How do you know if you have crabs. do they look like head lice?

See below: Crab lice look significantly different from head lice but your need a microscope to really see the differences other than general shape. Crab lice are shorter, thicker and broader that head lice when using the nacked eye. See your doc if you think you are affected.

Sort of: They belong to similar family of lice. Pediculosis pubis can be googled and in general are a little fatter than head lice. It is difficult to see as you will require a magnifying lense or microscope to determine them. I would see your pcp or dermatologist locally for this.

Crabs not same ...: A.K.A. Pubic lice, is caused by the crab louse called pthirus pubis. It is a parasitic insect that infests human pubic hair. Pediculus humanus (head lice)is a different parasitic insect that infests hairs in other parts of the body. Pubic lice is stubby and "crab- like" while body lice have an elongated body and narrower jaws.

Lice: They are not the same organism and look different microscopically. Body lice (Pediculosis humanus corporis) is most apt to present on the flank, neck or upper shoulders but might manifest in the axilla. Crabs or pubic lice (Phthirus pubis) might be found under the arm but is more likely to be found in the groin area. Blue spots may show on the skin where pubic lice have been feeding.