Caddis flies Order: Trichoptera

Caddis flies are aquatic insects found in fresh water. They are probably best known by fly fishing enthusiasts who tie lures to resemble both the flying adult and aquatic larval life stages. Ecologically, they are an important dietary component of fishes in these streams, while scientifically, their presence or absence in water bodies is used as an indicator of water quality.

Adult Caddis fly
Photographer: Arturs Neboiss / Source: Museum Victoria

Caddis flies have aquatic larvae (an immature stage in the life cycle), which are often enclosed within a case made from mineral, e.g. sand, or organic matter, e.g. leaves or twigs. Some larvae do not construct cases but are usually found within a silken retreat and others are free-living.

Most bodies of fresh water have caddis flies, but the largest number of species is found in rivers and streams. The larval life cycle varies from a few months to a year but these details are not well known for Victoria. Adults caddis flies resemble small dark winged moths. They live for much shorter periods than the larvae and are responsible for reproduction and dispersal.

Larval Caddis fly
Photographer: Dr Ken Walker / Source: Museum Victoria

One common form of caddis fly has quite large larvae (up to 25 mm long) with feathery gills attached to the abdomen. They live in retreats, part of which is modified as a net. The larvae are only found in running water, usually in areas with strong currents. These larvae rely on their net to trap small algae and invertebrates which they then eat. Other common Caddis larvae are quite small (less than 8 mm long) and live in dome-shaped stone cases commonly on the surface of rocks in the main channel of streams and rivers. These larvae eat algae, which they scrape from the surface of rocks.

A very common form of Caddis larva in Victoria lives in cases made of sand grains, hollowed twigs or plant material. These occur in streams, rivers, temporary streams, ponds, lakes, saline lakes and estuaries. The larvae are often found amongst detritus (rotting organic matter) or on aquatic plants, and usually in still or slow-flowing water. Most species eat detritus and algae, but a few are predators on other invertebrates.

Larval Caddis flies cemented to a rock
Photographer: Arturs Neboiss / Source: Museum Victoria