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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

What is that strange big insect? Dobsonflies, hellgrammites, and fishflies (Corydalidae)


An image of this lovely female dobsonfly was one of several recently sent to Bug of the Week for identification. Photo credit: Randy Taylor


Recently, Bug of the Week received several requests to identify large creepy looking insects discovered on vegetation and structures near streams in our region. These interesting creatures belong to an order of insects known as the Megaloptera – huge winged insects. Dobsonflies are among the largest of winged insects, found hereabouts ranking in size with large moths and butterflies.

Extremely long mandibles of male dobsonflies are used to battle rivals. Photo credit: Nolan Jenkins

While female dobsonflies are magnificent, males really have something special in their enormous sickle-shaped mandibles. Careful observations of mano-a-mano encounters between male dobsonflies reveal that their super large mandibles are used in combat to dislodge competitors from substrates where potential mates might be present. These mandibles are useless in capturing prey and both male and female dobsonflies, which have powerful jaws, are not predatory as adults. As adults their diet is likely a liquid one.

Juvenile dobsonflies go by the name of hellgrammites and live a life aquatic. These fierce predators roam the interstitial spaces between stones and vegetation at the bottom of rapidly flowing streams where they capture and dine on immature mayflies, stoneflies, and caddisflies. Their powerful jaws can deliver a memorable bite to unsuspecting humans attempting a capture. They are a key indicator of stream health and not found in polluted waters. Fish adore them and they are excellent bait. Like many aquatic insects, hellgrammites have gills lining the margins of the abdomen enabling them to extract oxygen from their watery habitat. In an unusual developmental twist, they also have spiracles, breathing ports, which allow them to obtain air on land. This adaptation is critical to their amphibious life style as they climb out of the water to build pupal chambers on land beneath stones, logs, or other moist protected structures. You may encounter dobsonflies in the morning near lighted buildings, as both sexes are attracted to light.  After mating, female dobsonflies deposit eggs on vegetation overhanging water. Hatchlings drop to the stream below to roam the benthos in search of prey.  Larval development can take from one to three years.


Watch as a hellgrammite demonstrates its gill action, ability to roll over and play dead, and then finally escapes the camera of the insect paparazzi.

Fishflies are close kin to dobsonflies. Like their cousins the dobsonflies, larvae are aquatic predators and adults can be found on vegetation near water. Smaller size, feathery antennae and less magnificent jaws distinguish them from their majestic relatives. Over the next several weeks as you wander the banks of freshwater streams and rivers in our region, keep an eye open for these marvelous giants of the insect world.      


Fishflies are close relatives of dobsonflies but are generally smaller and distinguished by their feathery antennae and lack of enormous jaws.


Bug of the Week thanks Randy and Nolan for providing images that were the inspiration for this episode. The wonderful publications “Behavioral Observations on the Dobsonfly, Corydalus cornutus (Megaloptera: Corydalidae) with Photographic Evidence of the Use of the Elongate Mandibles in the Male” by T. J. Simonsen, J. J. Dombroskie, and D. D. Lawrie, and “Featured Creatures, common name: eastern dobsonfly (adult), hellgrammite (larva) scientific name: Corydalus cornutus (Linnaeus) (Insecta: Megaloptera: Corydalidae: Corydalinae)” by D. Hall were used as references for this episode.