That will depend on how cooperative these creatures are. But do feel free to threaten it menacingly.

[ He slips the handle and its precious vial between the bars of the nearest trap and pokes the spider with the end. It shrinks in, its multitude of eyes glaring at him and at the intrusive item prodding it. ]

Hmm...

[ He gestures for his familiar to circle around. It snaps its jaws near the spider's huddled form. Thus provoked, it lunges forward and attempts to bite the stone arm Emet is holding through the bars. Venom trickles into the vial - and all around it as well. It's quick to let go and the Ascian quickly withdraws his makeshift tool, inspecting the takings. ]

Inelegant, but it will do. How do you fare?