Ovaries Of Black

FIGURE 15.6 Posterior ends of abdomens of Gltssina adults, ventral view, showing sexual differences. (A) Male, with knoblike appearance of hypopygium drawn up into the abdomen; (B) female, lacking knoblike hypopygium. (From Potts, 1973.)

FIGURE 15.7 Larva of tsetse fly (Glossina morsitans). (From Newstead etctl., 1924.)

mating lasts longer than an hour. Sperm are transferred in a spermatophore and are stored in the female's sper-mathecae. Once inseminated, the female remains fertile for life and rarely mates more than once in nature.

About 9 days after copulation, the first ovulation of a single egg occurs, and sperm are released through die spermathecal duct by dilation of a sphincter. The egg is positioned with the micropyle against the spermathecal duct opening, allowing for fertilization. The fertilized egg moves posteriorly into the uterus, where hatching occurs about 4 days later. The first-instar larva uses an "egg tooth" on its anterior end to rupture the chorion of the egg. The larva is retained in the uterus, where it is held against the uterine wall by a supporting structure called the cboriotbete. Secretions from the milk glands pool around the larval mouth and are easily ingested. The developing larva molts twice within the uterus, becoming a second-instar larva 1 day after hatching and a third instar about 1.5 days later. The third-instar larva is fully developed about 2.5 days after the second molt, at which time it occupies most of the female's abdomen and is about equal in weight to the rest of the female's body. The female continues to ingest blood, albeit in progressively smaller amounts, as the larva grows.

About 9 days after ovulation, the fully developed third-instar larva is deposited on the ground by the female. Shortly thereafter, the female ovulates again (within as little as 1 hr after larvipositiori). A well-nourished female, after this first larviposition, will deposit a third-instar larva about every 7—11 days, depending on the ambient temperature. The average interval for all tsetse species is 9— 12 days. The ovaries, and the ovarioles in each, alternate in releasing a single egg at each ovulation, starting with the right ovary. Follicular relicts seen in dissected flies reflect the ovulation history of individual females and can help in estimating the longevities of wild-caught female flies.

Tsetse females generally live for about 20 to 40 days, but may have a maximum life span of 3—4 months. The males typically mate only once or twice during their lives and apparently survive in the wild for 2—3 weeks (Glasgow, 1963; Potts, 1973). More accurate estimates of longevity will probably become possible with newly developed fluorescence techniques that measure accumulated pteridines in tsetse head capsules (Lehane, 1991).

The full-size third-instar larva is cream colored and oval-shaped. It measures 3—8.5 mm in length, depending upon the species, and has two prominent black knobs at the posterior end (Fig. 15.7). These conspicuous knobs are respiratory lobes that function only during, and for a short time following, intrauterine life. The active larva is deposited on the ground, usually in loose soil shaded by trees or other vegetation, The larva, which is negatively r i

FIGURE 15.7 Larva of tsetse fly (Glossina morsitans). (From Newstead etctl., 1924.)

phototactic and positively thigmotactic, quickly burrows to 1.5—2.5 cm below the soil surface. Within a few hours of deposition, the larval integument hardens and darkens, and the third-instar larva becomes an immobile brown to black puparium. About 2—4 days later, molting occurs within the puparial case and a true pupa is formed. A key for the identification of puparia to species is given by Jordan (1993).

Adult flies emerge about 30 days after formation of the puparium. As in all other cyclorrhaphan flies, eclosion involves the breaking of the circular puparial cap by a ptil-inum. The teneral adult pushes its way to the surface of the substrate, where it rests for a short time, usually less than an hour, before it can fly The teneral fly does not fully harden and the thoracic flight muscles do not completely develop until about 9 days later, after the fly has had at least a few blood meals (Glasgow, 1963; Lehane, 1991; Potts, 1973).

The low reproductive rate in tsetse is compensated by the extreme protection given to each larva by the female, by virtue of the viviparous mode of development. However, the low reproductive rate makes the impact of any loss of female flies greater than in species that mass produce eggs.

Although tsetse are found over an area estimated to be at least 10 million square kilometers, the distribution of the flies is discontinuous. The areas they inhabit may extend to several hundred kilometers and form what have been traditionally called fly belts. Within these belts are patches of forest and bush where environmental conditions, such as shade and high humidity, are suitable for tsetse survival and reproduction. Local residents living in their vicinity are often aware of these areas of high tsetse concentrations. One or more species of tsetse usually are found where woody vegetation is at least 4.5 m high. In many cases, Africans can predict the presence of particular species of tsetse by observing the types of shrubs and trees that occur in a given habitat. Rather than representing direct associations of tsetse species with specific plants, the plant communities observed probably reflect differences in a variety of microhabitat factors that directly affect the survival of tsetse, such as the water content of the soil, the availability of mammalian hosts, and the occurrence of natural predators. Remotely sensed satellite data that provide identification of different types of vegetation over large geographic areas have been used to estimate distributions of different species of tsetse (Rogers et al., 1994).

Tsetse flies are restricted in northern Africa by desert conditions, and in southern Africa, by the deserts of Namibia and Botswana and their lower ambient temperatures. Tsetse live in areas where the annual rainfall is at least 0.5 m per year. They require temperatures between 16 and 40° C, with optimal development occurring at 22—24°C; for this reason, the flies are not found at elevations above ca. 1500 m.

The potential difficulty of males and females finding each other in low-density populations is apparently overcome in some species by the attraction of both males and females to large moving animals. Mating usually occurs on or in the vicinity of a host. Once they have mated, however, females and males tend to be more attracted to stationary animals. Tsetse feed on an array of hosts including reptiles and mammals, but rarely birds. Individual species and species groups have definite host preferences. These preferences are of considerable epidemiological significance in relation to the reservoir hosts of the pathogenic trypanosomes transmitted by the flies to humans and domestic animals.

Host preferences v ary among tsetse species. Members of the palpalisgroup feed mostly on reptiles (e.g., crocodiles and monitor lizards) in their riverine and lacustrine habitats and on bushbuck, oxen, and occasionally smaller mammals and humans that visit these watering spots. Species of the morsitans group, living in scattered patches of vegetation in open country, feed mostly on the mammals of the savanna. In addition to showing a strong preference for warthogs, the savanna-dwelling tsetse feed on a diversity of mammalian species, including bush-buck, buffalo, giraffe, kudu, rhinoceros, duiker, bushpig, and oxen. The one forest-dwelling species in the savanna group, G. austeni, feeds almost exclusively on suids such as bushpigs and forest hogs. The fusca group feeds on a variety of host species, including bushbuck, buffalo and other cattle, giraffe, rhinoceros, elephant, hippopotamus, bushpig, river hog, porcupine, aardvark, and even the ostrich (Lehane, 1991). Humans are not the preferred hosts of any of these fly species. In some cases, people in the vicinity of other mammals will actually repel tsetse, whereas hungry flies will suck blood from humans who enter tsetse habitat.

Host attraction and host recognition are mediated by visual and olfactory cues. Their vision enables tsetse to react to a herd of moving cattle as far away as 180 m. The attraction of both male and female flies to large moving objects accounts for the common occurrence of tsetse attacking occupants of trucks and tourists in jeeps on safaris. When landing on the sides of vehicles, male flies that land with their heads directed upward are more likely to be hungry than those that land with their heads directed downward (Newberry, 1982). Tsetse species in the morsitans group, living in open spaces, have shown the greatest attraction to host odors. Certain tsetse species are attracted to components of ox breath, such as carbon dioxide, acetone and octenol, and phenols found in mammalian urine (Willemse and Takken, 1994). Host odors have been shown to be attractive to tsetse from distances up to 100 m away.

Although tsetse feed mostly in the daylight, feeding does occur at night, as in the case of G. medicorum, which feeds on the nocturnal aardvark. In general, tsetse adults are most active in the morning and late afternoon. They rarely fly for more than 30 min a day and are known to disperse up to about 1 km/day. They spend most of their time resting on vegetation. Some species, such as G. morsitans, rest as high as 12 m above the ground, while others, such as G. pallidipes, are seldom found above 3 m. When seeking a host, Glossina species can fly very rapidly, reaching speeds up to 6.5 m/sec (ca. 25 km/hr).

Host behavioral differences may account in part for the feeding preferences shown by tsetse species. Mammals that are heavily fed upon and irritated by other kinds of biting flies sometimes react with strong defensive behaviors, such as muscle twitching and rapid tail movements, that repel tsetse. Tsetse are more prone to start feeding on calm animals and often seem to prefer to feed on a host that is in the shade. The latter may be an adaptation to avoid reaching lethal body temperatures and may serve as a means of avoiding predation during feeding, or just after, when the fly takes off and alights a short distance from its host (Glasgow, 1963).

Upon landing on a host, a tsetse fly grips the skin with its claws and applies pressure to the skin surface with its proboscis. The teeth and rasps on the labeilum aid the labium in penetrating the skin. Strong back-and-forth movements of the fly's head cause the labium to rupture one or more capillaries in the skin, resulting in a hemorrhage within the bite site. The blood is rapidiy sucked into the food canal of the labrum by the negative pressure produced by the cibarial pump in the fly's head. Saliva is pumped intermittently through the salivary canal of the hypopharynx into the wound. The saliva contains anticoagulant substances, including an antithrom-bin and an apyrase that inhibit platelet aggregation. As in other hematophagous insects that have anticoagulins in their saliva, tsetse flies presumably benefit from these substances by their role in increasing blood flow at the feeding site, thereby reducing feeding time and the vulnerability of the fly to host defenses. If a tsetse fly is disturbed while penetrating the skin, it will rapidly withdraw the proboscis and fly away; however, once feeding begins, a tsetse is less likely to react to movement and physical stimuli that would normally cause it to escape (Glasgow, 1963; Lehane, 1991).

Tsetse engorge fully within aboutl-10min,the length of time depending in large part on how quickly the labium is able to rupture a capillary. The actual penetration of host skin occurs quite rapidly, whether it involves the thick hide of a rhino or a thin artificial feeding membrane. During feeding, a clear fluid is excreted from the anus. A tsetse fly imbibes about 0.03 ml of blood and when folly engorged weighs about 2-3 times its unfed body weight (Fig. 15.8), The ungainly fully fed insect slowly flies from the host (ca. 1.6 m/sec) and lands on a nearby tree or

other substrate. There the fly continues to excrete anal fluid as a means of ridding itself of excess water while concentrating its blood meal. About 40% of the blood meal weight is lost in the first 30 min after feeding. The rapid loss of excess fluid that begins during blood feeding helps the fed fly regain flight agility as quickly as possible. This helps it evade the defensive movements of the host and destruction by predatory flies, other arthropods, and vertebrate predators. A larva being carried by a female is especially vulnerable to loss just after the female has taken on the extra burden of a blood meal and has lost much of her maneuverability. Complete digestion of the biood meal occurs by about 48 hr. The interval between blood meals varies, with a mean of 3—5 days.

Because tsetse feed exclusively on blood, their main source of energy is derived from protein. They depend on the amino acid proline as the major energy source for flight. The energy is produced by the partial oxidation of proline to alanine in flight muscle (Glasgow, 1963; Lehane, 1991). The unique metabolism of tsetse flies enables them to live in dry habitats in which blood is their sole source of nutrition and water and to develop massive thoracic flight muscles that allow them to fly with heavy loads of blood and/or an internally developing larva.

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