So biologists have long been puzzled over the fact that the human head louse is a sister species to the chimpanzee louse, but the pubic louse is closely related to the gorilla louse.

所以生物学家长期为此困惑：为什么人类的头虱是黑猩猩虱子的近亲。而阴虱却是大猩猩虱子的近亲。