Larval development and emergence in Pyrrhosoma nymphula (Sulzer) (Zygoptera: Coenagrionidae)

Larval development and emergence were studied over 3 years in a population in Yorkshire, England. Artificial substrates were used as emergence sites in the field to obtain quantitative data on various aspects of emergence. Development was thought to be completely semivoltine at this location, although the possibility of univoltine development is discussed. The proportion of the senior cohort overwintering in the penultimate (F-l) instar varied from zero to approximately 35% in the three cohorts studied. Emergence was synchronised but was interrupted by adverse weather conditions. In the year when F-1 larvae constituted a large proportion of the overwintering senior cohort, their emergence was signified by a small second peak in numbers. The larval sex ratio and the sex ratio at emergence did not differ significantly from unity. Mortality at emergence ranged from 3% to 5% and was mainly due to incomplete eedysis, failure to expand the wings and predation by spiders. The reasons for the low emergence mortality in this population are discussed.