Sex ratio and adult emergence of the stonefly Perla bipunctata (Plecoptera: Perlidae) in the upper River Liffey, with notes from four additional rivers

摘要

The stonefly Perla bipunctata is a common component of fast-flowing, clean, stony rivers in Ireland. Living as a nymph for up to three years and reaching up to 33 mm in length little is known about their sex ratios and adult emergence behaviour. This study investigated variations in sex ratios of nymphs and adults, and adult emergence patterns in the upper River Liffey, Co. Wicklow. Nymph sex ratios were also assessed in four additional rivers in Co. Tipperary and Co. Laois. Results highlight contradictions between nymph and adult sex ratios, with a female bias recorded in nymphs and male bias recorded in adults. Male adults were observed to exhibit protandrous emergence which may explain the imbalance in adult sex ratios recorded but other possibilities are discussed. Overall this study sheds new light on P. bipunctata sex ratios and adds quantitative evidence to support observations previously made indicating early male emergence behaviour in Irish Perlidae.