Tsetse flies (UK: (T)SET-see or US: (T)SEET-see; sometimes spelled tzetze; also known as tik-tik flies) are large biting flies that inhabit much of tropical Africa. Tsetse flies include all the species in the genus Glossina, which are placed in their own family, Glossinidae. The tsetse is an obligate parasite that lives by feeding on the blood of vertebrate animals. Tsetse flies have been extensively studied because of their role in transmitting disease. They have pronounced economic and public health consequences in sub-Saharan Africa as the biological vectors of trypanosomes, causing human and animal trypanosomiasis. Tsetse flies can be distinguished from other large flies by two easily-observed features: primarily, tsetse flies fold their wings over their abdomens completely when they are resting (so that one wing rests directly on top of the other); Secondly, tsetse flies also have a long proboscis, extending directly forward, which is attached by a distinct bulb to the bottom of their heads. Fossilized tsetse specimens have been recovered from Paleogene rocks in the United States and Germany. Twenty-three extant species of tsetse flies are known from the African continent and the Arabian Peninsula.
Terminology Tsetse without the "fly" has become more common in English, particularly in the scientific and development communities. The word is pronounced [tsɛtsɛ] (tseh-tseh) in the Sotho languages and is easily rendered in other African languages. During World War II, a British de Havilland antisubmarine aircraft known as the Tsetse Mosquito helped establish the term in commonplace use among native English speakers.
Biology The biology of tsetse flies is relatively well understood by entomologists. They have been extensively studied because of their medical, veterinary, and economic importance, because the flies can be raised in a laboratory, and because they are relatively large, facilitating their analysis.
Morphology Tsetse flies can be seen as independent individuals in three forms: as third-instar larvae, pupae, and adults. Tsetse first become separate from their mothers during the third larval instar, during which they have the typical appearance of maggots. However, this life stage is short, lasting at most a few hours, and is almost never observed outside of the laboratory. Tsetse next develop a hard external case, the puparium, and become pupae – small, hard-shelled oblongs with two distinctively small, dark lobes at the tail (breathing) end. Tsetse pupae are under 1 centimetre (1⁄2 in) long. Within the puparial shell, tsetse complete the pupal stage. At the end of the pupal stage, tsetse emerges as adult flies. The adults are relatively large flies, with lengths of 0.5–1.5 centimetres (1⁄4–5⁄8 in), and have a recognizable shape, or bauplan, which makes them easy to distinguish from other flies. Tsetse have large heads, distinctly separated eyes, and unusual antennae. The thorax is quite large, while the abdomen is wider, rather than elongated, and shorter than the wings. Four characteristics collectively separate adult tsetse from other kinds of flies:
Anatomy Like all other insects, tsetse flies have an adult body comprising three visibly distinct parts: the head, the thorax, and the abdomen. The head has large eyes, distinctly separated on each side, and a distinct, forward-pointing proboscis attached underneath by a large bulb. The thorax is large, made of three fused segments. Three pairs of legs are attached to the thorax, as are two wings and two halteres. The abdomen is short but wide and changes dramatically in volume during feeding.
The internal anatomy of the tsetse fly is fairly typical of the insects; the crop is large enough to accommodate a huge increase in size during feeding, as tsetse can take a blood meal equal in weight to themselves. The dipteran crop is heavily understudied, with Glossina being one of the few genera having relatively reliable information available: Moloo and Kutuza 1970 for G. brevipalpis (including its innervation) and Langley 1965 for G. morsitans. The reproductive tract of adult females includes a uterus, which can become large enough to hold the third-instar larva at the end of each pregnancy. Most tsetse flies are, physically, very tough. Houseflies, and even horseflies, are easily killed with a flyswatter, for example; a great deal of effort is needed to crush a tsetse fly.
Life cycle
Tsetse flies have an unusual life cycle, which may be due to the richness of their blood food source. A female fertilizes only one egg at a time; she will retain each egg within her uterus, the offspring developing internally (during the first three larval stages), in an adaptation called adenotrophic viviparity. During this time, the female feeds the developing offspring with a milky substance (secreted by a modified gland) in the uterus. In the third larval stage, the tsetse larvae leave the uterus and begin an independent life. The newly-birthed larvae crawl into the ground and develop a hard outer shell (called the puparial case), within which they complete their morphological transformations into adult flies. The larval life stage has a variable duration, ranging from four to ten days, and the larvae must rely on the resources shared by the mother. The importance of the richness and quality of blood to this stage can be seen; all tsetse development (prior to emerging from the puparial case as a full adult after 35 days) occurs with only the nutrition provided by the mother fly. She must get enough energy for her own survival, as well as for the resources that her offspring, whose mass can exceed that of the mother, will require until it emerges as an adult. Both the male and female tsetse flies feed on blood, however. Technically, these insects undergo the standard development process of insects, beginning with oocyte formation, ovulation, fertilization, and development of the egg; following egg development and birth is the three larval stages, a pupal stage, and the emergence and maturation of the adult.
Hosts Overall Suidae are the most important hosts. Waterbuck (Kobus ellipsiprymnus) are unmolested by Glossina because they produce volatiles which act as repellents. Waterbuck odor volatiles are under testing and development as repellents to protect livestock. By species, bloodmeals are derived from:
Genetics The genome of Glossina morsitans was sequenced in 2014.
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