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Our eyes are small and covered by eyelids to protect its (in danger for eye or being asleep, when not used). Our eyes have cleaning mechanism ? tears. Still sometimes we use our hand as additional way of eye cleaning. And for some activities we still need protecting glass in addition to protection constructed by Nature.
Eyes of flies many times larger than ours in compare with body size. It seems that for fly eyes relatively even much more important than for us. And neither eyelids nor tear-cleaning exist. And feeding on flowers with pollen (or even dung or carrion, though it isn?t Syrphidae speciality :D ) requires perpetual cleaning.