Phylum Arthropoda
Subphylum Hexapoda
Class Insecta
Order Trichoptera
Common name: caddisflies
Overview
Trichoptera, or caddisflies, are one of the largest orders of aquatic insects. They are related to Lepidoptera, the adults resembling small fragile moths, but have the wings covered in dense hairs instead of scales, and lack the typical curled proboscis (mouthparts) of moths. In addition to the wings, the elongate bodies and slender legs are also covered in fine hairs. Adults have large eyes; reduced mouthparts; long, segmented antennae; and two pairs of membranous wings that are different in size. Most are dull coloured, and body length ranges from 2�40 mm. The aquatic larvae are caterpillar-like and can spin silk from specially modified salivary glands; typically larvae construct a case or retreat in which they live. They have well-developed chewing (mandibulate) mouthparts and, on the thoracic legs and the pair of prolegs on the last abdominal segment, they have a strong claw that is used to hold the larva in place inside their case or to the substrate.
Distribution and diversity
Caddisflies are found in freshwater lakes, rivers and streams around the world, often occurring in large numbers. One species in Australia is unusual in having larvae that are truly marine and live among algae in intertidal rockpools. Other species are found in coastal lakes, streams and one terrestrial species that also produces a case. Almost 12,000 caddisfly species have been described worldwide with estimates of as many as 50,000 species. The Australian fauna comprises 680 described species in 104 genera and 26 families; this probably represents about two-thirds of the total number of species.
Life cycle
Caddisflies mate during flight, on the ground or on riparian vegetation. Sperm transfer is direct. Females lay up to several hundred eggs on objects such as rocks, logs or plants protruding from the water, or scatter them directly on or in the water. Eggs are laid separately or enclosed in a gelatinous mass. The larvae are soft bodied, most sheathing the body in a casing or retreat made from silk that may be covered in various materials such as sand grains, small pebbles, or bits of leaves and twigs, to provide protection and camouflage. The shape and material used to construct the case is often specific to particular species taxonomic group. Caddisfly larvae usually develop through 5�7 instars; as case building forms grow, more material is added to the front of their case. Pupation takes place underwater within the larval case or in a pupal case made from silk. Most of the life cycle of caddisflies is spent in the larval stages; the adults are generally short lived.
Feeding
Adult caddisflies scarcely feed, although some take water or nectar. The larvae have a wide variety of feeding methods and diets, and may be either herbivores, scavengers or predators. Many feed on both living and dead plant material, some on algae and others pierce algal cells or aquatic plants, sucking out the cell contents. Other species filter organic particles from the surrounding water or scrape organic material from submerged rocks and plants. The predatory species feed mainly on other aquatic insects; some larvae spin silken nets in which they capture prey.
Ecology
Adult caddisflies are nocturnal and are generally found near water bodies where they can sometimes be found resting on tree trunks by streams and lakes during the day; some species are active diurnally. Larvae that are free-living and predaceous lay down a strand of silk as they move, similar to that of lepidopteran caterpillars. As they often occur in large numbers and in almost all types of freshwater habitats, caddisflies are important components of food webs in freshwater ecosystems. Water conditions are important to different caddisfly species and hydrological parameters such as oxygen levels, temperature, pH, salinity and presence of particulate matter, including man-made pollution, often correlate closely with the presence or absence of certain species. As such, the larvae are useful as biological indicators for assessing water quality. As has happened with many aquatic insects in Australia, the ranges of many species have contracted, due to habitat loss, and some species have even disappeared.
Adult trichopteran. South Perth, Western Australia.
Image credit: Photographer: E.S. Volschenk.
� Western Australian Museum
Trichopteran larva partially extended from its tube. Southeastern Queensland, Australia.
Image credit: Photographer: E.S. Volschenk.
� Western Australian Museum
