As Idzi mentioned, it is not possible to tell P. h. humanus and P. h. capitis apart by morphology alone since there is significant overlap in sizes between the 2 otherwise identical subspecies.
Richard, Florida Fan and Idzi nicely discussed the likely route of introduction into the nostril: little fingers, or possibly even inhalation. It makes me wonder though - could the lice ever live on nostril hair?
As a review, here are some of the key identifying features of Pediculus humanus:
- Small body (1-2 mm long)
- 6 legs
- Raptorial (grasping) claws
- Fused thorax, distinct from the head (with antennae) and abdomen
- Enlongated body
Thanks again to Kamran for donating this case!