Trichopterwhat? What is a caddisfly?

February 14, 2013

Caddisflies (Order Trichoptera) are not flies in the true sense of the word, but are rather a group of moth-like insects. The Latin name for this group comes from the Greek ‘Thrix’ ,meaning ‘hair’, and ‘Pteron’, meaning ‘wing’, while the common name (caddisfly) is thought to be derived from travelling sellers of yarn and ribbon from times gone by, known as cadice-men, who used to pin samples of their wares to the outside of their coats.

As mentioned above, adult caddisflies have the appearance of a moth-like insect and are generally dull brown in colour. They have two pairs of wings; with the forewings longer than the hind wings. The wings fold over the back of the insect when at rest, in an arrangement akin to an A-frame tent. The antennae are long and the mouth-parts are generally reduced. Click here to see some excellent photos of adult caddisflies.

The larvae of caddisflies are generally aquatic (though a wholly-terrestrial species, Enoicyla pusilla, does occur in Britain) and will be familiar to anyone who has carried out freshwater invertebrate sampling. The larvae of many species construct cases from various materials and may incorporate plant matter, sand and even snail shells, while others are free-living or construct webs, which they use to capture food. Caddisfly larvae are largely soft-bodied, with varying degrees of sclerotization resulting in harder parts and may have gills protruding from the abdominal segments.

The larvae of caddisflies are found in a wide range of aquatic habitats including streams, rivers, lakes, ponds, canals, ditches, seepages and even temporary waterbodies. Many factors dictate the species that may be present in a given waterbody, such as substratum type, vegetation, water flow, oxygen levels, salinity, etc. Given the aquatic nature of almost all caddisfly larvae, and the need for eggs to be laid in and around water, it should come as no surprise that adult caddisflies are most likely to be found in the vicinity of water. Adults will often be found in reed beds and other riparian vegetation and may be trapped in light-traps.

Caddisfly larvae exploit a range of food sources. Some species graze on algae or scrape biofilm from rocks and higher plants, others feed on plants and related detritus, some filter-feed, while others still are predatory. The feeding habit of various species can change as they moves through the larval stages (instars), while opportunistic feeding can also occur, with species normally considered herbivorous feeding on carrion when the opportunity arises. Adult caddisflies are not thought to feed; hence their reduced mouth-parts.

Caddisflies are important prey items for many species. Larvae are consumed by many fish species including salmon, trout and eel to name but a few, while they also feature in the diet of Dippers (Cinclus cinclus). As adults emerge from their pupae, they are preyed upon by fish as they swim to the surface, and once in the air, become the target of Daubenton’s and other bats.

The presence of caddisfly larvae in rivers can give information on the water quality of the river. In Ireland, biological water quality assessment is based on Q-values, with Q1 being seriously polluted and Q5 being unpolluted. Cased caddisfly larvae are considered to be indicators of good water quality, being assigned to Group B – Less sensitive to organic pollution (as compared to Group A, which are sensitive organic pollution), while uncased caddisfly are considered Group C – Tolerant of organic pollution.

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