Abstract: This contrivance includes a support (1), in which a tubular element (5) is mounted to slide, the tubular element containing a rod (6) mounted to slide in it and subjected to the action of a restoring spring (7) fixed to the support (1). At its lower end, the element (5) has two shells (10, 11) forming a cage (9) in cooperation with a belt (13) that is solid with the rod (6) and is moreover connected to the shell (11) by a second restoring spring (15). An element (17) is mounted to slide horizontally in the shell (10) and carries a hook (18) for grasping the thread, the movement of which hook is controlled by the joint action of a slideway (19), solid with the belt (13), and a dog point (17a) solid with the element (17).