Caddisfly

The architect larvae

Caddisfly larvae are among the most remarkable animal builders. Larvae spin silk from salivary glands and use it as cement to construct portable cases from their immediate environment. Different species use different materials: some species use small pebbles sorted by size, arranged with the largest at the front; others use sand grains, leaf fragments, twigs, pine needles, or plant stems, arranged in precise species-specific patterns. The cases are constructed new as the larva grows, and abandoned only for pupation.

Water quality indicators

Caddisfly larvae are bioindicators — their presence, abundance, and species diversity reliably indicate clean, well-oxygenated water. They are sensitive to pollution, organic enrichment, and oxygen depletion. River ecologists include caddisfly larvae alongside mayfly (Ephemeroptera) and stonefly (Plecoptera) in the standard EPT (Ephemeroptera-Plecoptera-Trichoptera) index used to assess river health. High EPT diversity indicates excellent water quality.

Fly fishing tradition

The adult caddisfly’s silhouette — wings held tent-like over the body, long antennae, small triangular outline on the water surface — is intimately familiar to fly-fishers. Caddis larvae, pupae, and adults are preyed on heavily by trout. Artificial fly patterns imitating caddis (dry “sedge” patterns, soft hackle wet flies) are fundamental to British chalk stream fishing. The word “cadis” itself is old English for silk ribbon — a reference to the silk-spinning larvae.

The artist’s caddisfly

French artist Hubert Duprat has collaborated with caddisfly larvae in a famous art project since the 1980s: he removes larvae from streams and places them in tanks with tiny gold pellets, rubies, turquoise, and pearls instead of natural materials. The larvae build cases from the precious materials, producing tiny jewelled sculptures.