Robber flies are members of the Asilidae family and are found throughout the world. There are over 7,000 species and while some of them resemble each other, there are a number of them that mimic flies, bees and other insects, to let them hide in plain sight while they stalk their victims. They're quick and powerful, making them a threat to insects quite a bit larger than themselves. And it isn't unusual to see one making a meal from a large grasshopper or beetle.
Robber flies have several common names. They are also known as assassin flies and my personal favorite, the Hanging Thief. They're capable of capturing their prey on the wing, or pouncing on unsuspecting victims like little dive bombers. They inject a neurotoxin with their proboscis that renders the quarry helpless, while its insides liquefy. The robber fly then sips the resulting goo while hanging by one leg under a twig. (That's why it is called the Hanging Thief) And while they are not a threat to humans, they can deliver a painful bite if mishandled, so let them go about their business.
These aggressive little insects are quite distinctive. They have two large compound eyes and can turn their heads like a dragonfly or praying mantis. It also has a growth of hairs that resemble a moustache on its face. In fact, the proper name for these hairs is ‘mystax' which is Greek for moustache. The hairs bristle up between the eyes and are probably designed to protect its face and eyes from struggling prey. Robber Flies prefer to hunt during the hottest part of a sunny day and in the evenings they tend to roost in dense vegetation. It is believed that they have excellent eyesight and are very raptor-like in their behavior of searching for prey and stalking it from above.
Robber flies are considered a beneficial insect, but they are not very choosy as to their meals. They have been known to capture honeybees, butterflies and ladybugs right along with the grasshoppers, gnats and mosquitoes. Fortunately, they are not present in large enough numbers to do lasting harm to the population of good guys. Even robber fly larvae are predatory and can be found in leaf litter and around fallen logs devouring grubs and similar creatures.
The most prevalent robber fly in my area is Diogmites missouriensis and those are the images in this article. They tend to hang out in my herb bed when my mint and oregano are blooming, because those blooms attract a huge number of flying insects. They are wary and difficult to photograph, but every now and then one will strike a pose while waiting on his next meal.
I allow the robber flies to go about their business on my property. A healthy population of predators is the sign of a healthy ecosystem and in turn a healthy garden.
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