This is the bowl vent and is open to the atmosphere. Usually it just lays on top of the air box. If you're worried about crud getting in, just wrap a small piece of nylon or pantyhose over the end and hold it with a rubber band, just don't crush the tube or it won't vent properly.
Livin' my life like a song.
1985 Honda Rebel 250 - "Birdie"
1979 CB750K - "Behemoth"