Physocephala burgessi (Thick-Headed Fly)


Found this cool Thick-headed Fly in the family Conopidae on my daisies while I was watering this morning. I believe this one to be Physocephala burgessi.

Thick headed flies are known for a behavior called “hilltopping” where flies will aggregate to find a mate. Adults are wasp mimics and seek out Hymenoptera species (bees/wasps) for development of their offspring.

A parasitoid of bees and wasps, Conopid flies will target an unsuspecting host typically while it is leaving its nest or nectaring on floral sources. The adult fly grabs the host and oviposits into the body of the individual.

The fly’s eggs hatch and migrate into the abdominal cavity of the bee/wasp where development continues as the fly larvae consume the contents within the abdominal cavity of the host. The bee or wasp host continues to live, and is able to fly throughout the duration of the larval development period.

Just before the end of larval development and transition to pupation, the host dies. The death of the host usually occurs somewhere near the entrance to its nest or within the nest itself. Pupation occurs in the abdomen of the now deceased bee or wasp host.

The adult fly typically emerges after overwintering in the abdominal puparium of the bee.

Note*** Typically, populations of these flies are fairly low. In twelve years on San Juan Island, this is the 2nd specimen I have ever seen. I photographed my first Conopid fly at American Camp, on the bluff overlooking Grannies Cove in 2010. In reviewing records on iNaturalist, it looks like my two are the only ones reported in San Juan County. This past year (2021), another resident sent in a photograph, bringing the total, that I know of, to 3.

References and Further Reading

https://bugguide.net/node/view/7190

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3014735/

https://www.biodiversitylibrary.org/page/2700384#page/126/mode/1up

http://staff.royalbcmuseum.bc.ca/wp-content/uploads/2018/02/Gibson_JESBC_2017.pdf

https://curve.carleton.ca/system/files/etd/6ab4c500-3013-43c0-aa9e-9f94353f1ff5/etd_pdf/139b167a8dbbb1d5ba97e1cd968dda3c/gibson-theevolutionarybiologyofconopidaedipteraa.pdf

https://link.springer.com/article/10.1007/s00114-019-1634-9

https://uwm.edu/field-station/tricks-of-the-trade-thick-headed-flies/

http://www.faculty.ucr.edu/~legneref/identify/conopid.htm

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