When I was in Oregon in '05, in the Eugene area, I didn't see any of the bright yellow ones, but there were a lot of dull yellow and olive yellow-green ones with and without spots. They were difficult to see in certain areas, like on moss and dead leaves, and some could pass for poo if you weren't looking carefully. I've never heard of any color of banana slug being poisonous, and considering I licked one, I feel assured that they're not. However, their slime does seem to have a slight numbing quality about it that I suppose might make them harder to eat, in theory.
As for the Arions with the orange belly, I assume you mean Arion hortensis. I only just found a few of these, two are slate gray with orange bellies and one is light gray with a pale belly. They seem to get along well. I find them sleeping near each other and I suspect that some mating between the two colors has already occurred. I've held them, and because I'm not famous for washing my hands after holding my gastropods and have yet to notice any effects, I don't think they're poisonous, either. Besides that, I have a fairly good knowledge of wild plants and there was nothing in their immediate environment that they could have eaten that would have bestowed them with such a quality, not even mushrooms.
That's not to say that there are no poisonous slugs, however. I've read limited (and questionable) information which suggested that slugs and snails could eat poisonous mushrooms and plants and then secrete the toxins through their skin as a defense measure. But I can't find examples or conclusive evidence of any sort.
I've not heard of a land slug that stores poison, but these guys (and other sea slugs) do. They're many times more dangerous than their main prey Portuguese men-o-war thanks to the concentrated toxins. All pictures of people holding Glaucus atlanticus are fake, you can't touch these with bare hands. Well OK, I guess technically you can but it's not smart. This inch-long animal can kill.
But they're cute as buttons. I choose you, G.atlanticus!
There's also a photosynthesising sea slug. I posted about it a while ago in the Snails in the media topic. They incorporate chlorophyll from algae into their bodies after hatching, kind of a start-up fueling, and then they can run (slime? swim? float?) on that and don't necessarily need to eat more, if I recall right. And sea hare ink apparently contains some algae toxins. The purple ink cloud they can release is a bit like noxious pee, unlike octopus ink which is used as camouflage for quick escapes. Sea hares, of course, don't do "quick escapes". But, SlugTech is waaaaaay cool. All they need is a big brain now to take over the world!
I gotta ask, though, is there a particular reason people kiss/lick banana slugs? I've heard that often. It's interesting to hear their slime numbs the tongue; that's another thing I've heard of some snails' and slugs' slime.
The camouflage mimicry is classic. Would be interesting to seea map of the variability.
The picture of the blue slugs above. I came across a yellow one under a brick in my garden, freaked me out, I had no Idea what it was, Thought it was in insect so I just left it. Later I researched and found it was a slug. It was a beautiful yellow. The blue are amazing as well.
3 species unidentified slugs
I'd heard that they would numb your tongue, and I just had to try it. Turns out that it's true, but the effects aren't as strong as people made it sound. It wasn't my slug, either. It was a wild one. He seemed awfully fond of me afterward, though.
LOL I bet he did! I imagine the human tongue to feel rather... sluggy to a slug. Very, very slimy and warm kind of sluggy. It might get them excited if there's no danger of getting eaten by the tongue's owner.
"Fabulous and advanced slug technology will rule the world."
My current snail family (& family album): A.achatina, A.fulica, A.hortensis, A.vulgaris C.nemoralis, C.lubrica D.invadens, E.vermiculata H.aspersa, H.pomatia L.alte, L.cinereoniger, L.maximus O.cellarius, R.decollata, T.pisana