Vertebrates respond to being fed upon by ectoparasitic arthropods through their immune responses, groom-ing and mobility. Arthropod ectoparasites have developed, among others, a series of chemosensory and be-havioural adaptations that are crucial to their resource tracking capabilities on vertebrates. Olfactory recep-tors in chemosensilla on appendages such as the antennae of insects and the first leg-pair of ticks permit these ectoparasites to find hosts from a distance. These include receptors for common respiratory products such as carbon dioxide and other breath components. Ticks, triatomine bugs and phlebotomine sandflies all possess receptors for volatile products common to vertebrates such as branched short-chain fatty acids and aliphatic aldehydes. Both tick and insect ectoparasites signal their presence on vertebrate hosts through the release of pheromones from exocrine glands. These species specific products serve to increase the apparency of the host to conspecifics and enhance encounter between sexes for mating. Constituents of sweat and skin lipids common to vertebrates affect the host attachment responses of arthropod ectoparasites. Receptor cells for these products reside in contact chemosensilla on the legs and mouthparts and these appendages are brought regularly into contact with host's surface as the ectoparasite searches for an attachment site from which to feed. The evidence suggests that evolution has forged a chemosensory disposition that is common to haematophagous arthropods, in which the primary chemostimulants are key by-products of host metabo-lism or host-associated microflora.