This page explores some of the lesser-known species of insects which can be found if you examine the stones in the rivers of Dartmoor and South Devon.
Mention caddisand most anglers assume you are talking about one of the numerous species of relatively large insects that form a protective case around their bodies from small stones, sand or pieces of vegetation.
But we also have a much smaller species of caddis - Glossosoma
In addition our rivers contain species of caddis that do not construct a protective case around their bodies (Rhyacophila and Hydropsyche),and - as any Summer visitor will confirm - clouds of biting midges (Simulium)which are eagerly consumed by trout as they struggle to break through the water surface.
It is easy to underestimate the difficulty insects experience breaking through the barrier between water and air, and consequently how exposed they are to being picked off by hungry trout. Gary LaFontaine points out that - in human terms - it is equivalent to breaking through three feet of soil above our heads!
(popular name: saddle case caddis, igloo caddis, turtle case caddis, little
black short-horned caddis)
Often writers of angling books overlook small flies such as midges and small caddis in favour of the larger and more glamorous flies. As Gary LaFontaine wrote, "A sad fact of modern fly-fishing is that so much of the lore is geared to one insect, mayflies.."
The rivers running off Dartmoor are home to large numbers of the inconspicuous Glossosoma caddis which - as this picture shows - are easy to overlook especially when seen alongside a larger mayfly nymph.
Glossosoma are small (3-7 mm, hook size 18-22) caddis often present in large numbers on the algae covered upper (illuminated) surface of stones in fast flowing sections of local rivers. They are thought to be the oldest (i.e. most primitive in evolutionary terms) type of case-making caddis. The more familiar caddis which make tubular cases from sand, stones or vegetaion are thought to be a later evolutionary development.
Glossosoma larvae have to abandon their cases - rather than add extra material - when they outgrow them. At this point they may drift, or be swept, downstream which would make them susceptible to predation by trout.
Adult Glossosoma females swim underwater to deposit their eggs. This may explain the effectiveness of 'wings' on some wet flies.