Strigomonas culicis is a protist and member of flagellated trypanosomatids. It is an obligate parasite in the gastrointestinal tract of mosquito, and is in turn a host to symbiotic bacteria. It maintains strict mutualistic relationship with the bacteria as a sort of cell organelle (endosymbiont) so that it cannot lead an independent life without the bacteria. This and other symbiont-harbouring trypanosomatids such as Angomonas deanei are considered as "excellent models for the study of cell evolution because the host protozoan co-evolves with an intracellular bacterium in a mutualistic relationship", and "the origin of new organelles" (i.e. symbiogenesis).