The natural history of ticks.

Ticks have evolved to become one of the most important groups of arthropod vectors of human pathogens. One or more of the approximately 840 known species of ticks are found in most terrestrial regions of the earth. Ticks are a highly specialized group of obligate, bloodsucking, nonpermanent ectoparasitic arthropods that feed on mammals, birds, and reptiles. They are classified into two major families, Ixodidae (hard-bodies ticks) and Argasidae (soft-bodied ticks). The Ixodidae is the largest and most important family. There are many taxonomic keys for identifying ticks to assist the serious investigator. Their life cycles are often complex, and even though ticks are associated with their parasitic habits, ticks spend most of their life off hosts and in vegetation or soil. Maintenance of water balance during periods of overhydration while feeding and periods of dehydration while fasting is significant in the distribution, survival, activity, and transmission of disease-causing pathogens to humans and animals. Ticks attach to skin of the host by using their hypostome as an anchor and create a feeding lesion to ingest blood or tissue fluids. Soft-bodied ticks feed relatively rapidly (hours or less) and ingest only blood. Hard-bodied ticks take days to complete feeding and feed on blood, lymph, and lysed tissues from a pool that forms around the mouthparts. Feeding causes direct damage to the skin of the host. Disease-causing organisms may be ingested or expelled during feeding. Ingestion of relatively enormous quantities of blood is characteristic of ticks.