use them as training, you should be able to kill or catch them with your hands alone. My technique is wait until they land, slowly approach with blade hands about 2 feet apart, then slap my hands together about 4-6 inches above the fly. More often than not the fly survives being smacked between palms, but it will be stunned for a few seconds, which is long enough to grab it by a wing and throw it outside, adding it back into the food chain.