A third instar stage is showed while it's grasping on child's hair. They can clearly see abdominal respiratory spiracles and a faecal tail coming out from cloaca. Young lice's feces have the particularity to be reunited in reddish spheres clusters. Such a characteristic makes them pathognomonic in comparison to any other exogenous deposit when found stuck to the hair or laying on the scalp surface.
Phthirus Pubis video in vivo (Dermlite, polarized light)
Human phthiriasis. Can dermoscopy really help dermatologists? Entodermoscopy: a new dermatological discipline on the edge of entomology.
G. Ital Dermatol Venereol. 2012 Feb ; 147(1):111-7