The trap comprises a lock which has a barrier grid (3) extending into the interior of the trap from the top and mounted in front of the rocker arm (7a). The locking mechanism (9) is located underneath the rocker arm and consists of two L-shaped spaced apart locking-pins (2) freely oscillating on an axle (10). The arm (2a) of the locking pins pointing towards the rocker-board (7) is shorter than the arm pointing towards the outlet. A release grid (1) is mounted under the longer arm and slopes to the bottom of the trap. The rocker is positioned eccentrically in relation to the trap entrance (8).
