Snakes are not poisonous...not even if you eat them. Neither are spiders, though I'm not sure there's enough people attempting to swallow either to be certain. Maybe a toxic species or two exists...but only if you eat it or rub it on your skin or all sorts of things you shouldn't do with snakes or spiders.
Poison, you see, is ingested or touched. VENOM is injected and that, my friends, is what those lovely fangs are for.
This is something that you need to get right, at least if you're writing about bites of any kind, evil giant insects, reptiles or anything venomous...VENOMOUS, meaning they inject an unlucky character through a bite of some kind.
Poisonous critters do exist. If your Heroine takes to licking random frogs in the Amazon, she might certainly be poisoned. In fact, she should probably keep her hands off unidentified amphibians as well, as some toxins can quite easily take effect through skin contact. So, feel free to kill by poison if you choose, but please, make sure it's not a biting injury..
Bite=fangs(or grooved teeth)=VENOM
If you think about it, it sounds cooler anyway. I mean, many butterflies are technically "poisonous" but if one bit you it would probably just tickle or something equally harmless.
Unless it was a venomous butterfly...and that's a story I just might have to write some day.