Predacious fungi form specialized hyphae structures to trap nematodes and other microscopic animals. Among the six kinds of trapping devices, the constricting ring is the only one that actively captures nematodes. When a nematode enters the aperture of the ring, which is formed by three cells, the cells rapidly triple their volume, close the aperture and hold the nematode in place. Hyphae then penetrate and consume the nematode. This paper reviews the data and hypotheses on conserving the evolution of constricting rings and their cytological and molecular mechanisms.