THE BIOLOGY OF TSETSE FLIES
Authors: JACKSON C. H. N. 
Published: January 2008

Summary1. The reproductive cycle of tsetse flies occupies a minimum of 55 or 56 days at 23o, made up of 12 days' ovarian development of the first egg (always in the right ovary), 3 or 4 days' embryonic development, 7 days' larval growth in utero, and 33 days as a pupa underground; the male pupal period is longer by about 6%. Second and later larvae follow the first at 11‐day intervals, equal numbers of each sex. A special uterine organ assists eclosion and ecdysis.2. The cycle varies with temperature but not with humidity. Failure to inseminate causes irregularities in ovulation; in nature the females almost invariably copulate once only, when seeking or taking the first meal. Males are potent after they are 4–6 days old. Some species will cross with each other but the male hybrids are infertile.3. During the rains pupae are widely scattered, which has led observers to suppose that breeding is then interrupted; breeding grounds may also shift with season.4. Pupae can survive slight frost; lethal high temperature varies from 40 to 57o according to shortness of exposure. They can develop within a range from about 16–31o, but at the extremes non‐viable flies result, and continual exposure to 30o sterilizes the ovaries of at least one species.5. Loss of weight during pupal life is nearly independent of initial weight; it depends on humidity but not on temperature.6. The size and weight of pupae depends on the temperature at which the mothers were living, and also on the timing of meals; three large meals are needed to produce a large larva, one shortly after larviposition and the third about 3 days before the larva is born.7. Natural mortality of pupae (excluding predation and parasitization) may be zero in certain sites, but normally varies from 8 to 30 or 50% with temperature.8. Before the first meal the ptilinum remains eversible, the chitin is soft and the fly is called teneral. The constitution of the pupa, and of the teneral fly of given age, varies with the species, but the water and fat proportions are fairly constant, and there is little difference between the sexes. Smaller pupae and flies have proportionally less fat. Mean weight may vary seasonally in the proportion of 4–5.9. Loss of water by starving teneral flies (and of course survival) is governed by temperature and humidity; loss of fat is governed by temperature; but the effect of humidity is controversial. Smaller individuals lose proportionally more weight than larger ones, and are less viable in the‐field. Death at low humidities is caused by water loss, before the fat reserves are used up.10. About 2 mg. of fat is probably synthesized at each meal in nature, but in captivity fat synthesis is poor. Fat is also accumulated over several meals, at least up to the age of 3 weeks. It is doubtful whether water loss is or could be compensated by metabolism of fat.11. Lethal high temperatures depend on humidity and time of exposure of the flies, as well as on species, but temperatures of 38–40o for 1–3 hr. may be lethal, especially at high humidity. As with pupae, light frost is tolerated for short periods, at least by some species.12. The salivary glands secrete an anticoagulin and the mesenteron a strong coagulin; species feeding much on reptilian or avian bloods, which tend to clot easily, have larger salivary glands. The digestive enzymes are mainly proteolytic. The chitinous peritrophic membrane is formed afresh from the proventriculus at each meal, keeping pace with the enclosed blood on its progress along the mesenteron.13. Precipitin tests have indicated that large bovids, giraffe and wart‐hog are important hosts of Glossina swynnertoni, but that zebra are not. Corpuscle measurements suggest that during the rains G. morsitans submorsitans feeds mainly on small antelope and wart‐hog; G. longipalpis can apparently survive on small antelope alone. Destruction or reduction of large ungulates sometimes eliminates G. morsitans; exceptionally this species maintains itself on man and his domestic animals. Five per cent of G. morsitans commonly take avian blood, but G. swynnertoni, G. longipalpis and G. pallidipes have an almost exclusively mammalian diet.14. On the other hand, G. palpalis and G. tachinoides take much nucleated blood, and are scarcely affected by the absence of ungulates.15. The eyes of Glossina are not more accurate than those of other Diptera, but may be adapted to perceive movement. G. swynnertoni can see large animals in a good light between 150 and 200 yards away and can scent them between 60 and 100 yards up‐wind; but G. morsitans with antennae occluded came to man as readily as normal flies.16. Flies of the morsitans group feed about every 4th day at 23o, but the hunger cycle depends on temperature and probably humidity; more weight is lost shortly after the meal than later on.17. Hungry G. morsitans (in East Africa at least) visit feeding grounds where animals are likely to be encountered; in so doing they are probably guided by a positive light reaction, which is reversed at high temperature (32o); they will follow artificial paths in absence of game.18. Feeding grounds are used less during the rains, when in East Africa flies are least hungry because the onset of hunger is slow, and they can survive 2 weeks without a meal. But in West Africa G. morsitans submorsitans is hungriest in the cool, long‐grass season at the end of the rains, when large animals are scarce in its habitat and difficult to see.19. The numbers of G. morsitans, G. stvynnertoni, G. palpalis and G. tachinoides caught by a standard method give a fair index of true density from time to time, but not necessarily from place to place. Attraction of G. pallidipes to man is usually very slight.20. Male G. morsitans and G. stvynnertoni live about 4 weeks in nature, but life depends on temperature, and on saturation deficit at low or moderate temperatures. Fortunate individuals may survive 100 days; expectation of life is scarcely affected by age, since mean life is so much less than the possible span. Females live at least twice as long as males, and probably half the larvae deposited fail to become adult flies.21. A mean saturation deficit of 7 mb. is optimal for G. morsitans, and 4 mb. for G. tachinoides; the level of density during the following year is determined by conditions at the end of the rains, when in a dry year density will rise and in a wet year it will fall.22. Densities of G. stvynnertoni and G. pallidipes fluctuate to a long‐term cycle which may or may not be associated with sunspots.23. G. morsitans, G. palpalis and G. tachinoides are most active at about 27o, and inactive outside the limits 15½ and 41½o. Some species are active in the dusk or at night, G. pallidipes especially on moonlight nights but G. austeni often in complete darkness. These two species are most active on the coast at high humidity, but inland on the arid plateau G. pallidipes is most active in dry weather. Attraction to man may be depressed by very high density of ungulates.24. There is some agreement between the proportion of teneral flies caught and the emergence rate. The proportion of females caught depends upon hunger, but also it is associated with the emergence rate because younger females are more active; younger males, on the other hand, are less active than older ones.25. The slow, coherent spread of G. morsitans fly‐belts advancing into favourable country argues that wandering animals do little to assist their spread; and quantitative estimates of dispersal from an area of 4 X 4 miles showed much to‐and‐fro movement over short distances, but only 3 % weekly loss from flies passing off the scene altogether.26. In open grassland G. swynnertoni will follow cattle more readily in the wet season, but even in the dry season may remain with the animals up to 1½ hr. This species will exceptionally travel, apparently unaided, up to 2 miles in 3 hr., and G. palpalis up to 5 miles a day.