Development of Leishmania in the sand fly digestive tract. Sand fly midgut is composed of a single layered epithelium with a brush border of microvilli lining the lumen. In contrast, the foregut (including the stomodeal valve) and the hindgut (including the pyloric triangle) are lined by chitin. Amastigotes (a) ingested along with a bloodmeal into abdominal midgut transform into procyclic promastigotes (b), these replicate and transform to long nectomonads (c). During the bloodmeal digestion the parasites are surrounded by peritrophic matrix (PM). When the PM is broken by sand fly enzymes, long nectomonads escape through the posterior opening and attach to midgut microvilli. The next stage are replicative short nectomonads called leptomonads (d); these transform into infective metacyclic promastigotes (e) or attach to the chitin lining of the stomodeal valve as haptomonads (f). In the late-stage development, masses of nectomonads secreting filamentous proteophosphoglycan obstruct the thoracic midgut. This, together with destruction of the valve, facititates reflux of parasites when the fly takes a subsequent bloodmeal. In subgenera Viannia and Sauroleishmania, haptomonads attach also to chitin lining of the pylorus region.