The New World "Leishmania" species transmitted by "Lutzomyia" sand flies are often classified in the subgenus, "Viannia". These genetically distinct parasites show markedly different patterns of development within the New World sand flies when compared to those seen in the Old World "Phlebotomus" sand flies. The first stages of development are similar between all "Leishmania" species, with the sandfly taking up the amastigote form of the parasite following a bite of an infected host. However, unlike in the Old World, the replicated parasites then migrate to the hindgut of the "Lutzomyia" sand fly, a feature which is thought to be essential in allowing the parasite to become established. The parasite then undergoes further development into the infective, promastigote stage as it migrates to anterior end of the insect, ready for inoculation into a new, susceptible host.