Megaloptera

(Dobsonflies, fishflies, and alderflies)

Class Insecta

Order Megaloptera

Number of families 2

Evolution and systematics

The oldest known fossils of Megaloptera date from the Permian. The Megaloptera are often considered to be the most primitive group of endopterygote (with internal development of wings as imaginal discs in the larvae) insects. The order Raphidioptera constitutes their sister group, and together with Neuroptera they form the monophyletic group of the Neuropterida. Megaloptera includes only two families, Corydalidae and Sialidae (alderflies). Corydalidae, in turn, comprises two subfamilies, Corydalinae (dobsonflies and hellgrammites) and Chauliodinae (fishflies), which are considered families by some authors.

Physical characteristics

Adult alderflies range from 0.4 to 0.6 in (10–15 mm), and dobsonflies range from 1.6 to 2.4 in (40–60 mm); larvae reach a maximum length of 1 in (25 mm) and 1.2–2.6 in (30–65 mm), respectively. Adults are soft-bodied insects, black, brown, or yellowish orange to dark green in color. They possess filiform, moniliform, or pectinate antennae and large compound eyes, and they either have (Corydalidae) or lack (Sialidae) ocelli. They have two pairs of membranous wings

with a complex nervelike pattern of veins, although longitudinal veins are not branched near the wing margin. The hind wings are broader at the base than the front wings, and this enlarged anal area is kept folded like a fan at rest. Larvae are elongated, flattened, and prognathous, with chewing mouth-parts. They have seven or eight lateral pairs of abdominal gills. In sialids the abdomen ends in a median unsegmented filament, whereas in corydalids it terminates in a pair of anal prolegs. Pupae have free appendages that are not fastened to the body and are not enclosed in a cocoon.

Distribution

They occur in the New World, South Africa, Madagascar, and Asian and Australian regions and are most diverse in temperate regions, with fewer species in the tropics.

Habitat

Larvae are aquatic, inhabiting both lentic (still water) and lotic (moving water) environments, including streams, spring seeps, rivers, lakes, ponds, swamps, and even temporarily dry streambeds. They burrow into soft substrate or in crevices or hide under stones or bark. Eggs, pupae, and adults are terrestrial. Eggs can be found in masses on leaves, branches, rocks, and bridges overhanging the aquatic habitat. Pupae are found in the shoreline soil and litter adjacent to the larval habitat, and adults are found in the same general area where larvae live.

Behavior

Adults seldom are seen in large numbers because they are short-lived and secretive. Sialids are active during the warm midday hours, engage infrequently in brief flights, and can be found resting near their larval habitats with the wings held rooflike over the abdomen. Most corydalids have nocturnal habits and are attracted to light, although there are some diurnal species. Their flight is slow and irregular, but some may fly considerable distances. At rest they keep the wings flat over the abdomen.

Feeding ecology and diet

Larvae are entirely predaceous, feeding nonselectively on a wide variety of small aquatic invertebrates, such as insect larvae, crustaceans, mollusks, and annelids. Adults apparently do not feed.

Reproductive biology

Communication between sexes before mating is known to occur in some sialids and corydalids. Corydalus species show sexual dimorphism; males have very long mandibles with which they duel with each other and prod the female. Copulation takes place in vegetation near the water. Females deposit masses of one to five layers of 200 to 3,000 eggs on objects overhanging the water, in locations protected from insolation during the hottest part of the day. After opening the egg with an egg burster, the larvae drop to the water after hatching, where they undergo 10 to 12 molts. Fully grown larvae leave the water and pupate in an unlined chamber in the soil or litter. Adults usually emerge from late spring to midsummer. Life cycles range from one to five years.

Conservation status

About 300 species of megalopterans have been described, none of which is listed by the IUCN. Some species with restricted distributions in small rivers and streams probably are sensitive to deforestation, pollution, and eutrophication (involving depletion of oxygen that is normally taken from the water by aquatic larvae) and could be potential candidates for conservation programs.

Significance to humans

Larvae are important predators in aquatic food chains. Some species are economically important as trout food, and their larvae are used as fish bait (i.e., Archichauliodes diversus from New Zealand). In the Japanese tradition, dried larvae of some dobsonflies are believed to be a remedy for emotional problems in children.

Species accounts

List of Species

Eastern dobsonfly

Corydalus cornutus

family

Corydalidae (Corydalinae)

taxonomy

Hemerobius cornutus Linneus, 1758, Pennsylvania.

other common names

English: Hellgrammite, toebiter, bass bait (larva); French: Grande mouche Dobson (adult).

physical characteristics

The adult is 2 in (50 mm) long, with a wingspan up to 5 in (125 mm), and the larva is 2.6 in (65 mm) long. The head is almost circular and the prothorax square and slightly narrower than the head. The wings are translucent gray with dark veins and cells with white spots. The mandibles of the male are as long as half of the body length, curved and tapering to the tips and held crossing each other. The mandibles of the female are shorter.

distribution

Occur east of the Rocky Mountains in the United States and Canada.

habitat

Larvae live in fast-flowing water.

behavior

Adults are nocturnal and secretive and are seldom seen during the daytime, when they hide under leaves in the canopy of trees. Larvae have been seen swimming forward or backward in a snakelike fashion, but they usually crawl.

feeding ecology and diet

Adults do not feed; larvae consume other insects and small invertebrates.

reproductive biology

Mating behavior is stereotypical; males flutter their wings, display genital appendages, and fight over females. Rounded masses containing 100–1,000 or more eggs are laid on rocks, branches, and objects close to the water. Each mass is coated with a whitish secretion. Larvae drop into the water or crawl to reach feeding grounds. After two or three years, they crawl out of the water and prepare pupal chambers under stones or logs, where they overwinter. Adults emerge in early summer.

conservation status

Not threatened.

significance to humans

Fishermen use dobsonfly larvae, called hellgrammites, as bait for trout, largemouth bass, catfish, and other fishes. Hellgrammites also help control populations of pest aquatic insects, such as the Asian tiger mosquito.

Black creeper (larva), Dobsonfly (adult)

Archichauliodes diversus

family

Corydalidae (Chauliodinae)

taxonomy

Chauliodes diversus Walker, 1853, New Zealand.

other common names

English: Toebiter, black fellow (larva).

physical characteristics

Larvae have a long, thin, gray body up to 2 in (50 mm) long and a subtriangular jet-black head. Adults are about 2–3 in (50–75 mm) long with large, clear wings with a 2–3.2 in (50–80 mm) wingspan.

distribution

New Zealand.

habitat

Stony streams and rivers. Older larvae are found beneath dry stones close to the water's edge.

behavior

Larvae cling to the stream bottom in all but the strongest flows and become dislodged only with flooding. Adults fly in a slow, clumsy manner and only for short distances at low heights over streams.

feeding ecology and diet

Larvae feed on mayfly larvae; adults do not feed.

reproductive biology

Larvae take from 18 months to three years to reach full size. Adults emerge in summer and live for only six to 10 days. Eggs that are laid late in the season undergo obligate diapause (suspension of development, which starts again once climatic conditions become more favorable).

conservation status

Not threatened.

significance to humans

Larvae often are used by anglers as trout bait.

Alderfly

Sialis lutaria

family

Sialidae

taxonomy

Hemerobius lutaria Linnaeus, 1758, Europe.

other common names

French: Mouche du Saule; German: Gemeine Wasserflorfliege; Danish: Dovenflue.

physical characteristics

Adults are 0.52–0.72 in (13–18 mm) in length and blackish-brown in color.

distribution

Occurs in Europe and into Russia.

habitat

Larvae inhabit the depths of still waters and muddy backwaters of rivers.

behavior

During spring and early summer adults are found on plants near the water; they fly only when it is sunny and warm.

feeding ecology and diet

Larvae feed on worms, insect larvae, and other small freshwater animals. Adults occasionally take nectar from flowers with easily accessible nectaries.

reproductive biology

After mating, the female deposits dark gray eggs on leaves of littoral vegetation and then cleans the newly laid eggs. Larvae crawl into the water and scurry to the bottom, where they tunnel in the silt. After two winters the larva leaves the water and pupates for two weeks. The greatest numbers of adults are seen on the wing from May through mid-June.

conservation status

Not threatened.

significance to humans

They are bioindicators in water quality assessment.

Resources

Books

Brigham, W. U. "Megaloptera." In Aquatic Insects and Oligochaetes of North and South Carolina, edited by A. R. Brigham, W. U. Brigham, and A. Gnilka. Mahomet, IL: Midwest Aquatic Enterprises, 1982.

Chandler, H. P. "Megaloptera." In Aquatic Insects of California, edited by R. L. Usinger. Berkeley: University of California Press, 1956.

Contreras-Ramos, A. Systematics of the Dobsonfly Genus Corydalus Latreille (Megaloptera: Corydalidae). Thomas Say Monographs. Lanham, MD: Entomological Society of America, 1998.

Evans, E. D., and H. H. Neunzig. "Megaloptera and Aquatic Neuroptera." In An Introduction to Aquatic Insects of North America, edited by R. W. Merritt and K. W. Cummins. 3rd. edition. Dubuque, IA: Kendall/Hunt Publishing Company, 1996.

Henry, C. S., N. D. Penny, and P. A. Adams. "The Neuropteroid Orders of Central America (Neuroptera and Megaloptera)." In Insects of Panama and Mesoamerica, edited by D. Quintero and A. Aiello. Oxford: Oxford University Press, 1992.

Penny, N. D. "Neuroptera." In Aquatic Biota of Tropical South America. Part 1, Arthropoda, edited by S. B. Hurlbert, G. Rodríguez, and N. Dias dos Santos. San Diego: San Diego State University, 1981.

——. "Neuroptera." In Aquatic Biota of Mexico, Central America and the West Indies, edited by S. B. Hurlbert and A. Villalobos-Figueroa. San Diego: San Diego State University, 1982.

Periodicals

Contreras-Ramos, A. "Mating Behavior of Platyneuromus (Megaloptera: Corydalidae), with Life History Notes on Dobsonflies from Mexico and Costa Rica." Entomological News 110 (1999): 125–135.

Davis, K. C. "Sialididae of North and South America." New York State Museum Bulletin 68 (1903): 442–486, 499.

Stewart, K. W., G. P. Friday, and R. E. Rhame. "Food Habits of Hellgrammite Larvae, Corydalus cornutus (Megaloptera: Corydalidae), in the Brazos River, Texas." Annals of the Entomological Society of America 66 (1973): 959–963.

Natalia von Ellenrieder, PhD