Hematophagy is the practice of certain animals (mostly "parasites") of feeding on blood. Small parasites have existed for a long time because it is an efficient method to obtain food. Blood is a fluid tissue rich in nutritious proteins and lipids that can be taken without enormous effort and without killing the host. Parasites that feed on blood include worms (e.g. leeches) and arthropods (e.g. mosquitoes).