Tandems aggregate during oviposition. Discrimination experiments show that a motionless S, fixed to a floating leaf of Potamogeton natans in the typical vertical position of a tandem c?, attracts conspeciftc pairs to land and oviposit at the same site. Successful oviposition occurs only within distinct plant parts, in the case of P. natans almost exclusively into leaf base and petiole, where tandems most frequently land. In the choice of oviposition site, sparkling reflections from the uneven water surface, caused by the edges of water plants, seem to play an important role. On P. natans petioles actual oviposition lasts on average 9.9 min, with a mean egg deposition rate of 5.56 eggs/min. During short stays of less than one minute no actual oviposition normally takes place.