Jeffrey Hahn, Asst. Extension Entomologist
Some residents have been recently encountering a large (and to some people a scary) insect larva. This dark colored insect, a hellgrammite (Corydalus cornutus), has an elongate and somewhat flattened body. It has six conspicuous legs as well as a series of filaments on the sides of its abdomen with small finger-like gills clustered at the base. A hellgrammite also has very prominent, strong mandibles (jaws). It reaches about two to three inches in length when fully grown. Despite its appearance, hellgrammites are not aggressive or dangerous to people, although it is possible that they can bite if they are handled carelessly.
A hellgrammite lives in running fresh water, such as streams or rivers, often hiding around rocks or debris. They are predaceous, feeding on insects and other aquatic invertebrates. They have the interesting ability of being able to swim backwards as well as forward. It is generally believed that they take one to three years before crawling out onto dry land to finish their development. They will usually create a cell in the soil near the water to pupate. They are sometimes used by fishermen as bait.
As an adult, a hellgrammite becomes a dobsonfly. A dobsonfly is brown up to two inches long with long slender antennae and long wings with conspicuous mandibles. You can distinguish between the sexes as males possess mandibles up to 3/4 inch long, making them look quite fierce and dangerous. Fortunately they are not able to bite people, using their mandibles only for fighting other male dobsonflies. Females have smaller mandibles but could bite people if given a chance. Although adults are usually found near water, they are attracted to lights and can be found a fair distance away. Dobsonflies are harmless and are just a curiosity.