Haller's organ

Haller's organ is a complex sensory organ possessed by ticks. The organ detects hosts – which the tick, being an obligate parasite, must find in order to survive – via olfaction and the sensing of humidity, temperature, and carbon dioxide. Haller's organ is a minute cavity at the terminal segment of the first pair of a tick's legs (not the pedipalps). Each one is composed of a pit and a capsule, which contain sensory setae.