Fish flies are common insects found near freshwater environments. Despite their name, they are not true flies, which belong to a different insect order. Their presence can also indicate the health of the aquatic ecosystems they inhabit.
What Exactly Are Fish Flies?
Fish flies are part of the insect order Megaloptera, which includes both dobsonflies and alderflies. These insects are among the larger non-Lepidopteran insects found in temperate regions, with some dobsonfly species having wingspans up to 18 cm (7.1 inches). Adults typically have soft, brownish-gray bodies and large, veined wings that are held roof-like over their backs when at rest.
Their larval stage, commonly known as hellgrammites, inhabits aquatic environments. Hellgrammites are robust, segmented, and can reach lengths of up to 7.5 cm (3 inches), featuring strong mandibles and numerous gill tufts along their abdomen for breathing underwater. They are often mistaken for centipedes due to their elongated bodies and multiple leg-like appendages. Unlike mayflies or caddisflies, which have different wing structures and life cycle details, fish flies and dobsonflies are distinguished by their large, often mottled wings and the prominent mandibles, especially in male dobsonflies. Male dobsonflies possess exceptionally long, sickle-shaped mandibles, reaching up to 40 mm (1.5 inches).
Life Cycle and Habitat
Fish flies undergo complete metamorphosis, progressing through four distinct stages: egg, larva, pupa, and adult. The life cycle begins when female fish flies lay numerous egg masses on surfaces overhanging water, such as rocks, leaves, or bridge abutments. These egg masses are covered with a protective fluid. Eggs hatch within one to two weeks, with the newly emerged larvae dropping directly into the water.
The larval stage, the hellgrammite, is aquatic and can last for several years. Hellgrammites thrive in clean, well-oxygenated freshwater environments like swift-flowing streams and rivers, where they cling to rocks and debris on the bottom. As they grow, hellgrammites molt their exoskeletons multiple times. When ready to pupate, the larvae crawl out of the water to burrow into moist soil, under rocks, or in decaying logs.
The pupal stage is relatively short, lasting from seven to 14 days. During this phase, the insect transforms into its adult form. Adult fish flies emerge with a lifespan of a short period, primarily focused on reproduction. Adults are generally nocturnal and are found in thick vegetation near water bodies, though they are weak fliers and often attracted to lights.
Ecological Role and Interaction with Humans
Their larvae, hellgrammites, are aquatic predators, feeding on a variety of smaller aquatic insects and invertebrates. This predatory behavior helps regulate populations of other aquatic organisms. In turn, hellgrammites serve as a food source for various aquatic and terrestrial predators, particularly fish such as smallmouth bass, as well as birds and bats.
Beyond their role in the food web, hellgrammites are recognized as bioindicators of water quality. Their presence typically signifies clean, well-oxygenated, and relatively unpolluted freshwater environments, making them valuable in assessing ecological health. They are sensitive to pollution and habitat disturbances, so their absence can indicate environmental degradation.
Interaction with humans mostly involves the adult stage, which can be quite large and intimidating in appearance. Despite their size and the prominent mandibles of the males, adult fish flies are generally harmless and do not bite or sting. The exceptionally long mandibles of male dobsonflies are primarily for mating and display, not defense, and are too unwieldy to inflict a painful bite on humans. Female dobsonflies and hellgrammites do have shorter, sharper mandibles capable of delivering a pinch or bite if handled, which can be painful, but typically without lasting effects. Adults are strongly attracted to artificial lights at night, which can lead to large numbers congregating around homes or businesses, occasionally causing a nuisance due to their sheer numbers or by creating slick surfaces when crushed. This attraction to light is likely a disorientation of their natural navigation by celestial light sources.