The Stealthy Assassin Robberfly

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These striped slender robberflies (Leptogaster cylindrica) may look like they’re competing to see who reaches the top of the leaf first, but in fact they’re at rest, having roosted in this position for the night. Photographer Oliver Wright captured this rare image on a visit to his local pond in Yorkshire, United Kingdom.

Robberflies are known for their stealthy hunting habits. Also called “assassin flies,” the predatory insects wing slowly and quietly through the grass, searching for aphids, flies, and even small spiders. They typically seize their prey in mid-flight, biting with their stout, piercing mouthparts and injecting paralyzing neurotoxins (which can also deliver painful bites to humans). Then, they liquify their victims’ insides, and suck them in.

“The striped slender robberfly is not a commonly found species,” says Wright. He has photographed the insects, which are native to the tall grasslands of southern England, only at his local pond. “This might be due to the fact that I’ve spent more time here than anywhere else.”

Also called “assassin flies,” the predatory insects wing slowly and quietly through the grass.

Over the years, Wright has learned that a dawn arrival is key to photographing the elusive insects, which measure less than half an inch long. “I’ve only ever been able to find them very early in the morning when they have roosted for the night and they have yet to wake up and take flight,” he says. “They are torpid with the cold morning and will not be ready for flight until the dew has gone.”

Only twice in 10 years of visiting the pond has he found two striped slender robberflies roosting on the same blade of grass, as in this image, which he took using a high-magnification lens and tripod in the low light.

After the flies have awakened from their awkward slumber and brushed off the morning dew with their legs, they get to work finding food. Rising languidly into the cool morning air, the tiny but fierce creatures prepare for their next ambush.

This story originally appeared in  bioGraphic, an independent magazine about nature and regeneration powered by the California Academy of Sciences.

• April Reese

  Posted on September 13, 2024

  April Reese is a freelance science writer and editor based in Portugal. Her reporting has appeared in Scientific American, Discover, bioGraphic, Science, Aeon, and many other outlets. She holds a master’s degree in Environmental Studies from the Yale School of the Environment.

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