Dorset Wildlife

A Dip in the Stour

As the evenings become longer and touched with golden light, and the air becomes warmer and dares us to leave our jackets at home, some of us migrate towards the riverside. The irresistible draw of the tinkling flow over rocks or the reassuring glossy slide of deeper waters calms the mind and soul.  Those who stop to peer beneath the water’s mirror sheen will be rewarded with a glimpse of a busy and bustling world, with inhabitants going about their multitudinous tasks, oblivious to the land of giants above. One of these inhabitants, the caddisfly larvae, has a fascinating story.   

Caddisfly adult, credit: Magnus Hagdorn, CC BY-SA 2.0 via Wikimedia Commons

My first introduction to caddisfly larvae was when my son showed me one in his hand. “They’re everywhere”, he told me. “There’s a creature inside. I think the creature makes its own case to hide in.” As usual, when it comes to matters of nature, he was right. The caddisfly, or sedge fly, is a large order of insects which can be found in all sorts of wetland. They are known for building cases around their bodies to shelter and then pupate inside. The caddisfly larvae gather sand, small stones and pieces of wood which they spin into a tube-like case with silk secreted from glands in their mouths. In this way, the caddisfly larvae create a portable shelter for themselves that perfectly matches the riverbed. The larvae can emerge their head and legs from the case and scuttle around, ready to shrink back inside at the first sign of danger, looking exactly like a piece of gravel.

Caddisfly Larva, credit: marsupium photography, CC BY-SA 2.0 via Wikimedia Commons

There are almost 200 species of caddisfly in the UK. The adults are moth-like but with fine hairs on their wings instead of scales, and wings that fold back along their bodies. Swarms of adult caddisfly can be seen flying above the surface of water in late spring: a delectable buffet for lurking fish. The adult caddisfly will live for around a month, during which time the females lay eggs on vegetation just by the water’s surface. On hatching, the larvae fall into the water and begin immediately building a case to live in. When they are ready to pupate, the caddisfly larva will seal its case with a stone or piece of wood. It will then spin a cocoon of silk around its body as it undergoes metamorphosis into its adult state, and the cycle begins once again.

River dipping, credit: Sue Crookes

Next time you’re picnicking, paddling or messing about in boats, take a moment to look at the riverbed. There might be more going on than you’d think.

Find out what you can do to help insects on the Dorset Wildlife Trust website: dorsetwildlifetrust.org.uk/action-insects.

Melanie Fermor, Dorset Wildlife Trust volunteer

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