Three major human diseases, malaria, sleeping sickness and leishmaniasis, are caused by protozoan parasites that are transmitted by blood-sucking insects. These insects are not mere 'flying syringes' that mechanically transfer parasites from one mammal to the next. Instead, they provide a specific environment--albeit not a particularly hospitable one--in which the parasites differentiate, proliferate and migrate to the correct tissues to ensure transmission to the next mammalian host. Recent studies on the role of parasite surface molecules in insect vectors have delivered some surprises and could provide insights on ways to interrupt transmission.