Robber Flies: What Are They all About? Back »

Figure 1. Some robber flies mimic bee coloration. Courtesy: Adam Varenhorst

Written collaboratively by Adam Varenhorst, Amanda Bachmann, Philip Rozeboom, and Patrick Wagner.

During summer, you may come across some rather large flies that are often carrying other insects. Although these flies may appear threatening, and sometimes even mimic other insects such as bees (Figure 1) or wasps, they are generally benign to humans. The insects in question are commonly referred to as robber flies or assassin flies. They get these names based on their highly aggressive feeding habits and, in some of the smaller species, their ability to steal prey (kleptoparasites) from spider webs.

There is a lot of variation in size, amount of body hair, and coloration among robber fly species However, there are some commonalities that can be useful when trying to identify these fascinating insects. Robber flies have a distinct “beard” that covers their piercing beak-like mouthparts. In addition, robber flies also have very long legs and long abdomens. Another distinct characteristic of robber flies is the concave space that is present between their circular eyes (Figure 2).

Small fly with red eyes holding a spider in its mouth.
Figure 2. Robber flies have a concave space between their circular eyes. Courtesy: Adam Varenhorst

Robber flies will feed on a wide variety of hosts. Some common insects in their diet include dragon flies, damsel flies (Figure 1), spiders (Figure 2), wasps, bees (Figure 3), beetles (Figure 4), grasshoppers, and even other robber flies. Robber flies will often perch on plants and wait for prey to pass by. Once a suitable target is observed, the robber fly will swoop out to catch the prey and bite it using a piercing mouthpart. The robber fly then injects saliva into the hosts that paralyzes it along with enzymes that liquefy the internal organs. The robber fly then finds a suitable perch to consume its prey.

Although robber flies can inflict a painful bite, they usually ignore humans and will only bite when they feel threatened. These flies are generally fun to watch, but shouldn’t be considered as a potential natural enemy for pest management as their host range is too broad.

Orange fly with green eyes holding a honey bee on a green soybean leaf.
Figure 3. Hanging thief robber fly carrying a honey bee. Courtesy: Adam Varenhorst

Grey robber fly eating a beetle on a green corn leaf.
Figure 4. Giant robber fly eating a beetle. Courtesy: Adam Varenhorst

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