Hundreds of years ago in England, itinerant sellers of cloth fastened scrap pieces of cotton and silk to their clothing to advertise their wares. In older English these scraps are called cadices, so these salesmen were dubbed “caddice men”. The little caddisfly larvae that labour away in streams and ponds, covering themselves with bits of sand and plants, probably got their name from these once-familiar figures.
Caddisflies belong to the insect order Trichoptera, which means “hairy wings”. Although small, moth-like, and mostly obscure as adults (Figure 1), when immature, caddisflies are among the most bizarre and wonderful inhabitants of fresh water habitats – from tumbling creeks to soupy, plant-choked ponds. Sticky silk extruded from modified salivary glands helps these larvae (Figure 2) build an amazing array of structures: cases and retreats that offer protection, camouflage, stability in currents, and efficiency in respiration and feeding. The behaviour involved in making their constructions is similar to that of the cocoon building performed by many Lepidoptera (moths and butterflies) larvae; the two orders are closely related. Only a few examples of caddisfly architecture are mentioned here.
Many species build to catch food. Using plant debris and silk, larvae in the genus Hydropsyche gain both food and shelter by constructing a net-walled chamber in front of a tube-like retreat. Polycentropus makes a silken tube set in a maze of threads spread over the stream bottom. Small animals crawling across the threads are seized and devoured. Neureclipsis conceals itself at the narrow end of a trumpet-shaped tube. Guy lines of silk support the opening, which filters food from the current.
Micro-caddisflies are also called purse-case makers because they mould bivalve (two-sided) silk pouches. Hydroptila’s clamshell-like cases are covered with a single layer of sand grains. Ozyethira has a flattened, bottle-shaped case made entirely of silk (Figure 3).
If you are a tiny aquatic creature, it’s not easy to live in the rushing waters of mountain streams. Many caddisflies use their cases to help them stay put. Most stream-dwelling larvae build their cases from small pebbles or sand grains. Obviously, this is readily available material, but also its weight acts as ballast and prevents the insects from being swept away. In some groups, the faster the current, the larger are the stones incorporated in the case. In cold, rapid streams Glossoma sticks together small stones into a turtle-like shell, which protects the larva on the exposed rocks where it likes to graze on diatoms. When the larva pupates, it glues the case to a rock and spins a cocoon inside. Many stream-dwelling species build long and tapering cases; these act as vanes to keep the insect’s front end turned into the current, reducing resistance to the water and increasing stability, much as do the long, stabilizing tails of many larval mayflies and stoneflies. Many caddisfly groups construct smooth, curved cones of sand. Some are made by Ecclisocosmoecus (Figure 4), which conceals itself in sand and gravel on the bottom of mountain streams. Heteroplectron gets by with a hollowed-out twig lined with silk. Brachycentrus makes distinctive, four-sided cases from thin pieces of plant material (Figure 5) and often glues them, facing into the current, to rocks or plants.
Camouflage, or disguise, is an important function of many larval cases. Helicopsyche makes one of sand that is a dead ringer for a snail shell. There is a single species of this group known in British Columbia; it has been found in the Cariboo region. Ceraclea cases are also made of sand, but resemble slippers (Figure 6); the overhanging lip covers the head of the larva. Most Ceraclea larvae feed on detritus, but some eat fresh-water sponges. The beautifully camouflaged sand tubes of Mystacides are decorated with conifer needles (Figure 7). Most caddisfly larvae crawl on the bottom of streams and ponds, but Trianodes species swim high among the rooted plants on which they feed. They make their cases from pieces of these plants, arranging them in intricate spirals (Figure 8). Along the quiet edges of ponds and marshes, Lenarchus glues sedge leaves lengthwise into a tube (Figure 9) and the many species of the diverse genus Limnephilus often patch together a bristly brush of wood and leaf bits (Figure 10). Large flat pieces of leaf moving along the bottom of seepage pools belong to Clostoeca, and in waters with abundant snails, Philarctus adds these tiny shells to its tubes of sand and sedge seeds.
Tube-like cases offer not only shelter, camouflage, and stability, they also make it easier for the larva to extract oxygen from the water. Flowing water carries more oxygen than still water and, inside its case, a larva can produce its own current. Undulating its abdomen, the larva draws water in through the front opening of the tube and out the back one, bathing the abdomen and gills in constantly renewed water. Inside a case, a caddisfly larva can better survive periods of unusually high temperatures or other conditions where oxygen levels drop below normal. Thus, case-making has also allowed some species of caddisflies to exploit the still waters of ponds and other habitats far from the oxygen-rich, running waters in which their ancestors lived – and in which many modern species still dwell.
The caddisfly section of the Royal BC Museum Entomology collection is small. No entomologist working at the Museum has ever been an expert on this insect order, and the collection has grown only as a byproduct of other aquatic insect research. Caddisflies, both adult and larvae, are usually stored in glass vials containing preservative fluid, such as 70% ethyl alcohol. There are about 500 vials of caddisflies accessioned in the collection. Some vials contain only one specimen, others hold many specimens of the same species collected at the same place on the same date. Most of the specimens are adults, but some of the collections represent the larvae (and their cases) discussed in this article.
There is plenty of scope for useful scientific collecting and study of British Columbia’s caddisfly species, and the RBCM should increase the size and representation (both of species and geographical areas) of its collection in the future.
A checklist of the 275 known caddisfly species in British Columbia has been produced by the RBCM. It is available here.
This information was gleaned from many collections, including that of the RBCM. One of the most comprehensive collections of caddisflies anywhere is in the Royal Ontario Museum, Toronto, where staff researched the fauna of Canada and the world in detail for many years.