Waltzing flies breed in early spring, generally on the carcasses of moose. By midspring of a good year, they are one of the most abundant insect species in the local forests. Nubile females bask on vegetation around carcasses, and males aggregate in their sun to court and to defend territories. Competing males usually begin by spreading their forelegs and holding each other's foretarsi. This recalls the competitive behaviour and adaptations of another fly family, the Diopsidae, in which the males match the spread of each other's eyes. Such behaviour may permit the adversaries to assess each other's size, thereby avoiding futile and possibly damaging conflict with a larger opponent. When neither of two competing males of "P. xanthostoma" is intimidated, they fight, each striking the other with his head and antennae. Such fights may last for several minutes, a long conflict for such small, active, and delicate animals.