Tandems of C. puella accumulate during opposition. The presence of an ovipositing tandem attracts other pairs to the same site. Discrimination experiments show that a single motionless male in the typical vertical position of a tandem male stimulates aggregation and oviposition behaviour of other pairs. — In Potamogeton natans eggs are nearly always inserted into the basis of the leaf and the petiole, with a mean egg deposition rate of 7.07 eggs/min. During short stays no eggs are laid.