Hey there !
"Hey human, I'm looking for your blood ! Here... just here, right here !" Nymphal instar of tick, waiting for an host to feed on. Castor bean tick (Ixodes ricinus, L. 1758)
Not much is known about their abilities to detect the presence of the desired host. Some organs are present at the last segment of their frontal legs. They could help to do the job. Described for the first time by G. Haller in 1881, this authors initially thought it was an organ of hearing due to the similarity with auditory organs of many crustacea. You should read the note writtten in 1907 by G.H.F. Nuttal, W.F. Cooper and L.E. Robinson, there is some nice planche describing the anatomy.