If you cycle in the summer without glasses or goggles you'll certainly get flies in your eyes. Your eye is wet, and any fly you happen to hit with it will stick. Then you blink, and the fly ends up under your eyelid. It's most noticeable when cycling between large cornfields, which usually have lots of aphids flying about.
Some flies get in there on their own, too. Here in the New Forest, cattle and horses' heads are always surrounded by clouds of tiny flies trying to sip their tears (and bite the inside of their ears). If you walk near they'll try it on you, and quite often end up stuck in your eyes.