Swimming is the most important escape mechanism in lestid damselflies. The effect of lamellae autotomy and sexual size dimorphism on startle-response performance was studied experimentally in larvae of the lestid damselfly Lestes sponsa. In contrast with the prediction of McNeill (1960) that lamellae loss would not affect swimming speed in fast swimmers such as Lestes larvae, swimming performance decreased in a quadratic way with the removal of subsequent lamellae. Lamellae autotomy therefore will considerably reduce the probability of escape from a predator. Larger larvae swim faster than smaller ones of the same instar. This may contribute to a higher survival of larger larvae compared with smaller ones when confronted with both conspecific and heterospecific predators. Despite larvae showing sexual size dimorphism, with females being larger, no difference in swimming speed between the sexes was found.