I have 3 cats at different ages, all 3 will go for anything which crawls, jumps, hops or flies. My female is probable the worst of the bunch, her favorite is moths no matter what size, and has a look of heaven when shes munching them! Yuk!!! Summer is the worst time she brings frogs home to play with and do they scream or what ( the frog that is.! ) My other half has to go to the rescue, I wont touch them. As for flies no they wont harm your cat, let your cat have fun while at the same time he / she is getting rid of unwanted parasites.