Not Australian myself, but I'm completely used to them, too, and not afraid either (I've even had a tarantula on my hand once!). That said... arachnophobia is certainly a legit thing, and I've got family who is afraid of spiders, and I don't find them "pathetic" or anything, so I can't say rightly the same thing about Murtagh.

Come to think of it, its "resolution" is indeed pathetic. Thorn at least "gets over" his claustrophobia, but in Murtagh's case, he goes on killing spiders, but without being disgusted by them? It's absolutely not an improvement!