Thick-headed flies are aptly named, sporting disproportionately large heads. At first glance, flies within this genus might be mistaken for solitary wasps, resembling them in general shape and color. When at rest they often wave their forelegs above their heads…presumably to simulate hymenopteran antennal movements…thus reinforcing their protection through wasp-mimicry. Adults fly from late spring to late summer and often visit flowers to take nectar. Females may also be observed engaging in a behavior that could serve as inspiration for a horror movie: grabbing a host, which is generally a wasp or bee (including honey bees and bumble bees), and inserting an egg into it while it’s visiting flowers or in flight. And we’ll let BugGuide take it away from here:

“The egg subsequently hatches within the host and begins feeding…first on haemolymph, and later on internal tissues…eventually killing the host (i.e. it’s a parasitoid). This usually takes 10-12 days, though a study in Alberta showed that bumble bees parasitized by P. texana had the same lifespan as unparasitized individuals (Otterstatter et al. 2002)(3).

Bumblebee hosts have been documented to bury themselves under dirt before dying, a behavior presumably induced by the Physocephala larva, which benefits thereby from having a safer underground environment in which to overwinter before pupating and eclosing the following spring.

As with all members of Schizophora, the emerging adult rhythmically inflates its ptilinum to break out of its puparium…and, in Physocephala, also to break out of the host’s corpse as well as to dig to the surface. (Video of emerging P. tibialis, from Gibson et. al.(2014).)

Once out of the ground, it finds a place to perch while it expands its wings (by pumping haemolymph into them) and “cures” (i.e. dries and hardens its exoskeleton).

Adults then go about mating, and females seek out hosts…repeating the cycle.”

Size: 5 – 14 mm for genus 

Photo by: Lindsay Guttermuth on 7/29/22 in Missoula, MT