Are there any 'anti-fly' masks on the market? I keep swallowing flies, because I'm mostly cycling along a canal.  I'm cycling quite hard, and mild hayfever means that I can't just breathe in through my nose.

I've bought a 'buff', but I'm getting too hot in that, and I'm not worried about pollution (small particles), so I'm after a lightweight mask that I can breathe through easily.  Are there any masks on the market suitable for this?
Buffs are useful for a time, but I understand that they can get uncomfortable after a few kilometres. Try something like this mask from Respro - you can buy replaceable filters and I think they look kind of cool too; you can even shove them in the washer after a particularly muddy or sweaty ride.