Dance flies give worthless gifts to get on a girl’s good side, or even just steal from other males
Male flies impress females with nutritional gifts, like juicy flies. These tasty treats are supposed to convince a female to allow the male to mate with her, and often a female has an entire swarm of gift-bearing males to choose from. Depending on the species, gifts can be wrapped in silk produced by glands on the front feet or encased in a balloon of saliva. Some flies even encase their gifts in bubbles of anal secretions.
However, these flies have learned how to lie. Some males catch an insect and suck out the insides, leaving an empty shell. They wrap this hollow exoskeleton up and present it to the female, who allows him to mate. Afterwards, the male flees and the female finds her present is worthless. The boldest flies give empty packages of silk or saliva, making sure the gift is extremely large to throw the female off.
One dance fly species has taken this a step further. A male fly poses as a female and waits for another male to approach. When the interested male gets close enough, the impostor snatches the gift and makes a hasty escape. It then uses this gift to woo a female for itself.