TSETSE FLIES (GLOSSINIDAE)

Tsetse flies belong to the sub-order Cyclorrhapha, and form, along with the families Hippoboscidae, Streblidae and Nycteribiidae, the superfamily Hippoboscoidea. They are now placed within their own family the Glossinidae, with a single genus the Glossina, circumscribed by Wiedemann 1830. Before this allocation they were placed in the subfamily Stomoxinae or in the sub-family Glossinae of the family Muscidae. The name tsetse is derived from the noise these flies make when flying. The name itself means "fly" in Tswana, the language of the land formerly known as Bechuanaland. Tsetse range in size from 6-14mm and are robust in form. There are 31 members in the genus Glossina, 23 species and 8 sub-species. The proboscis of tsetse flies projects straight out front from the head and is simple in structure consisting only of labrum, hypopharynx and labium. These structures are ensheathed by the equally long palps. Tsetse flies are extremely important vectors of African trypanosomiasis, causing nagana in livestock, a fatal disease of horses and cattle, and sleeping sickness in humans. Apart from two localities in the Arabian peninsula tsetse flies are found only in sub-Saharan Africa. Fossil records have shown that it once had a wider distribution into the Neartic. The 31 members of the Glossinidae are divided further into three sub-genera, the Glossina, Nemorhina and Austenina. The sub-genera are also called morsitans, palpalis, and fusca respectively and refer to the commonest species in each of the sub-genera.

The subgenus Glossina contains a total of seven species and sub-species. Those of major economic importance are G. morsitans and G. pallidipes with G. swynnertoni and G. austeni being important in specific regions. The species of the Glossina sub-genus are called the savannah flies due to their preference for this environment. It is this fact that makes them the most important vectors as the African savannah is a vast area and the flies come into contact with both man, his animals and wild game animals. Flies of this sub-genus are totally dependent on wild animals for their blood meals. These animals act as a reservoir of trypanosomes; with time, wild animals developed immunity to trypanosomiasis, but domestic livestock have not.

The sub-genus Fusca or Austenina group are also referred to as forest flies. The 15 species and sub-species which comprise this group are mainly of no medical or veterinary importance. These larger and also more primitive flies feed on small mammals in forested areas. The main exceptions are G. longipennis and G.brevipalpis which do not live in the dense equatorial forest but G. longipennis in more arid areas and G. brevipalpis in secondary forest. They have very little contact with man and his livestock due to their habitat. None of this group are vectors of sleeping sickness.

Flies of the sub-genus Nemorhina or Palpalis group are often called the riverine flies. There are nine species and sub-species in this group which are found in close association with local patches of dense vegetation along the banks of rivers and lakes in arid country, and also in dense, wet, heavily forested equatorial rain forest. Species of this group occur throughout much of western Africa. The principal hosts of these flies are reptiles, especially monitor lizards and crocodiles. They will also bite ungulates (hoofed animals). G. palpalis is the commonest and all species are vectors of trypanosomiasis.

Morphological features Contents

Tsetse flies are yellowish to brown, the abdomen may be uniformly coloured or transversed by stripes according to species. Two visually distinctive characters that are apparent to the eye are the forward projecting proboscis and the unusual hatchet shaped cell formed by the wing venation. A closer examination also reveals that the arista arising from the third antennal segment (a characteristic of Cylorrhapha) has branched setae which is not seen on any other fly. The hatchet cell is found in the center of each wing, inbetween veins 4 and 5. The resting position of the wings is also unusual although not unique in that they are folded across the abdomen one over laying the other. Both sexes have dichoptic eyes and it is olfaction that plays the main part in host location.

Internally the alimentary canal begins with the formation of the food canal by the labrum and labium. This canal leads to the oesophagus. Behind the oesophagus is a slightly larger area the proventriculus. The oesophagus is also the region from which the oesophageal diverticulum arises. The proventriculus rear portion forms the fore-gut and the beginning of the mid-gut. In the tsetse fly the mid-gut is long and convoluted. In the anterior part of the proventriculus epithelial cells secrete the peritrophic membrane. This membrane is secreted around a blood meal to aid digestion. At the proventriculus end this membrane is soft and becomes progressively harder further along the mid-gut, the entire length of the gut is lined with this membrane. The mid-gut, hind-gut regions are distinguished by the four malpighian tubes. These tubes are similar to a kidney. The salvary glands in tsetse flies are narrow but extremely long, extending back into the abdominal cavity. In the head the glands form a common salivary duct which passes down the hypopharynx.

Life cycle Contents

Female tsetse flies are unusual in reproducing by adenotrophic viviparity a method similar to that seen in Hippoboscidae (sheep keds). This method involves the retention of a single egg which develops to the third larval stage before being deposited. Adults of other fly species also do this, such as Sarcophaga but in these the first stage larval are deposited. Reproduction begins in a newly emerged female when an ova is released from the right ovary. From the ovary it passes down the oviduct to the uterus. Here the micropyle, which is a small aperture at one end of the egg, comes into a position with the opening of the spermathecal duct. This duct is controlled by a sphincter and opens only at ovulation. The eggs within the uterus hatches in 3-4 days giving rise to the first stage larva. The larva exits the egg using a special labral tooth and once out the remains of the egg are shed through the vaginal opening. As the larva is retained within the uterus for almost its whole larval development the female tsetse has to provide nourishment. Opening into the uterus at the internal end is a duct through which secretions from a pair of uterine glands pass. These glands are often reffered to as milk glands. In order to feed upon these secretions the larva faces head forward so it is facing the same way as the adult female. To maintain its position, most important when at the small first larval stage, is the choriothete. This region produces a sticky secretion which helps to maintain the larva in the correct position. It does not have a role in ecdysis of the larva as previously thought. Development through the three instar stages varies in length according to the ambient temperature. At 25⁰C the first instar lasts 24hrs the second 36hrs and the third 60hrs. Just before depositing her larva at the third instar the adult female actually weighs less than her offspring. The uterine gland has greatly enlarged to supply the nutrients for larval development.

Whilst within the uterine cavity the larva respires through a pair of posterior spiracles and in the third stage via a pair of polypneustic lobes. These lobes contain three air chambers and open through numerous supernumerary stigmata. These lobes become darkened some 24 to 48 hours before the female deposits the larva. The larva shows negative phototaxis and is positively thigmotactic. The larva is deposited onto friable soil and burrows down into the soil. After several hours the puparium darkens with the larval cuticle becoming sclerotised. Later, usually within 48-72hrs pupal apolysis occurs with formation of the pupal cuticle. Development within the puparium is relatively long frequently taking 4-5 weeks although this period is reduced at higher temperatures taking around three weeks at 30⁰C, and extending to 7 weeks at 20⁰C. Apart from temperature the abiotic environment, namely the substrate, along with light intensity also effect development time. The young adult emerges from the puparium in a typical cyclorrhaphan way using its ptilinum. This structure is inflated and forces off the cap of the puparium. The fly then uses it to move up through the soil. This stage of the fly's life cycle probably has a high mortality due to the struggle up to the surface. If the ground has become compacted or a surface crust has formed after rain the young fly may not be able to emerge. On emergence the teneral fly is unable to fly immediately and has to wait until the wings are expanded. It takes a further ten or so days until the complete endocuticle is secreted and the exocuticle hardens.

Having emerged both sexes will seek a host to gain a blood meal. In the females this provides energy and material to build flight muscle mass and rear her larva. In males feeding also develops muscle mass and energy for spermatogenesis. Males are not fully fertile until several days after emergence whilst females are able to mate two too three days after emergence. Studies have shown that certain olfactory cues are the prime responses involved in host finding. The three most important olfactory cues are CO₂, acetone, and octonal. These chemicals emanate from livestock and humans from the breath and urine. Other factors of host finding will be discussed in the control section but such factors as movement and colour also act as cues to feeding behaviour. Teneral flies, i.e. those that have recently emerged and so have low energy reserves show the least repellency to hosts that are not usually fed upon. These hosts include humans which are generally repellent to mature tsetse flies. Studies have shown that non-teneral flies that have been captured feeding on humans have had low energy reserves and so less choice of feeding on their preferred host. Tsetse feed every two to three days and the first larval offspring is deposited about 9-12 days after the female emerged. Due to the length of development tsetse flies are relatively long lived, upto 14 weeks for females, with males having shorter lives of around 6 weeks. Hence the rate of reproduction is extremely slow compared to other diptera, like the prodigiously prolific house-fly