The Cow - 2:26
"Allah does not hesitate to give the example of a mosquito or even the one on it. The believers know that this is a truth from their Lord. But the disbelievers say, "What did Allah intend by this analogy?"He leads many astray by it and guides many to the truth. He leads astray with it only the disbelievers."
Example of a fly:
The Pilgrimage/22:73
People, here is an example for you that you may listen attentively: Those whom you serve beside Allah, even if they were gathered together, they could not create even a fly. Even if a fly snatches something from them, they cannot take it back from him. Both the one who asks and the one who is asked are powerless...