This unusual insect was found on a University sidewalk, swarmed by scavenging ants, as the final moments of its life slipped away. A curious passer-by interrupted the insecticidal carnage to bring him into the Home & Garden Center for identification. Corydalus cornutus commands attention because of the large size of its adult phase, and, in the male of the species, those formidable, sickle-shaped mandibles.
The Dobsonfly begins life as an egg deposited on rocks or vegetation close to fast-moving stream. The grayish-white egg mass resembles a bird dropping, likely an evolutionary adaptation to discourage predators. When the egg hatches in 1-2 weeks, a tiny larva falls or crawls into the water and undergoes an aquatic phase for 1-3 years. There, a predator itself, it sheds its exoskeleton 10-12 times, until it achieves a size of nearly 4” in length and a prehistoric appearance straight out of a Hollywood horror film. It’s armored with hard dorsal plates for protection and a pair of hooks at the end of its abdomen for stability in moving water. When fully grown, its fierce mandibles are powerful enough to draw blood. Although aquatic, the hellgrammite’s respiratory system allows it to breathe in or out of water. The name Dobsonfly comes from a fisherman’s term of the late 19th century, “dobsons.” Much prized as bait, its tough exoskeleton endures through several catches.
Having completed the larval stage, the insect leaves the stream to find a suitable damp location under a rock or log to pupate. Talk about horror films – this emergence is a curious synchronous event of mature larvae called a “hellgrammite crawl, believed to be triggered by the vibrations caused by thunderstorms. The pupa is unusual in that the insect has full mobility and is able to defend itself against predators. This phase may vary in duration from two weeks to overwintering, depending on climate.
Adult dobsonflies live only about three days (males) to 10 days (females) and do not feed. Normally found in vegetation bordering streams, they are active at night and are sometimes seen near dwellings because of their strong attraction to lights. The male’s pincers are used in the courtship ritual and for jousting with other males, but are not for grasping females during mating as was previously believed.
Dobsonflies are members of the order Megaloptera (“large wing”). Relatives of the lacewings (Neuropterida) which they closely resemble, they are North America’s largest winged insects other than the Lepidoptera (butterflies and moths). They are not well known because of their brief lifespan in the adult phase.
Corydalus cornutus is found throughout the eastern U.S., from Canada to Mexico. A top predator in its aquatic phase, it plays an important role in riparian biodiversity. Its presence is an indicator of unpolluted water.