tsetse fly

tsetse fly, (genus Glossina), any of about two to three dozen species of bloodsucking flies in the housefly family, Muscidae (order Diptera), that occur only in Africa and transmit sleeping sickness (African trypanosomiasis) in humans and a similar disease called nagana in domestic animals. Tsetse flies are distinguished in part by a forward-projecting piercing proboscis on the head that is capable of puncturing skin. They readily feed on the blood of humans, domestic animals, and wild game.

Tsetse flies are robust, sparsely bristled insects that usually range from 6 to 16 mm (0.2 to 0.6 inch) in length. Tsetse flies are rather drab in appearance: their colour varies from yellowish brown to dark brown, and they have a gray thorax that often has dark markings. The abdomen may be banded. The stiff, piercing mouthparts, directed downward as the fly bites, are held horizontally at other times. While resting, the wings are held flat over the back, folded one over the other. A bristlelike appendage (arista) on each antenna bears one row of long, branched hairs on its upper edge, differentiating the tsetse fly from all other flies.

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Male tsetse fly adults may live two to three weeks, while females can live for one to four months. Tsetse flies are larviparous—the larva hatches from an egg within the female—and the young develop singly within the female’s uterus, feeding on a nutrient fluid secreted by paired milk glands on her uterine wall. The ensuing three stages of larval growth require about nine days. The larva is deposited on the ground, where it burrows into the soil and pupates within one to five hours. Adults emerge after several weeks. When adequately fed, a female tsetse fly will produce one larva about every 9 or 10 days throughout her life. Without a sufficient blood meal, however, the female fly will produce a small, underdeveloped, and nonviable larva.

In general, tsetse flies occur in woodlands, though they may fly out a short distance into open grasslands when attracted by a host animal. Both sexes suck blood almost daily, but daily activity patterns vary by species and environmental factors (e.g., temperature); some species, for example, are especially active in the morning, whereas others are more active at midday. In general, tsetse fly activity declines soon after sunset. In woodland environments, male tsetse flies are responsible for the majority of attacks on humans; the females usually feed on larger animals.

Tsetse flies are arranged taxonomically and ecologically into three groups: the fusca, or forest, group (subgenus Austenina); the morsitans, or savanna, group (subgenus Glossina); and the palpalis, or riverine, group (subgenus Nemorhina). The medically important species and subspecies belong to the morsitans and palpalis groups. Two of the most significant vectors of sleeping sickness are Glossina palpalis, which occurs primarily in dense streamside vegetation, and G. morsitans, which feeds in more open woodlands. G. palpalis is the chief carrier of the parasite Trypanosoma brucei gambiense, which causes sleeping sickness throughout western and central Africa. G. morsitans is the chief carrier of T. brucei rhodesiense, which causes sleeping sickness in the highlands of eastern Africa. G. morsitans also carries the trypanosomes that cause nagana.

Historically, the widespread presence of the tsetse fly inhibited human settlement and agriculture in large areas of sub-Saharan Africa. In the 20th century, efforts to control tsetse flies were implemented with varying degrees of success. By the mid-1960s, however, human African trypanosomiasis was largely under control. Eradication from areas where the disease was a cause of epidemics enabled settlement (or resettlement) and the development of livestock production. In the early 21st century, following a lapse in surveillance that allowed reemergence in some areas, annual new cases of the disease in Africa reached all-time lows.

The most-effective control measures for tsetse flies have been environmental ones: destruction of the wild game upon which the flies feed, clearing of woodlands, and periodic burning to prevent the growth of brush. Trapping of flies, control by natural parasites, and the spraying or other application of insecticides usually reduce fly populations in a locality but have difficulty eliminating them altogether. An alternative method is the introduction of large numbers of sterilized male tsetse flies into a wild population. Exposure to gamma radiation in laboratory facilities renders the flies sterile but does not interfere with their ability to mate. The sterile males’ unions with females produce no offspring, and, since female tsetse flies mate only a single time in life, those that mate with sterile males are themselves rendered sterile for all practical purposes. The method has been found to totally eradicate tsetse flies in localities where their populations have already been significantly reduced by conventional methods.