And, no, it wasn't the frogs. Those flies would sit on their heads. It was actually the pennies in water.
House fly eyes are compound organs that are comprised of thousands of individual lenses. Compound eyes are capable of detecting both the polarization of light and color spectrums unseen by humans. Can you imagine how magnified the perceived "preditor eyes" must seem in that bag of water? A monster head with a million orange eyes. Let me know if it worked for you.