TRICHOPTERA

T.J. Renals

With the exception of a single, rare terrestrial species (not found in Cornwall), the 199 species of caddis flies in the British Isles possess an aquatic, larval stage. Eggs are laid in the water or, in the case of species that inhabit water-bodies that dry up in the summer, on damp ground that will flood later. Two species lay on overhanging vegetation. The larvae fall into two basic groups. The majority use silk to bind together vegetable debris or cut pieces of plant, or sand grains, or a mixture of all three, to make a cylindrical case. A few species make cases entirely of silk. The larva lives inside this case and carries it around. The other large group uses silk to make fixed shelters attached to stones or vegetation; within the shelter, the larva moves freely. Some spin silken snares from the shelter to catch other water creatures while others make nets or bags to sieve vegetable and animal material being washed down a stream or river. A group of four species (three of which live in Cornwall) live entirely free-roaming predacious lives and the bright green larvae only spin a shelter at pupation. The caddis flies are very much an under-recorded group in Cornwall. Wallace et al. (1990) and Edington & Hildrew (1995) provide keys that will enable identification of most larvae. Macan’s 1973 key to adults is out of print (a new edition is planned). Marshall’s 1979 key to the micro-caddis adults is still available. A few later papers to individual difficult groups are listed in Wallace (1991). At present, there is no key that includes wing markings (although they can be used in the field once familiarity with them is achieved). Although the cases of many species are distinct, they are too plastic in form to be reliable (although with familiarity many species can be named in the field from their cases). The national classification is according to Wallace (1991). Only four RDB species are listed for Cornwall. Only one of these is a recent confirmed record, the others are listed here for completeness. There are no confirmed notable species. The notable species Ylodes conspersus (Rambur) recorded for Truro is not supported by a specimen. It may have been that species, but could also have been the related Y. simulans (Tjeder), a RDB3 species recorded in Cornwall. Many species of caddis undoubtedly await recording from Cornwall, but it is unlikely to ever have a large list or many rarities because of the absence of large natural lakes and long lowland rivers. The legacy of mining activity and resultant pollution by heavy metals will also probably have taken it toll. Studies on the affects of climate change on freshwater macroinvertebrates reveal a trend complicated by the North Atlantic Oscillation and underlying differences in stream acidity (Durance & Ormerod, 2007). Caddisflies are amongst a broad range of aquatic invertebrates that are already giving indications of sensitivity to climate change, particularly in headwater communities. The continued spread of non-native species of crayfish, such as Signal Crayfish Pacifastacus leniusculus also threaten native freshwater invertebrate communities which can become seriously depleted by crayfish predation. The author would like to acknowledge the valuable help from Dr Ian Wallace of Liverpool Museum in preparing this account.

Ylodes simulans

Glossosoma intermedium

Leptocerus interuptus

Setodes argentipunctellus