Parasitic insects are small creatures that feed off of the blood of other animals, including humans. Two of the most common parasitic insects are nits and LICE. Nits are the eggs of LICE, which are tiny wingless insects that live on the skin and hair of mammals. LICE feed on the blood of their hosts and can cause itching and irritation. Both nits and LICE can be treated with special shampoos and medications.