The stuff your dreams are made of.
Real answer: Each strain of Sepsis/Degenesis have a certain affinity to and control over certain types of animals. Biokinetics have arachnids, Pheromancers have insects, and the Precogs happen to have shells, ammonites and suchlike. They often adorn themselves with these animals, sometimes just carrying them around and sometimes even having them become part of themselves. The Pheromancers are far and away the most extreme with this, though.