DiscussionWhen Bruce first implicated tsetse flies as the carriers of pathogenic trypanosomes in 1895, he did not appreciate that the trypanosomes undergo a cycle of development inside the fly rather than just being transmitted mechanically from animal to animal. By the 1970's the complexity of the developmental cycle of Trypanosoma brucei had been revealed by light and electron microscopy and it was thought that most of the details were known. Now fluorescent proteins have opened a new window on the development of trypanosomes inside the fly. By genetically engineering trypanosomes to express fluorescent proteins, we can directly observe these parasite cells inside the fly. Even a single trypanosome can be seen. This approach has allowed us to track the trypanosomes as they migrate through the alimentary tract of the fly and identify the developmental stages found in different organs. We can analyse how different strains and species interact along their shared migration routes. By tagging stage-specific genes, we can locate particular life cycle stages in the fly and for example pinpoint the location where genetic exchange takes place.