Eastern Dobsonfly

(Corydalus corntus)

Kingdom: Animalia

Phylum: Arthropoda

Class: Insecta

Order: Megaloptera

Family: Corydalidae

Genus: Corydalus

Species: cornutus

Introduction

Corydalus corntus is more commonly known as the Eastern Dobsonfly. This species is the most widely distributed member of its genus. Adults are winged but are not very competent fliers due to their large size. The Dobsonfly is a large insect with a wingspan measuring 13 centimeters. They are an aquatic species, meaning part of their lifecycle is completed in freshwater environments. They are most easily recognized by their large head and mandibles. This species is prized by insect collectors, and is often captured and studied using ultraviolet light traps, which attract these nocturnal animals.

Life Stages

Corydalus corntus is a holometabolous insect, meaning it undergoes complete metamorphosis. Immature stages of holometabolous insects are known as larvae, and the larvae of Corydalus corntus are known as Hellgrammites. Corydalus corntus spends the vast majority of its lifespan in the Hellgrammite form, which can last 1-3 years depending on latitude. Larvae are aquatic and undergo 10-12 larval instars before they leave the water to molt into adults. Once the adults emerge they breed and lay their eggs on structures above a waterway, covering them in a chalky white secretion for protection. Adults die shortly after emerging, following a reproductive period of about one week.

Habitat

The Corydalus corntus is found in much of the United States, found from Canada and Mexico East of the continental divide. Larvae are found under large rocks or woody debris where they can cling to in fast current and search for prey. Larvae require clean flowing water to live, so will only be found in rivers and streams with low levels of pollution. They are so sensitive to pollutants that their presence in a waterway is used as an index to measure the health of the waterway.

Feeding

Adult life stages of Corydalus corntus do not feed, they only live long enough to mate. The larvae on the other hand, are voracious predators. Their flat heads contain large mandibular muscles giving the larvae a powerful bite. They are opportunistic feeders, feeding primarily on other aquatic insects. Larger instars may even eat small vertebrates like fish or tadpoles. Larger fish are known feed on the dobsonfly larvae as well. Fishermen may know the species well because it is often used as bait for fish like smallmouth.

Sexual Dimorphism

Corydalus corntus exhibits drastic sexual dimorphism. The male's mandibles are highly elongated. These mandibles are rendered useless due to their enormous size: males are unable to feed or bite with much force. Female mandibles more closely resemble those of the larval stage. Like the larval stage these are powerful and can deliver a strong bite, though females do not typically feed either.

Reproductive Behavior

These sexual structures are used for two main sexual behaviors; Courtship and Fighting. Males use their antenna to track down females that are releasing attractant pheromones. Males will then battle eachother with their mandibles. They attempt to grab the other with their mandibles like a claw machine. The objective of the fight is to trow the opponent from the perching sight. Males will often swing their mandibles and flap their wings to intimidate would-be challengers. Victorious males win the right to breed.

Copulation begins when the male approaches from the side and they touch each other with their antenna. If the female is receptive, the male will place its mandibles perpendicular to the wings of the female. This complex behavior is important to get correctly, as the female possesses powerful mandibles which they use to fend off unwanted males. If the female is still receptive mating will occur.

References

Brown, Arthur V., and Lloyd C. Fitzpatrick. “Life History and Population Energetics of the Dobson Fly, Corydalus Cornutus.” Ecology, vol. 59, no. 6, 1978, pp. 1091–1108., https://doi.org/10.2307/1938225.

Simonsen, Thomas J., et al. “Behavioral Observations on the Dobsonfly, Corydalus Cornutus (Megaloptera: Corydalidae) with Photographic Evidence of the Use of the Elongate Mandibles in the Male.” American Entomologist, vol. 54, no. 3, 2008, pp. 167–169., https://doi.org/10.1093/ae/54.3.167.

Bowles, David E., and Gregory W. Courtney. “The Aquatic Neuropterida of Iowa.” Proceedings of the Entomological Society of Washington, vol. 122, no. 3, 2020, p. 556., https://doi.org/10.4289/0013-8797.122.3.556.

Short, Robert A., et al. “Production of Corydalus Cornutus (Megaloptera) in Four Streams Differing in Size, Flow, and Temperature.” Journal of the North American Benthological Society, vol. 6, no. 2, 1987, pp. 105–114., https://doi.org/10.2307/1467220.

Bowles, David E., et al. “Gynomorphic Mandible Morphology in the Dobsonfly,Corydalus Cornutus.” Journal of Insect Science, vol. 7, no. 23, 2007, pp. 1–5., https://doi.org/10.1673/031.007.2301.

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