One of the overlooked points in mosquito blood feeding research is a final step before blood feeding such as finding a best position of blood feeding site and blood vessels underneath. How mosquitoes detect blood vessels especially using chemosensory organs prior to a final stage of blood feeding is totally unknown. Here we provide the anatomical and chemosensory evidence that a piecing structure of the mouthpart of the mosquitoes is an essential apparatus for the penultimate stage in blood feeding in mosquitoes. Indeed, mosquito mouthparts possesses a set of olfactory receptor neurons in sensory hairs, which are sensitive to volatile compounds present in host blood. Furthermore, the inhibition of gene expression of these odorant receptors delayed blood feeding of the mosquito from host animals. Taken together, these results identify that chemosensory perception in mouthpart is involved in mosquito blood feeding behaviors, which in turn allows mosquito to locate a feeding site more precisely.