Invertebrates
Caddisfly
Trichoptera · also called caddisflies, sedge flies
A caddisfly larva builds its own house out of whatever is lying around a streambed, gluing sand, gravel, twigs, or tiny shells into a protective tube with silk it spins from a gland near its mouth. Give one gold flakes and crushed pearls instead of sand, and it will build with those too, which is exactly what a few jewellery artists have done.
Overview
Caddisflies are the order Trichoptera, over 14,000 species and the closest living relatives of moths and butterflies, found in fresh water nearly everywhere. Most larvae build a portable case from silk and local debris, dragging it along and retreating inside when threatened; others spin fixed silk nets in flowing water to filter food from the current.
Cases, nets, and jewellery
Case designs vary by species and even by population, built from sand, leaf fragments, twigs, or snail shells. The trick has been put to unusual use: reared with gold flakes or gem fragments instead of sand, caddisfly larvae will incorporate them into cases later collected and set as jewellery.
A sentinel of clean water
Larvae are highly sensitive to pollution and low oxygen, so caddisfly diversity is used alongside mayflies and stoneflies as a standard indicator of stream health — their decline is often an early sign of wider water-quality problems.
Status
Least Concern overall and often abundant in clean water, but declining in many industrialised regions from pollution, damming, and warming rivers. Some cold-water and cave-stream specialists are more narrowly threatened.
Caddisfly: common questions
Why do caddisfly larvae build cases?
The case serves mainly as portable armour and camouflage, protecting the larva's soft, vulnerable body from fish, other predatory invertebrates, and physical damage while it moves around and feeds, and because it is built from whatever sand, gravel, leaf fragments, or shell pieces are locally available, it also blends in closely with the stream or pond bed, making the larva harder for a predator to spot even outside the case's physical protection. Some case designs are also shaped to improve water flow across the larva's gills, helping with respiration as well as defence.
What is the connection between caddisflies and gold jewellery?
Certain artists and jewellery makers rear caddisfly larvae in tanks provided with gold flakes, crushed pearls, opal fragments, turquoise, or other valuable materials instead of ordinary sand and gravel, and the larvae, following their instinct to build a case from whatever fine material surrounds them, incorporate these materials into their cases just as they would use sand or gravel in the wild. Once the larva has finished using the case and emerged as an adult, the case is carefully collected, sometimes reinforced, and set into pendants, earrings, or other jewellery, an unusual practice that has been marketed as a form of collaborative or bio-art.
Are caddisflies closely related to moths?
Yes, caddisflies are considered the closest living relatives of the moths and butterflies, sharing a common ancestor with them and showing clear similarities as winged adults, including a broadly similar body plan and wings held tent-like over the back at rest. The clearest structural difference is that a caddisfly's wings are covered in fine hairs, reflected in the order name Trichoptera, meaning hairy wings, while a moth or butterfly's wings are covered in tiny overlapping scales, reflected in their order name Lepidoptera, meaning scaled wings.
Why are caddisflies used to check water quality?
Caddisfly larvae are particularly sensitive to pollution, low dissolved oxygen, and excess sediment, and different species have different tolerances, so the presence, abundance, and diversity of caddisflies in a stream or river gives ecologists a fairly direct and cost-effective read on water quality without relying solely on chemical testing. A stream with many different, pollution-sensitive caddisfly species present is generally a strong sign of clean, healthy water, while a decline in caddisfly diversity, or the disappearance of the most sensitive species, often signals pollution or habitat degradation, which is why caddisflies are surveyed alongside mayflies and stoneflies as standard freshwater health indicators.