Ticks are small arachnids. Like mites, they have bodies which are divided into two primary sections: the anterior capitulum, which contains the head and mouthparts, and the posterior idiosoma which contains the legs, digestive tract and reproduction organs. Ticks are external parasites, living on the blood of mammals, birds and sometimes reptiles and amphibians. Ticks are vectors of a number of diseases that affect both humans and other animals. Despite their poor reputation among human communities, ticks may play and ecological role by ailing infirm animals and preventing overgrazing of plant resources.