Twenty generations of Pyrrhosoma nymphula (Sulzer) and Enallagma cyathigerum (Charpentier) (Zygoptera: Coenagrionidae)

Quantitative studies have been carried out on the larvae of P. nymphula and E. cyathigerum in a small artificial moorland pond. A double-cylinder toothed sampler appeared to give good quantitative results but it was not a rapid method of collecting and it removed part of the environment. A net caught a higher proportion of larger than of smaller larvae. Artificial vegetation harboured more specimens per unit area than real vegetation. A patch of Potamogeton natans is the main egg-laying site of Pyrrhosoma. Colonization of mats of artificial Littorella indicate passive dispersal soon after hatching. There is little wandering later in life. Development can be completed in one year, but generally requires two. It takes three years when larvae are numerous. Numbers, particularly at the end of a generation, did not vary much from 1955 to 1965, although predation by trout was heavy in the second half of the period and absent during the first. In 1966 and thereafter larvae have been scarcer, probably because there has been much less rooted vegetation.