Dobsonflies are a species of insect that lives in and around bodies of moving water like rivers and streams. They’re large, with a wingspan of around 5 inches. Dobsonflies spend most of their life as larvae and only live for a few days as adults. Only one species is found in the U.S., and it only inhabits the eastern side of the country.
What Is a Dobsonfly?
Dobsonflies aren’t truly flies, not like houseflies or horseflies. While all are insects, they belong to different orders. Flies like houseflies belong to the order Diptera with mosquitoes and crane flies, while dobsonflies belong to the order Megaloptera with other large-winged creatures like alderflies and fishflies. Megaloptera means “large wing.”
Dobsonflies are one of the largest types of insects after butterflies and moths. The bodies of adult dobsonflies are usually 2 to 2.5 inches (50.8 to 63.5 millimeters) long, though their wings can span 4 to 5.5 inches (114 to 127 millimeters). Male dobsonflies have long, thick mandibles (jawbones) that almost look like tusks and can be up to an inch and a half in length (40 millimeters). They use these for self-defense and to hold the females while they mate. The mandibles on female dobsonflies are much smaller.
Dobsonflies can be found in most of the world except Europe and Antarctica. There are 30 species of dobsonflies in the Western Hemisphere, but only one in the U.S.: Corydalus cornutus, the eastern dobsonfly.
The eastern dobsonfly is usually a drab shade of black, brown, or gray. Their wings fold down the length of their back when at rest and have obvious veining. The larvae of eastern dobsonflies are often black, brown, or tan.
Dobsonfly Life Cycle
Adult dobsonflies don’t live very long, only three to 10 days. Despite their short time as adults, their overall lifespan is one to three years. There are four stages of dobsonfly life: egg, larva, pupa, and adult.
Dobsonfly eggs are tiny, less than 1.5 millimeters long and 0.5 millimeters wide. The female dobsonfly arranges the eggs in three layers and covers the cluster with a clear fluid that dries white. The eggs incubate for two or three weeks before hatching, and the eggs always hatch at night.
The larvae, called hellgrammites, are dark brown, segmented, and can be long, up to 3 inches. They look somewhat like centipedes. They have hardened plates on their thorax and strong jaws. Hooks on their front legs keep them from floating away in the water.
Dobsonfly larvae live in the water but can breathe both in the water and on land. Most of their development happens during the larval stage, which in itself is made up of 10 to 12 stages. Dobsonfly larva may shed their skin up to 10 times as they grow. The length of time they spend in the larval stage depends on the temperature. In warmer areas, they may complete their cycle in a year, but in colder areas, it can take two or three.
Hellgrammites are sometimes sold as fish bait.
In the pupa stage, dobsonflies begin to lighten in color to a yellowish-orange with dark splotches. They spend two weeks in the prepupa stage, after which they shed their exoskeleton. They then begin to develop adult features like antennae and wings, and males grow their long mandibles. After seven to 14 days in this phase, the dobsonfly emerges as an adult.
Almost immediately after reaching the adult stage, the dobsonfly mates. The males die within a few days, while the females live longer to lay their eggs, usually eight to 10 days. Female dobsonflies can lay up to three groups of eggs, and each group can have up to 1,000 eggs in it. Females die shortly after laying their eggs.
Dobsonfly Habitat
Dobsonflies are found in or near moving water, like rivers and streams. Eastern dobsonflies are found specifically near rivers and streams of eastern North America from Mexico through Canada.
Female dobsonflies lay their eggs on structures like tree branches or rocks that are next to or over the river or stream. When the eggs hatch, the larvae move into the water, where they’ll live along the bottom of the river or stream, usually under rocks. They usually crawl along the gravel, organic debris, sand, and various sediments, and aren’t often found on the plants of the river or stream.
To pupate, dobsonflies move to land, often hiding in decaying wood or muddy soil near the stream. They’ll dig a hole with their legs and mouths to stay in as they go through the pupal stage.
Once they become adults, they’ll stay near the stream to mate and, for the females, lay eggs. They’re usually active at night.
What Do Dobsonflies Eat?
Dobsonflies do most of their eating while in their larval stage. They feed on insects and insect larvae, small worms, and small mollusks that they find in the water. They’re willing to eat a range of foods but tend to prefer blackfly larvae, net-spinning caddisfly larvae, and mayfly larvae.
The consensus among the entomology community is that pupa and adult dobsonflies don’t eat any solid food. The adult males, with such a short lifespan, likely don’t eat at all. Adult females have been reported to feed on nectar from flowers. In captivity, they’ve been known to accept sugar water.
Do Dobsonflies Bite?
Dobsonflies only bite when threatened. Both the larvae and the adult females have strong mandibles that can deliver a painful bite. The adult female is known to be able to draw blood with her bite. The male, with his long, extended mandibles, is unable to deliver a strong or painful bite.
Are Dobsonflies Dangerous?
While running into a dobsonfly might be initially alarming because of its size, if you leave it alone, it'll leave you alone. Despite their bite, dobsonflies aren’t dangerous.
Dobsonflies play an important role in the ecosystem as mid-level predators. The larvae feed on small animals, but they also have predators. Fish and crayfish eat dobsonfly larvae, while creatures like birds and bats are natural predators of adult dobsonflies.
While dobsonflies are not endangered, they are beneficial insects that should be conserved.