Life history patterns of stream-dwelling caddis

Abstract

Full life histories of fourteen coexisting species of caddis in an Irish stream are described. The study is based on growth patterns and distribution of larval head-capsule width through time. It also notes adult flight periods. Most species had simple life histories and were univoltine (Agapetus fuscipes, Hydropsyche siltalai, H. instabilis, Philopotamus montanus, Ecclisopteryx guttulata, Glossosoma conformis, Drusus annulatus, Potamophylax cingulatus and Halesus radiatus). In the latter four species the occurrence of instar-1 larvae indicated that egg hatching was delayed or prolonged over several months. Complex univoltine life histories were found in three species (Sericostoma personatum, Plectrocnemia conspersa and Silo pallipes) where, generally, a one-year life cycle occurred, although some individuals required more than one year to emerge. Data for two species (Rhyacophila dorsalis, Odontocerum albicorne) suggested the possibility of bivoltine life histories. Both types of complex life history led to overlapping generations. This is the most comprehensive information to date on the life histories of most of the species. Intraspecific variation in development rates, the wide range of size classes present at one time, the ability to overwinter in different larval stages and the asynchronous extended flight periods may be evidence of a spreading of risk within life histories. Without knowledge of whether the variations in development rates are genetically determined, it is difficult to advance a hypothesis based on an adaptive strategy to explain such variation as opposed to attribution to environmentally-controlled variation in growth, influenced by such factors as temperature and food.