The reproductive morphology and behaviour of 6 and 9 at the northern edge of the sp. range, in Britain are described. Copula duration was relatively short and occurred in tussocks around the stream and oviposition usually occurred in tandem. Although there was no significant relationship between body size and clutch size, large clutches were only found in the largest individuals and larger 2 9 produced larger, and thus better-provisioned eggs. Examination of the dimensions of the aedeagus and the 9 sperm storage organs revealed that <J 6 could remove rival sperm from both the bursa copulatrix and spermatheca consistent with the presence of extensive proximally oriented microspination with sperm masses on the surface of the aedeagi. 9 9 store sperm from previous matings and a few partition clutches between consecutive oviposition episodes. The fulfilment of these ecological and behavioural pre-requisites and the predominance of mate guarding during oviposition suggests that sperm competition by sperm displacement is prevalent in this sp. and constitutes a substantial selection pressure.