As a confirmed arachniphobe,I can vouch for the imagined creepy feeling that would be produced by their legs moving over me, combined with an irrantional disgust at the speed with which they move. Strangely, the larger slower moving varieties like tarantulas I find interesting, and would not object to holding at all, but leave me in the room with one of those little tennis ball-shaped ones with the really long legs that move like Sebastian Coe with his shorts on fire, and I can't even get close enough to put a glass over it! It is irrantional, but that's the definition of a phobia, so everyone can leave out the "It's more frightened of you..." argument, because a phobia is not subject to rational statements, thank you.