I think, as you say, it's just arachnophobia, officially. An arachnophobic doesn't need to fear everything in the arachnid category. I'm scared of spiders, but not scorpions (though, if I lived in an area with them, who knows..?) If we were to build a term using Ancient Greek, it may be scorpiophobia. read more
A phobia is an irrational fear. If you are afraid when you see a scorpion near you and you know it's poisonous, that's completely rational. If you see a picture of a scorpion, and you get more than mild heebie-jeebies, it could be a phobia. If you hide under the sofa during The Mummy Returns, I'd say phobia. read more