Mechanism for the deceleration of a vertically retracting, upwardly moving insect-blocking roller screen (23) winding onto drum (24), the mechanism comprising a cylinder (1) containing a fluid mass (11), a shaft (2) equipped with blades rotatably mounted within cylinder (1) and means for the engagement of drum (24) with the shaft (2) slowly rotatable within fluid mass (11) during the retracting upward movement of the insect-blocking roller screen. No bolt whatsoever is employed neither in the linear assembly of parts of the mechanism nor in the mounting of the mechanism onto the side plug means (12). The linear assembly is effected by a circumferential end annular rib (22a) provided at the edge of shaft portion (22) protruding through the open outlet of cylinder (1) clipping onto a circumferential groove (9) of an end stabilizing washer means (8), whilst mounting of the mechanism onto the side plug means (12) is effected by at least one longitudinal slot (19a) along a shaft extension (19) through the closed end of cylinder (1) clipping with at least one rib (14a) of a tube projection (14) of the side plug means (12).