Evolutionary Biology Lab

Evolution & Ecology Research Centre and School of Biological, Earth and Environmental Sciences, University of New South Wales


WALTZING FLIES (Prochyliza xanthostoma)

Extreme sexual dimorphism and remarkable behaviours

The waltzing fly is a piophilid carrion fly remarkable for its extreme sexual dimorphism. Males possess several grotesquely exaggerated morphological traits (antenna, head capsule, forelegs) which they employ in complex combat and courtship behaviours.

In Algonquin Park, Ontario, waltzing flies typically breed in early spring on the carcasses of moose. In March and early April, they are one of the most abundant insect species in the Algonquin forest. The photograph on the right shows a copulating pair (male on top of female). 

P. xanthostoma mating



Courtship          Combat          Ejaculate feeding          On carrion1         On carrion2          On carrion3


Females with immature ovules can be found basking in sun-spots on vegetation in the vicinity of carcasses, and males aggregate in these sun-spots to defend territories and court the females. The sexual behaviour of P. xanthostoma is described in detail in Bonduriansky (2003) and Bonduriansky and Rowe (2003). Combat interactions between males often escalate into spectacular battles lasting up to 2 minutes: the opponents begin by spreading their forelegs and holding each other's fore-tarsi, perhaps in order to assess relative body size, after which each male attempts to strike the other with his head and antennae (see photos below). Male courtship consists of a side-to-side 'zig-zag' dance in front of or beside the female. As he dances, the male holds the front part of his body elevated above the substrate and his elongated antennae raised and spread, apparently 'showing off' this trait to the female. He also intermittently vibrates his elongated forelegs (see photo below). If she wants to mate, the female orients to the male and spreads her forelegs, to which the male responds in the same way. After touching forelegs, the male somersaults over the female's body, spins around 180 degrees, and attempts to land on the female's back and immediately initiate genital lock. Following copulation (~ 6 minutes duration), the female expels and ingests much of the male's ejaculate. Females oviposit on carcasses in various stages of decomposition, and larvae often develop inside the bones. As in other piophilid species, final instar larvae leap off the surface of their feeding substrate and pupate in the surrounding soil (see the Antler Fly page for a description and illustration of larval behaviour).


The Waltzing fly provides an ideal model system for research on the evolution of sexual dimorphism.


Prochyliza xanthostoma

Prochyliza xanthostoma

Prochyliza xanthostoma


P. xanthostoma male (left) and female (middle), and two males in combat (right).