A lancet body for a skin pricker has resiliently flexible fingers (9) with outwardly splayed tips (10). The device is fired by pressing a trigger (13) and a spring (5) urges the lancet forwards and the fingers (9) flex inwards as the tips (10) snap past barbs (11), so that the tips (10) of the arms (9) engage behind the barbs (11). If the user tries to push the lancet back into the cocked condition, the fingers (9) are trapped and will flex as the tips (10) press against the barbs (11) until the arms buckle and the tips (10) are pushed out through openings (14). Thus the lancet is arrested and immobilized before a re-cocked position is reached, and re-firing is prevented.
