Key to Australian Freshwater and Terrestrial Invertebrates

Phylum Arthropoda
Subphylum Hexapoda
Class Insecta
Order Megaloptera

Common name: alderflies, dobsonflies

Overview

Megaloptera, commonly known as alderflies or dobsonflies, form a small order of relatively large, but soft and fragile, insects with aquatic larvae. Adult alderflies are weak fliers; they have two pairs of membranous wings that are similar in length, but the hindwing usually wider due to a large, folded, fanlike area that at rest is hidden by the forewing. They have large, bulging eyes; long, segmented antennae; and chewing (mandibulate) mouthparts; some males have extremely long mandibles. At the end of the abdomen, males may have claspers, which they use to hold the female during mating. Adult wingspan ranges from 20�100 mm. The aquatic larvae are elongate with well-developed mandibulate mouthparts and gills along the side of their abdomen. Adults are short-lived (around a week).

Distribution and diversity

Alderflies are found in running and still aquatic habitats in tropical and, more commonly, temperate areas worldwide. Megalopteran species are more common along the eastern side of Australia from Cape York to Tasmania, particularly in the upper regions of cool streams along the Great Dividing Range; only one species occurs in the west, in south-western Western Australia. Worldwide around 300 species have been described. Twenty-six species, in five genera from two families are known from Australia.

Life cycle

Courtship behaviour occurs between males and females and may involve vibration of the abdomen, wing fluttering and touching of the antennae. Males may also use pheromones to attract females. Sperm transfer is direct; mating takes place on branches or shrubs near the aquatic habitat. A female may lay up to 3,000 eggs in masses on vegetation, rocks overhanging water or wood and rocks projecting from the water. Larvae hatch in 1�4 weeks and fall or crawl into the water shortly after hatching. Mature larvae crawl out of the water to pupate in the soil or litter adjacent to the larval habitat. The complete life cycle may take only one year in warmer areas or up to five years in colder climates. The short-lived adults emerge from late spring to autumn.

Feeding

Adults are short-lived and generally do not feed, although they may drink water or nectar. However, the larvae are active predators of aquatic invertebrates, including other alderfly larvae, and have strong mandibles with which they grasp their prey. They have also been recorded as scavengers on dead invertebrates.

Ecology

Adult alderflies are found on plants close by the freshwater streams that are the habitat of their aquatic larvae. Typically, Australian species are active at dusk and at night, and are attracted to light. They are generally found in low numbers, however females may group together to lay eggs. The larvae can be found on the bottom of clear, cool, freshwater creeks, seepages, rivers, swamps, pools and lakes, and may survive even in temporarily dry creek beds. As with many aquatic insects, human activities, particularly clearing of forests and pollution of freshwater habitats, have strongly affected the distribution and abundance of alderflies in Australia.