Dobsonflies are a type of flying insect with large, clumsy wings. These bugs are not often seen because they are most often active at night or at twilight. They are part of the Megalopteran family Corydalidae and fall within the subfamily of Corydalinae. Their aquatic larvae, referred to as hellgrammites, are coveted by fisherman as bait for trout and fresh water bass.
Adult dobsonflies are some of the largest non-Lepidopteran insects of temperate zones such as the United States and Canada, with a wingspan of up to seven inches in some species. The wings vary from a grayish to translucent shade, depending on the species, and the anal region of the hindwing is wide and folded at rest. Despite the large wings, adults are weak, fluttery fliers. The body is soft and reaches up to three inches in length, and coloration varies from yellow to dark shades of brown.
Male adults are easily recognized by their long, curving mandibles, reaching up to an inch and three-fifths in length, which they use in competition for females. These oversized mandibles have developed as secondary sex characteristics used by females to evaluate males during courtship. Males cannot use these mandibles to bite because they are too long; on the other hand, females have short, heavily sclerotized mandibles which enable them to deliver powerful bites when threatened. The antennae of males are also noticeably elongated, even longer than the mandibles.
Dobsonflies have quadrate heads with a postocular spine, ridge, and plane, non-pectinate antennae, four crossveins between the radius and the radial sector, and distinctive male terminalia with a well developed sexual appendage.
Dobsonflies and their larvae are aquatic, living in streams. The larvae, commonly called hellgrammites, are perhaps better known than the adults due to their more readily findable nature. In the water, they absorb dissolved oxygen through abdominal lateral filaments and tracheal gills. They also have spiracles that allow them to take in air directly when above water.
Larvae of dobsonflies bear eight pairs of lateral processes as well as anal prolegs with a pair of terminal hooks used to hold themselves to a stream bottom or other surface. At the end of the abdomen is a pair of claw-like structures. Body color is black or dark brown.
The larvae of dobsonflies live along the rocky bottoms of streams, and are chiefly active during the night, when they ambush prey in the middle of riffles, shallow landforms in a flowing channel, which supply plenty of oxygen and stir up prey. They are generalist predators; dissections have revealed that they primarily eat aquatic immatures of mayflies, caddisflies, stoneflies, and chironomid midges. Although the larvae spend most of their lives under rocks below water, emergences, known as “hellgrammite crawlings,” occur during thunderstorms or when at night a bright light source is placed atop a boulder or large rock that is partially immersed in and partially out of the water.
Dobsonfly adults are nocturnal, and are seldom seen as they hide under leaves in the canopy during the daytime. However, they do sometimes form aggregations under bridges or other structures along streams. Since the adults live only about a week, they are not known to eat anything, although they have been reported to drink sweet solution in captivity.
Males compete with each other for females, aggressively fluttering their wings and trying to place their long mandibles underneath the body of the opponent in order to flip him into the air. Afterwards, the male approaches a female from the side and touches her with his antennae. At first the female reacts somewhat aggressively, moving the head from side to side with mandibles wide apart. However, she then allows the male to come closer and place his mandibles over her wings in a perpendicular position, a position he holds until the female signals reception to mating.
During copulation the male attaches to the female’s genitalia a large, globular spermatophore about 4 mm long and wide. The spermatophore consists of two parts: a large gelatinous mass, and a smaller seminal duct containing the sperm. After copulation, the female proceeds to spread her legs wide apart, curl the abdomen under the chest, and eat the gelatinous part of the spermatophore.
Oviposition occurs along rocky walls of streams at night, most often from May to September. The females deposit egg masses containing on average one thousand grey, cylindrical eggs, each egg about 1.5 mm long and 0.5 mm wide. This mass is covered by a layer of a chalky, white substance, which probably protects the eggs from desiccation and overheating. Females tend to deposit egg masses at relatively few sites, resulting in grouped egg masses.
One to two weeks after oviposition, the eggs hatch and the first instar larvae either fall directly into the stream or if not, immediately search for water. There the larvae live for up to five years, going through 10-12 instar molts. Large, older larvae, which may be up to 3.15 inches long, have strong biting mouthparts and are ferocious predators on other aquatic insects and small invertebrates. When they have finally reached maturity, larvae migrate from their freshwater habitat to wet soil, moss, or decaying vegetation, a rock, log, or other anthropogenic debris, typically close to the stream but sometimes up to 130 feet away. There they construct a chamber for pupation and spend several days as prepupae before shedding the exoskeleton and spending about a week as pupae. The pupae are yellow-orange with dark spots on the dorsum of the abdomen. Although the males have a small tubercle on the prothoracic sternum and a slightly wider head than the females, the mandibles are not as noticeably divergent as in the adults. Finally, the pupae emerge from the chamber, leaving behind the larval and pupal skins.
From Wikipedia and Encyclopedia Britannica