the brown recluse to develop it’s powerful, but slow acting venom? They said that the initial bite was painless, so an animal that was “messing with” the spider would not get an associable (is that a word?) negative reinforcement. Since the poison acts so slowly (takes about an hour before you even feel it), the little spider would have to catch a ride if it wanted to eat a bird or a cow ( 
).
For want of a better answer, I imagine that its just a coincidence that the brown violin’s venom affects people powerfully but slowly, cause of amount of blood/tissue, etc. relative to a itty bitty bug.
As a WAG, how about stealth warfare? Bite an animal at night shortly after it goes to sleep. The painless bite won’t waken the target. Then wait a few hours and start munching.
I saw the same program, and I have often wondered the same thing. One thought occurred to me though, one of the main effects of the venom is that is kills tissue. Perhaps the venom also acts as an aid in digestion, breaking down most of its prey’s substance and making it easy to slurp up.
Now as to why it’s so powerful, I haven’t a clue. When I was in school and working at McD’s one of my co-workers was bitten on the hip and she was out for a month.
Keep in mind that “powerful” can be a relative term. I seem to recall reading that the venom of the Australian funnel web spider, while almighty nasty, affects only humans and cats among all mammals. Go figure.
There ya go andyman. I like it. I can buy it.
Even more curious is the Comodo Dragon, which bite large animals and leave. The saliva is full of bacteria so the animal dies a few days later from infection, to be eaten by the dragon - or any dragon who happens to be around. It’s good for the species to have a constant supply of infected, dying animals around, but it’s hard to see how that can result from evolution.
I admit I got this information from “Last Chance to See” by Douglas Adams, but most factual information in there seems fairly accurate.
Well, I didn’t see this show, but here goes:
andyman - that could very well be. That’s a common use of venom in the bug world.
Max Torque - that’s what I was thinking. Did they say that it was only slow-working in humans? My WAG would be that in much smaller prey animals, it would act much more quickly.
scr4 - yes, that information is true. It was in a recent issue of Scientific American as well as innumberable Discovery channel documentaries, including a Crocodile Hunter.
Well, that was fun.
They did not specify how quickly the venom worked on other creatures or bugs. I would assume that it acts very quickly on smaller creatures such as bugs. From what I understand, the brown recluse is a spider that does not build a web to trap its prey. It is an ambush killer. If its venom worked slowly on bugs, the spider would have to either pin his prey or follow it until it died. This being so, the question would still stand. Other venoms that are not very powerful (to us) will still kill bugs very quickly, so why did the bad-a spiders develop such powerful venom?
I think a combo of andyman’s and Max Torque’s answers probably get it. The venom disolves tissue to aid in digestion, and just by coincidence, the venom has a really bad effect on humans relative to other spider venoms.
Another spider bite variable is fang strength/size. Most spiders’ bites aren’t dangerous to humans simply because their little fangs can’t penetrate thick human skin. The recluse and the black widow are among the only small spiders with fangs sturdy and long enough to penetrate in the first place, thus allowing the venom to get where it can do damage.
If ya wanna talk dangerous spiders, learn all you can about the funnel web (atrax robustus), generally considered the world’s most dangerous, due to three factors: fang strength (capable of penetrating a human fingernail), hostility (they’ll rear up and strike repeatedly at anything they find remotely bothersome), and venom toxicity (funnel web venom is capable of killing a child within hours; thank goodness there’s an antidote nowadays).
Final note: Alas, it looks like I was in error. A web page I just discovered tells me that humans and other primates are strongly adversely affected by funnel web venom, but toads, cats and rabbits are almost completely unaffected by it. Sorry, looks like I misremembered what I’d read. Anyway, the point still stands that whether a spider bite will affect you depends greatly upon your species.