Dance flies are a very diverse group of predatory flies mostly feeding on other flies. They often gather in 'dancing' swarms you can see backlit in forests and meadows, though many other fly groups show the same behaviour.
Much more unusual is the practice of 'gift-giving' by males of some species. They bring a dead insect to the female for her to dine on while they mate. Some species even wrap their gift in a silken cocoon and believe it or not, in some of those species shifty males offer empty cocoons. Females of the Long-tailed Dance Fly (Rhamphomyia longicauda) are completely dependent on male food gifts for nourishment. Since males bring prey only to females they think have eggs, females inflate sacs in their abdomen to appear egg-laden even if they don't have any.