Understanding the relationship between the gender of insects and their ability to act as vectors of insect-borne diseases (IBDs) could provide clues as to the origin of the intimate interplay among insect, pathogen and vertebrate hosts. The vector activity of several species of blood-feeding insects is linked to adult females. Interestingly, the only known exception is the transmission of canine and human thelaziosis by a male dipteran fly. This biological difference raises the question as to whether the parasitic behaviour of male and female insects transmitting IBDs is an expression of a co-evolution of vectors and pathogens.