Nemacytes are parasitic organisms that dwell within the body of the colossal Riftworm in Gears of War 2. They are six feet in length and move around on two clawed legs, scouring the Riftworm's innards and destroying any foreign bodies they find, such as living creatures that the worm has swallowed. In this sense, Nemacytes function in a similar manner to white blood cells in the human body.
During the events of Gears of War 2, Marcus Fenix and the other members of Delta Squad found themselves being swallowed by the Riftworm. As they explored the insides of the monster, they encountered Nemacytes that attacked them by biting and spitting acid. Ferocious and numerous as they were, the Nemacytes were also physically weak and could be easily killed by the Gears' weaponry. Marcus and the others were able to slay the Riftworm from within by destroying each of its hearts, and when the gargantuan annelid died, all the Nemacytes that thrived within its body died as well.