The mechanism of sex recognition in adult males of C. a. amurensis was studied in an experiment in which the movement patterns of intact and modified test individuals were manipulated. Patrolling males usually behave agonistically (i.e. attack and retreat) against an individual which rushes at them, without perceiving sexual differences in external morphology. On the other hand, such males respond sexually to (i.e. try to copulate with) an individual which makes swinging (egg-laying) or escape movements and which has an abdomen thick at the base. Sexual differences in external morphology are perceived after an approach has been made.