At the rendezvous, d 6 prefer stones as perches. Discrimination experiments with pairs of substrates showed that they land preferentially on perches that correspond in height to the flight level of $ 9 appearing at the water. When they first landed, d 6 preferred perches in the middle of the stream, but afterwards they also used those near the stream margin. The results are interpreted in terms of early recognition of 9 9 and rapid formation of tandem linkage.