Lemures have some sort of debuff that they give you such as sapping your nutrition when nearby. This stacks with other lemures, so it is insignificant if you are facing only one or two, but can get perilous if you are surrounded by a lot of them, as may happen in Gehennom.

Manes and lemures should count as undead rather than demons, and should perhaps be moved to the ghost or wraith monster class.