Tsetse fly
Tsetse 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 impacts 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 ; 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 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 end. Tsetse pupae are under long. Within the puparial shell, tsetse complete the last two larval instars and the pupal stage.
At the end of the pupal stage, tsetse emerges as adult flies. The adults are relatively large flies, with lengths of, 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:
| Proboscis | Tsetse have a distinct proboscis, a long and thin structure attached to the bottom of the head, pointing forward. | Image:tsetse head-proboscis.jpeg|center|70px|A photograph of the head of a tsetse illustrating the forward pointing proboscis |
| Folded wings | When at rest, tsetse fold their wings completely, one-on-top of the other. | Image:tsetse foldedWings.jpeg|center|200px|A photograph of the whole body of a tsetse illustrating the folded wings when at rest |
| Hatchet cell | The discal medial cell of the wing has a characteristic hatchet shape, resembling a meat cleaver or a hatchet. | Image:tsetse hatchetCell.jpeg|center|140px|A photograph of the wing of a tsetse illustrating the hatchet shaped central cell |
| Branched arista hairs | The antennae have arista with hairs which are, themselves, branched. | Image:tsetse aristaHairs labeled.jpeg|center|140px|A photograph and diagram of the head of a tsetse illustrating the branched hairs of the antenna's arista |
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 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, in an adaptation called adenotrophic viviparity. During this time, the female feeds the developing offspring with a milky substance 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, 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 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 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:| Species | Hosts |
| G. swynnertoni | |
| G. austeni | |
| G. fuscipleuris | |
| G. tabaniformis | |
| G. morsitans | |
| G. fusca | |
| G. brevipalpis | |
| G. palpalis | |
| G. fuscipes | |
| G. tachinoides | |
| G. pallidipes | |
| G. longipalpis | |
| G. longipennis | |
| G. m. submorsitans |
Genetics
The genome of Glossina morsitans was sequenced in 2014.Symbionts
Tsetse flies have at least three bacterial symbionts. The primary symbiont is Wigglesworthia, which live within the fly's bacteriocytes. The second symbiont is Sodalis intercellularly or intracellularly, and the third is some kind of Wolbachia.Diseases
The salivary gland hypertrophy virus causes abnormal bleeding in the lobes of the crop of G. m. centralis and G. m. morsitans.Systematics
Tsetse flies are members of the order Diptera, the true flies. They belong to the superfamily Hippoboscoidea, in which the tsetse's family, the Glossinidae, is one of four families of blood-feeding obligate parasites.Up to 34 species and subspecies of tsetse flies are recognized, depending on the particular classification used.
Current classifications place all species of tsetse fly in a single genus named Glossina, with most considering the genus as the sole member of the family Glossinidae.