Copulation in the damselfly Ischnura graellsii lasted 1-5 h, and took place in the afternoon and evening at the study site in north-west Spain. Copula duration was measured in the laboratory under controlled temperature, humidity, photoperiod and density. At high density (15-20 males/insectary), copulations that started early in the day were always long, while at low density (two males/insectary) they could be short or long. At both densities, copulations with previously mated females were longer than with unmated females. The duration of stage II of copulation, when the male transfers sperm to the female, was constant and independent of density and time of day. A study of egg production in females mated only once indicated that copula duration is not correlated with the proportion of fertile eggs laid, and that females start running out of sperm after about 15 days. These results indicate that prolonged copulations of I. graellsii have a guarding function, but the existence of more sperm displacement in long copulations cannot be rejected. Guarding takes place during stage I of copulation, before the males invest sperm in the female, which is unusual. The relation between copula duration and postcopulatory behaviour in Ischnura species so far studied is discussed. © 1990 The Association for the Study of Animal Behaviour.
