From Lexicanum

The Psychneuein is a warp creature similar to a large wasp. These creatures are sensitive to the presence of a Psyker, and reproduce by planting eggs in the brain of the Psyker, if allowed to do so. Once hatched and having killed the victim, the creatures will grow to become large meat eating predators.¹

Sources

• ¹: Warhammer 40,000: Rogue Trader, p. 205