Bearded Bobby the Brigand

Kelly’s Field Notes

Common Name: Robber Fly

Order: Diptera

Family: Asilidae

Species of Note Near You (There are more than 7,000 species of robberfly! 1,000 are native to North America.):

• Efferia aestuans - common in the northeastern USA. This species will often try to perch on people! They prefer horizontal or vertical spots to perch on. They don’t bite unless harmed.

• Laphria fernaldi - common in the western USA.

• Stripe-legged robber fly (Dioctria hyalipennis) - native throughout Europe.

• Giant yellow robber fly (Blepharotes coriarius) - one of the largest known robber flies, found throughout Australia.

• Mallophora ruficauda -  a bumblebee mimic (to avoid predation), this robber fly is found throughout Central and South America. Their larvae parasitize scarab beetles.

• Shioya-abu (塩屋虻, シオヤアブ) (Promachus yesonicus) - the Japanese common name shioya (salt merchant) abu (horse fly) is due to the white tip on its abdomen, which resembles salt. This species is found throughout Japan.

Description: 

Robber flies are fairly large, generally, with a body length of about 5cm (2 in ) in length. They get their name from their ambush approach to hunting other flying insects. Robber flies have short antennae, elongated abdomens, and occasionally an arista (a bristle located on the third antennal segment).The arista is an extra sensory organ that can sensecues and/or changes in the environment (scent, humidity, heat, etc.). Their faces are covered in small hairs (sensila), giving the robber fly a bearded appearance (this is called the mystax, located between the eyes). These hairs protect the robber fly’s face from struggling prey. They also have “fleshy” ends to their feet, likely to aid in grasping, and many spikey sensilla on their legs. They have narrow wings, built for speed!

Most robber flies come in colors that help them blend in with their environments, such as black, tan, or brown. Some species, such as Wyliea mydas, have brightly colored wings to ward off predators. This species mimics the tarantula hawk moth.

Life Cycle: 

Females oviposit (lay eggs) in grasses and plants low to the ground, as well as in soil or in crevices in bark or wood. Eggs are laid singly, or in clusters of around 300 or more depending on species. The eggs will hatch in a few hours to a day, the larvae are white grubby looking little guys. They sort of look like mealworms but softer. 

Robber fly larvae can take anywhere from a year to three years to become adults, pupating before their final stage. Some larvae are predatory, feeding on eggs and other soft bodied insects. Others feed on decaying plant matter. Some species, such as Mallophora  ruficauda, are parasitoids, paralyzing and feeding on beetle larvae. Once they have fed enough, going through 4 instars, they pupate. Unlike some other species we’ve discussed, robber fly (and fly pupae in general) can move! As they get closer to becoming adults, they will move towards the surface of whatever substrate they are in.

Adults are active during the hottest part of the day. They mostly capture flying prey, but will also go for insects or other arthropods on the ground or on leaves. They likely have good sight which is common in active hunters. Males are territorial and will chase each other away or try to kill each other. 

Super Powers:

• Toxic Saliva - robber flies have short beaklike mouth parts they use to inject prey with neurotoxins and digestive enzymes. The “beak” is used as a dagger to stab, prey, then a needle to inject, and finally a straw to suck up the prey’s bodily fluids. 

• Mimicry - Wyliea mydas mimics tarantula hawk wasps not only in color, but in behavior. It displays its genitalia as if it were a stinger, to ward off predators. Laphria posticata, among other robber flies, mimic bumble bees. This species goes the extra mile with bright yellow halteres (they help flies stabilize and know their body rotation mid-flight, light a gyroscope) , which resemble pollen grains.

• Deception - males initiate mating by grasping females mid flight. If she is not into it, she will play dead, causing him to drop her.

• Toxin Protection - robber flies prey on a variety of arthropods, not bothered by any caustic chemicals the prey may have (stink bugs, etc.).

• Flight - they’ve got two wings and know how to use them.

• Speed - robber flies are speedy flyers, making them excellent predators.

Robber Flies in Culture:

• The Hopi people represent robber flies in their Kachina dolls, which represent the personification of benevolent spirits. Wearing the costume of the Kachina doll imbued the person wearing it with the characteristics the spirit represented.

• It has been theorized that Kokopelli, a Hopi God of childbirth and agriculture, might look a little like a desert robber fly. Kokpelli can be found represented in many cave drawings.

References:

AMNH. “A Born Mimic: Toxic Robber Fly Mimics Tarantula Hawk Wasp.” American Museum of Natural History, 14 Nov. 2014, www.amnh.org/explore/news-blogs/from-the-collections-posts/a-born-mimic-toxic-robber-fly-mimics-tarantula-hawk-wasp. Accessed 1 Feb. 2024.

‌“Arista - Entomologists’ Glossary - Amateur Entomologists’ Society (AES).” Www.amentsoc.org, www.amentsoc.org/insects/glossary/terms/arista/. Accessed 1 Feb. 2024.

“Asilid, Bumblebee Mimic | U.S. Geological Survey.” Www.usgs.gov, www.usgs.gov/media/images/asilid-bumblebee-mimic. Accessed 1 Feb. 2024.

‌‌“Blepharotes Coriarius.” Wikipedia, 27 Sept. 2023, en.wikipedia.org/wiki/Blepharotes_coriarius. Accessed 31 Jan. 2024.

Castelo, Marcela karina, and Juan Carlos Corley. "Oviposition behavior in the robber fly Mallaphora ruficauda (Diptera: Asilidae)." Annals of the Entomological Society of America 97.5 (2004): 1050-1054.

Crespo, Jose E., and Marcela K. Castelo. "Life‐history traits in a parasitoid dipteran species with free‐living and obligate parasitic immature stages." Physiological Entomology 35.2 (2010): 160-167.

“Mallophora Ruficauda.” Wikipedia, 25 Nov. 2023, en.wikipedia.org/wiki/Mallophora_ruficauda. Accessed 31 Jan. 2024.

Mitton, Jeff. “Robber Flies Are Fierce Predators and Resourceful Lovers.” Colorado Arts and Sciences Magazine, University of Colorado Boulder, 28 Feb. 2020, www.colorado.edu/asmagazine/2020/02/28/robber-flies-are-fierce-predators-and-resourceful-lovers. Accessed 1 Feb. 2024.

‌“Promachus Yesonicus.” Wikipedia, 9 Oct. 2021, en.wikipedia.org/wiki/Promachus_yesonicus. Accessed 31 Jan. 2024.

“Robber Flies - Asilidae.” Entnemdept.ufl.edu, entnemdept.ufl.edu/creatures/beneficial/flies/robber_flies.htm.

‌‌‌‌“Species Efferia Aestuans.” Bugguide.net, bugguide.net/node/view/20274. Accessed 31 Jan. 2024.

Wikipedia Contributors. “Kokopelli.” Wikipedia, Wikimedia Foundation, 3 Nov. 2019, en.wikipedia.org/wiki/Kokopelli.