Contrary to the definition provided below by Webster 1913, the tsetse fly (Glossina sp.) is a vector of a deadly disease. There are 23 species in the genus, and 20 of these are capable of transmitting trypanosomes to mammals. Several of these are important transmitters of the parasites responsible for African trypanosomiasis, or sleeping sickness.

These animals are true dipterans, and typically measure 7 to 15 millimeters long. They are ovoviviparous, which is odd for insects. Females give birth to only one progeny at a time, resulting in a very slow reproductive rate. Both males and females are ectoparasites of humans and other mammals.

The insects have traditionally been controlled by discriminate (sometimes in-) spraying with DDT. Despite the fact that the tsetse fly is responsible for up to 300 000 infections per year for humans, and up to 1.5 billion dollars (US) is losses for infected livestock, political instability and conflict has hampered a thorough and sustained attempt at control.