Prochyliza xanthostoma, the waltzing fly, is a species of carrion-feeding cheese skipper, insects in the family Piophilidae and the order Diptera.

The adult flies are found through North America and are brown-bodied, with orange and black coloring.

[9] P. xanthostoma is an important model organism for sexual selection, larval behavior, and adult reproductive success and survivability.

The waltzing flies are distributed throughout North America, mostly near the carcasses where they lay their eggs, during the spring, summer, and fall.

[1][13] The natural larval substrate is carcasses, but waltzing fly larvae are also found in cheeses and cured meats, which makes them a pest.

[9] Adult waltzing flies eat primarily carrion, but will also consume fungi, cheese, and many other foods.

[9] Laboratory experiments on P. xanthostoma approximate cheese fly diets by feeding them lean ground beef that had aged for several days.

[6] Males may benefit from ejaculate feeding by decreasing the number of eggs females have available for other mates, thereby increasing his paternity.

[6] Females decrease re-mating rate by exhibiting resistance behaviors such as moving or flying away when a male is attempting to copulate.

[14] Researchers therefore hypothesize there is antagonistic coevolution between the ejaculate in male waltzing flies and the female re-mating rate.

Male head capsules, forelegs, and antennae are long and narrow, which helps increase success in male-to-male combat as well as in courting female waltzing flies for mating.

[12] Female P. xanthostoma flies that were fed a high quality diet exhibited shorter antennae with respect to their body size relative to female P. xanthostoma flies that were fed a low quality diet.

[7] Genetic variation at loci on the chromosomes of P. xanthostoma flies are correlated with efficiency of the acquisition of resources as well as male body shape.

[12] More specifically, genetically, head length and width in males are negatively correlated to condition (high or low).