Giant Dobsonflies: Freshwater Species of the Week

An Eastern dobsonfly, Corydalus cornutus. Photo: Didier Descouens, Museum of Toulouse, Wikimedia Commons

A couple of months ago, I went camping in Harpers Ferry, West Virginia. Not exactly roughing it, I stayed at a campground with such amenities as a pancake house, a giant trampoline, and a pool. One evening, while relaxing in the pool, enjoying the views of the surrounding hills, I got a start.

A giant insect floated toward me. It was several inches long and had extremely large, pincher-like mandibles. I had never seen a bug like this before, and it triggered something in my brain, perhaps in my genetic memory. It looked menacing, even evil, and I recoiled.

freshwater species of the weekI couldn’t tell if the thing was still alive. But I also felt bad for it, because it was listless in the chlorinated water. Not wanting to touch it, I splashed it up on the concrete edge of the pool.

My companion and I leaned over it, trying to see if it would move. “Is it some type of giant ant or termite?” I asked.

“Give it mouth to mouth!” a guy cracked as he walked by. “I think it’s one of those cicadas,” his companion said.

“It’s definitely NOT a cicada,” I replied.

The bug didn’t move a millimeter. I dried off and looked it up on my iPhone, Googling “giant insect with big jaws in West Virginia.” The first results showed pictures of exactly what I had found, and identified it as a dobsonfly.

What’s a Dobsonfly?

Dobsonflies are a primarily aquatic group in the insect subfamily Corydalinae, part of the family Corydalidae, which also includes bugs called fishflies. There are more than 200 species of dobsonfly, distributed in the Americas, Asia, and South Africa.

Dobsonflies can reach lengths up to five inches (12.5 centimeters). They have long antenna and wings that fold back. But their most noticeable trait are enormous, sharp mandibles, which can grow up to one-inch long (25 millimeters) in males.

Those mandibles are so unwieldy that male dobsonflies have trouble getting leverage, so they can’t usually break the skin of a human being. The smaller, but still impressive, jaws on the females can reportedly inflict a sharp bite, although the insects aren’t generally thought of as aggressive.

Mostly, the male uses his bulging jaws to impress the females, and to position her during mating.

Cycle of Life

Dobsonflies spend most of their lives as larvae, which live in lakes and streams (probably not treated swimming pools!), especially under rocks. The larvae can reach a length of three inches (7.6 centimeters), and they devour other insects. They are known as hellgrammites, grampus, go-devils, or crawlerbottoms.

The larvae live in the water for a few years. When they are ready to grow up, in late spring or early to mid summer, they crawl unto land. They pupate, molt, and then take on the fearsome appearance of mature adults.

The adults typically only live seven days, and are not thought to feed. They hang out near the water where they grew up, mate, and die. Females often lay their eggs on vegetation overhanging the water.

Have you ever seen a dobsonfly?


Brian Clark Howard covers the environment for National Geographic. He previously served as an editor for and E/The Environmental Magazine, and has written for Popular Science,,,, Yahoo!, MSN, and elsewhere. He is the co-author of six books, including Geothermal HVACGreen LightingBuild Your Own Small Wind Power System, and Rock Your Ugly Christmas Sweater.