Leishmania spp. use blood-feeding insects as a vector. In the insect gut, amastigotes that have been ingested along with a blood meal transform into promastigotes and divide by binary fission. The promastigotes are inoculated into another mammal when the insect takes a second blood meal, and seek to enter host macrophages.
This photo shows four promastigotes of L. major in a culture situation, (3 indicated by arrows) flanked by three macrophages. The morphology of the promastigotes is distinctive. The tapered body contains a large central nucleus. Midways between the nucleus and one end is a small, darkly staining kinetoplast. A flagellum arises from near the kinetoplast and remains internal until it emerges as a free flagellum at the end of the body.