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| Robber Flies are also called Assassin Flies, and for good reason! |
What is that powerfully built, fly-like creature that waits in ambush to catch their prey in flight? It’s a robber fly, otherwise known as an assassin fly, which feeds almost exclusively on other insects, including other flies, beetles, butterflies, moths, bees, ants, dragonflies, damselflies, wasps, grasshoppers, and even some spiders.
Robber flies have stout, spiny legs, short antennae, three simple eyes on the top of their head between two large compound eyes, bristly bodies, and a short, stout proboscis that encloses a sharp, sucking mouthpart called a hypopharynx. They also have a dense moustache of bristles on the face called the mystax, which appears to offer some protection from their struggling prey. These flies attack their prey by stabbing it with the proboscis and injecting it with a saliva that contains both a neurotoxin and proteolytic enzymes. This paralyzes the prey and liquifies the insides, allowing the robber fly to suck the material up through their proboscis.
In spring and summer in our area, some of the more common robber flies include the Bee-like Robber Fly (Laphria macquarti), Prairie Robber Fly (Diogmites angustipennis), and the Maroon-legged Robber Fly (Promachus hinei).
The Bee-like Robber Fly is large (up to 1.25” long), hairy, mostly black with a yellow thorax, basal abdominal segments, and some yellow on the legs. It mimics the American Bumblebee, and while it prefers to feed on small beetles, it will take a wide range of prey, often perching on tree stumps in the open.
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| Bee-like Robber Fly |
The Prairie Robber Fly, also called the Slender-winged Hanging Thief, is about 1” long, with greenish-red eyes, tan to reddish-brown body, thorax with darker brown longitudinal stripes, and abdomen with darker lateral stripes. It is common in both woodland and meadows, hunting bees and other insects, and typically hangs from vegetation by its forelegs while it consumes its prey.
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| Prairie Robber Fly |
Another large robber fly is the Maroon-legged Robber Fly, up to 1.5” in length. It has dark eyes, light-colored bristles around the face, a brown thorax, a cream-colored abdomen with thick black bands and a black tip, and hairy reddish-brown legs. It prefers hunting bees and wasps, and can be found flying around a variety of habitats, perching on the ground or in vegetation.
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| Mating Maroon-legged Robber Flies |
Female robber flies deposit tiny, whitish eggs on low-lying plants and grasses, or in cervices in wood or bark, or in soil. After hatching, the larvae live in this detritus, feeding on other soft-bodied insects. Robber flies overwinter as larvae, pupate in the soil, and eventually emerge as adults. This developmental cycle can take one to three years, depending on the species and environmental conditions. Robber flies generally occur in relatively open habitats with scattered vegetation, or in forested margins and woodland edges.
Robber flies are generally considered beneficial insects due to their predatory nature, as they primarily feed on other insects, many of which are considered pests. These aerial assassins help to naturally regulate insect populations thereby contributing to a more balanced ecosystem!
