Dobsonfly larvae, sometimes called hellgrammites, are flattened and elongate, dark brown in color, with a segmented body. They have a wide head with strong biting mouthparts, 3 pairs of thoracic legs, and a eight pairs lateral filaments, one to a segment, down each side of the body, each with a gill tuft at the base of the filament. They are distinguished from stonefly (Plecoptera) larvae by the pair of prolegs at the hind end of the abdomen, each of which has two terminal hooks. Fully-grown larvae may be as long as 90 mm.
Pupae and adults have large mandibles, also a wide head, and an elongate abdomen. Adults are tan or light brown, with darker mottling, and are up to 75 mm long. They have two pairs of large, strongly-veined wings. The forewings are translucent grey-brown, with darker markings, especially on the veins. At rest they are held folded over the back, in a roof-like arrangment. The mandibles of adult males are extremely long (up to half the body length, and horn-like. (Evans and Neunzig, 1996; McCafferty, 1983)
The eggs of Corydalus cornutus are laid tree branches, rocks, or other structures that are over or immediately adjacent to moving water. The larvae live on the bottoms of fast-moving (well-oxygenated) streams and rivers, climbing over gravel, cobbles, sand, soft sediments, and organic debris. They are not usually found on living aquatic plants. Dobsonflies pupate on land, usually hidden in muddy soil or decaying wood near a streambank. Adults tend to stay near to streams, mating occurs on the ground or on vegetation.
This is a holometabolous species, with four life stages: egg, larva, pupa, and adult. The eggs, pupae, and adults breath air and live on land. The larvae are aquatic, and take their oxygen from the water. Nearly all feeding and growth occurs in the larval stage, which may molt as many as 10 times as it grows. Larval development is strongly affected by temperature: larvae in colder climates and colder streams take longer to grow (sometimes spending two winters in the larval stage before) and may be larger when they transform. (Anderson, 2003; Evans and Neunzig, 1996; McCafferty, 1983)
Adults mate within days of emergence, in late spring or summer. Mating occurs near streams, on the ground or on vegetation. Males may use their elongate mandibles in contests with other males. They also use them in courtship and mating behavior with females. (Anderson, 2003; Evans and Neunzig, 1996; McCafferty, 1983)
Female dobsonflies lay eggs very soon after mating. They produce up to three masses of eggs, usually on the undersides of leaves, branches, or other structures over-hanging a stream. Each mass may contain as many as 1000 eggs, laid in 1-5 layers and covered with a white protective material. Eggs incubate for 2-3 weeks before the new larvae hatch and drop or crawl to water.
The life-cycle of this species is strongly affected by temperature -- in the southern part of the range they can complete a generation in less than a year, but further north it may take 2-3 years. Adults only live for a few days -- females die after laying their eggs. (Anderson, 2003)
Only parental investment is in choosing egg-laying site, and provisioning eggs. (Anderson, 2003)
Corydalus cornutus larvae and adults are most active in twilight or at night. Except for mating, they are not social. Larvae mostly crawl, but can swim forward or backward by undulation. Adults are sometimes attracted to artifical lights at night, and may fly some distance from their emergence site. (Anderson, 2003; Evans and Neunzig, 1996; McCafferty, 1983)
Hellgrammites, the larval stage of Corydalus cornutus, probably rely mainly on touch and chemical sensing to locate prey. They do have eyes though and can at least detect motion and shadow.
Adult male dobsonflies have scent glands on their abdomen that apparently play some role in mating. They also lay their mandibles over females when courting them, so touch is relevant too. (Anderson, 2003; McCafferty, 1983)
Hellgrammites, the larvae of Corydalus cornutus and other corydalids, are active predators that feed on a wide variety of small stream invertebrates, including insects and other arthropods, small worms, and small molluscs. They are generalists, whose diet choices probably reflect relative abundance of different prey types rather than specialization. They are known to particularly feed on blackfly larvae (Simuliidae) and the larvae of net-spinning caddisflies (several familiies in the Trichoptera) and mayflies (Ephemeroptera).
Corydalus cornutus avoids predators by limiting its activity in daylight, by hiding, and by biting in self-defense if necessary. Adults and larvae are cryptically colored. We have no information on specific predator species. Stream fish eat them, and probably some crayfish. Birds and bats are probably natural enemies of adult dobsonflies. (Anderson, 2003; Evans and Neunzig, 1996; McCafferty, 1983)
This species is a mid-level predator, feeding on smaller animals, but also fed upon by larger predators. In small stream where fish are small or rare, large hellgrammites may be some of the largest predators in the water.
Some very small parasitoid wasp species in the genus Trichogramma are known to lay their eggs in the eggs of Corydalus cornutus. The wasp larvae consume the host egg, and emerge as adult wasps. (Anderson, 2003; Evans and Neunzig, 1996; McCafferty, 1983)
Though people are sometimes frightened by the large size and fierce appearance of adult dobsonflies, they are quite harmless. The larvae can deliver a painful bite in self-defense.
George Hammond (author), Animal Diversity Web.
Anderson, N. 2003. Megaloptera (Alderflies, Dobsonflies). Pp. 700-703 in V Resh, R Cardé, eds. Encyclopedia of Insects. New York City, New York, USA: Academic Press.
Evans, E., H. Neunzig. 1996. Megaloptera and Aquatic Neuroptera. Pp. 298-308 in R Merritt, K Cummins, eds. An Introduction to the Aquatic Insects of North America. Dubuque, Iowa, USA: Kendall/Hunt Publishing Company.
McCafferty, W. 1983. Aquatic Entomology: The Fishermen's and Ecologists' Illustrated Guide to Insects and Their Relatives. Boston, Massachusetts, USA: Jones and Bartlett Publishers, Inc..