year to develop through each of the three feeding stages. Amblyomma species may also have a three-year lifecycle, and the adults can live off-host, without feeding, for up to 2 years. Ornithodoros and Argas ticks are particularly adapted to wait for their hosts to arrive by being able to survive for years between blood meals as adults, 18 years have been recorded for O. lahorensis. Food reserves for survival of off-host ticks include large membrane-bound vesicles of lipid in the digestive cells of their gut. Further adaptations include a thick integument with waxy waterproofing combined with ability to secrete hygroscopic salts from specialized parts of their salivary glands (type 1 acini) out to the exterior of their mouthparts and then suck back in the watery solution that develops around the salty material. At least some species of ticks have fairly stable populations, with variations in population densities from year to year varying by roughly 10- to 100-fold. The abiotic (environmental) factor that appears to have most influence on tick distribution and abundance is dehydration stress from the combination of high temperatures and low moisture (measured as relative humidity or low saturation deficit, usually resulting from a climate of low rainfall). Computer models to test hypotheses and to predict the distribution of ticks reflect these stresses, using meteorological information where available, or indices of vegetation type as an analogue of temperature and dehydration stress. Biotic (host related) factors that influence tick distribution and abundance include the obvious need for the hosts to which the ticks are adapted to be present, and also the defenses that the hosts have against the ticks. During the lifecycle of a three-host tick feeding on its natural host that has acquired immunological resistance to the feeding of the ticks, tick mortality can be high. This mortality is highest for the larvae which are easily killed by the immune reactions in the host's skin. It is lowest in the feeding adults. However, even if the ticks do feed, detach, and moult, the size of newly moulted adults is smaller, and the reproductive capacity of the females is reduced. These biotic factors are likely to produce a pattern of mortality of the ticks that is density-dependent. A high density of ticks attempting to feed induces strong immune resistance in their hosts, but a low density of ticks does not induce such strong resistance. Ticks combine long life of