What causes these tracks?

[Caro]

I originally posted this as a semi-jokey item, but now it's bugging me (no pun intended). Can anyone enlighten me as to what causes these tracks? They are on the outside of my greenhouse, on the roof panes. The tracks are essentially caused by the dust being cleaned away *ahem I've taken the photos from the inside, looking up and out.

It must be slugs or snails, surely, but I don't think I've seen enough around to cause so many tracks? Maybe it's just a couple of them over the course of a month or two. Or something completely different?

Thanks for any help.

[rana]

I have always thought these were snail tracks. The zigzag pattern is down to the way the snail head sweeps slowly from side to side across the glass while searching for food (algae).

[Capsid]

Yes, rana is correct, you can see similar patterns caused by snails on algae covered aquarium glass.

[rustylady]

Snails I would think. They are climbers, whereas slugs tend to stay fairly close to ground level.

[Caro]

Thanks for your replies. I really was stumped as to why the pattern would be so intricate, but I guess the zig-zagging head covers it.

Thanks everyone