In their initial search for blood, mosquitoes scan a cone-shaped segment of host tissue by using a rhythmic series of probing thrusts and retreats, but without removing the fascicle from the skin. The saliva of mosquitoes constitutes the richest known source of apyrase, which contributes to the formation of hemorrhages around host’s lacerated vessels mainly by hydrolyzing ADP, an important stimulus for platelet aggregation. When the mouthparts contact blood either in a hematoma or a vessel, feeding follows. Desistance—the aborting of a failed probe sequence—occurs after the host’s skin-site has been thoroughly probed, but before allergic reactions irritate the host and trigger its defensive behavior.