Towards a general rule for estimating the stage of pregnancy in field-caught tsetse flies

Abstract. Ovarian dissections were performed on the tsetse flies Glossina morsitans morsitans Westwood and G.pallidipes Austen of known ages, maintained in the laboratory or on an island in Lake Kariba, Zimbabwe. The lengths ( l ₂ and l _u) of the second largest oocyte and of the larva in utero were found to increase approximately exponentially during pregnancy. The length ( l ₁) of the largest oocyte increased exponentially for about the first 80% of pregnancy. Linear relationships between the log values of l ₁, l ₂ and l _u in field-caught flies, of unknown chronological age, are consistent with the idea that growth patterns are similar in laboratory, island and open field situations. The egg phase takes up c. 45% of pregnancy in both species, regardless of season and the absolute duration of pregnancy. The changes in the log values of l ₁, l ₂ and l _u, over the ranges within which they change linearly, can be used to assign flies to their stage of pregnancy. When applied to field data the rule showed that G.pallidipes caught in odour-baited traps, and on a mobile electric net, exhibited major activity peaks shortly before and after parturition. Flies from the trap (but not the net) showed a smaller peak of activity near the middle of pregnancy. The egg and the three larval phases in utero take up c. 45%, 25%, 20% and 10% of pregnancy respectively.     

Reproductive abnormalities in tsetse flies in Zimbabwe

Between November 1988 and July 1995 five technicians carried out ovarian dissections on 16,013 Glossina morsitans morsitans Westwood and 123,848 G. pallidipes Austen captured at Rekomitjie Research Station, Zambezi Valley, Zimbabwe. The ovarian age and uterine content were recorded, as were the lengths (l₁, l₂ and l_u) of the largest and second largest oocyte, and of any uterine inclusion. Major abnormalities and abnormal spermathecal contents were found in <0.1% of all flies dissected. Apparent abortions rates varied significantly between dissectors and occurred at frequencies of 0.8–4.5% in G. m. morsitans and 0.3–2.8% in G. pallidipes. The lowest estimates give the best picture of the field situation. Abortion rates were higher in flies caught on electric nets than in trapped flies where the rate was only 0.15%, indicating that reproductive losses are negligible for most of the year at Rekomitjie. The rates did, however, increase to >2% when mean temperatures exceeded 27°C and flies were captured in artificial refuges. There was little effect of ovarian age on the abortion rate, but the frequency of empty uteri declined markedly with age – with a suggestion, however, that it might increase again in the oldest flies. A knowledge of the rates of reproductive loss is important for the construction of realistic models of the dynamics of tsetse populations.     

Reproductive under-performance of tsetse flies in the laboratory, related to feeding frequency

Abstract. The rates of development of the eggs and larvae in utero and the next two developing ovarioles were measured by ovarian dissection on each day of the pregnancy cycle in tsetse, Glossina morsitans, subject to different feeding regimes. Compared with flies fed four times per pregnancy cycle, flies fed three times per cycle showed a lower pupal production rate (70%), the same (zero) adult mortality, a slightly slower growth rate of the larva and second ovariole only from day 8 onwards, but the same growth rate of the first ovariole. Flies fed only twice per pregnancy cycle produced no pupae, suffered 18% adult mortality and showed a significantly slower growth rate of the larva and second ovariole from days 6 and 7 respectively, but still the growth rate of the first ovariole was barely affected. Flies offered food three times or twice per pregnancy cycle engorged fully at every opportunity, but 16.5% of the flies offered food four times per cycle did not feed on every occasion, while 12–22% did not engorge fully on days 3, 5 or 7. In assessing the applicability of these laboratory results to the field situation the following points must be borne in mind: in the laboratory flies take smaller mean blood meals than in the field; during protein production associated with larval growth the proportion of the blood meal lost to transformation and excretory costs is less than during normal lipid metabolism; the balance between the tsetse's known fertility rate and adult and pupal mortality rates reveals that the abortion rate in the field must be extremely low. The high abortion rates usually observed in laboratory colonies, even when flies are offered food daily_l would be quite untenable in the field and indicate that laboratory conditions impose physiological stresses on the flies that are quite different from those in the field. These facts indicate that three field-sized meals may be sufficient to meet the energy demands of normal larval development in the field.     

The mating behaviour of tsetse flies (Glossina): a review

Abstract. The mode of reproduction of tsetse flies, by adenotrophic viviparity, is unusual among the Diptera and is associated with many unique aspects of the tsetse's mating system. Tsetse exist at relatively low densities in the environment but a combination of olfactory and visual stimuli brings males and virgin females together on or around host animals. The behavioural repertoire associated with mate location and identification, courtship and copulation is regulated by external physical and chemical stimuli as well as by internal physiological mechanisms. With a view to identifying stimuli that could be used to manipulate tsetse behaviour and exploited for control purposes, much progress has been made in recent years in elucidating the mating behaviour of tsetse and its regulatory mechanisms. This progress and the current state of understanding of tsetse mating behaviour is reviewed.     

Monitoring tsetse fly populations.:II. The effect of climate on trap catches of Glossina pallidipes

In Part I it was shown that the sampling distribution of trap catches of tsetse flies, Glossina pallidipes Austen, at Nguruman, Kenya, using unbaited biconical traps follows a Poisson distribution. In this paper we examine the effect of humidity and temperature on day-to-day and seasonal variations in the trap catches. It is shown that the seasonal variation is significantly correlated with maximum daily temperature, the catches increasing with temperature when the maximum temperature is below 34 degrees C and decreasing with temperature when it is above 34 degrees C. The correlation between trap catches and relative humidity is not as good as the correlation with the maximum temperature, and the two together do not improve the fit to the trap catches. The day-to-day variation is significantly greater than the intrinsic variation due to the stochastic nature of the sampling process and for some traps it is correlated with temperature and humidity. An autoregressive model gives a half-life for the decay of departures from the mean of about 1 day and it is suggested that this indicates the movement of flies in response to animal movement or to climatic factors other than temperature or humidity. After removing the temperature dependent part of the seasonal variation and the autoregressive component of the data, the male and female catches are still significantly correlated.     

Monitoring tsetse fly populations. I. The intrinsic variability of trap catches of Glossina pallidipes at Nguruman, Kenya

During 1986 the tsetse fly Glossina pallidipes Austen was monitored daily at Nguruman, southwestern Kenya, using three unbaited biconical traps. This was done to investigate the nature and causes of daily variation in trap catches. The variability of the observed catches was compared to a model which includes the trapping probability and the stochastic variation in the sex-ratio. By comparing the catches of male and female flies we are able to establish the sampling distribution of the trap catches. In addition to seasonal changes in the trap catches, day-to-day variations are observed and these are considered greater than the variation arising from the stochastic nature of the sampling process. Recommendations are made in relation to sampling tsetse fly populations.     

Nutritional levels of female tsetse Glossina pallidipes from artificial refuges

Female tsetse flies, Glossina pallidipes Austen, caught in artificial refuges were subjected to ovarian dissection and analysed for levels of fat, residual dry weight (RDW) and haematin. There were rather small proportions of flies in ovarian categories 0 and 1, in part due to large losses in the immature and teneral stages at the hottest time of year. The distribution of the female catch among pregnancy days was close to uniform. The wet and dry weights (WW and DW) and RDW of eggs, larvae and pupae increased by 0.821, 0.303 and 0.204 mg respectively, with each mm3 increase in volume. Water accounted for 71.7% of the fat-free WW, and fat for 32.7% of the DW. Between birth and ovulation, fat increased from 2 to 4 mg and RDW from 7 to 11 mg; thoracic RDW increased by 2.5mg and changed little thereafter. Fat levels increased 3.5mg by day 6 of pregnancy, but only 0.5 mg thereafter. Over the same periods RDW corrected to zero haematin (CRDW) increased by 1 and 8 mg respectively. Full-term fat and CRDW levels were 8.2 and 19.4 mg respectively. Cumulative haematin frequencies formed a smooth curve with a slope that increased continuously. The raw data were well fitted by a model where feeding rates increased exponentially and capture probability was independent of haematin content. The mean feeding interval was 60 h; feeding probabilities of >0.9/day were only found in flies that had failed to feed for>72 h. In early pregnancy, fat levels declined with haematin for flies that had fed>36 h previously; by days 5-7 fat levels were maintained at a constant high level for 60 h post-feeding. Fat-haematin graphs for female tsetse cannot be used to estimate rates of fat utilization. Traps sample tsetse with below-average fat and RDW in early and late pregnancy respectively. Refuge samples are less biased than those from traps; they give a better picture of the dynamics of pregnancy in normal flies and facilitate the explanation of existing anomalies.     

Lifetime changes in the nutritional characteristics of female tsetse Glossina pallidipes caught in odour-baited traps

Female Glossina pallidipes Austen were captured in odour-baited traps at Rekomitjie Research Station, Zambezi Valley, Zimbabwe during February 1994; 2890 were dissected and assigned to their ovarian age category and day of pregnancy using the lengths of the oocytes and uterine content. For 1838 of these flies, the nutritional state of the mother and her uterine content were estimated separately. It was thereby possible to see how, during pregnancy, the females acquired fat and residual dry weight (RDW) and transfered them to the larva. Newly emerged flies contained 1 mg fat and 6 mg RDW, of which 4 mg was in the thorax (TRDW). Fat hardly increased by the first ovulation; RDW increased by 2.5 mg and 1.5 mg of this increase was in TRDW. Mean haematin levels increased from 2 to 8 microg during each pregnancy. Fat increased from 1.2 mg to 4.5-5 mg by day 7 and was then rapidly transferred to the larva. RDW increased by only 1.8 mg by day 7, but larval RDW increased thereafter by > 6 mg. Amino acids from late-pregnancy bloodmeals are incorporated directly, in the uterine gland, into 'milk' that is taken up rapidly by the larva. Capture probability was highest on day 1 of pregnancy, when nutritional levels were lowest, with lesser peaks on days 5 and 8 when the fly was nourishing a rapidly developing larva. On day 1, the peak of the logarithm of the haematin distribution corresponded to flies estimated to have fed approximately 75 h previously; by day 8 it had shifted to approximately 60 h post-feeding. A model in which feeding rates and capture probabilities increased exponentially with time since feeding accounted for 97% of the variance in log haematin frequencies. On 4/9 days of pregnancy there was no significant decline in fat with haematin content during the lipolytic phase. The rate of decline is not a satisfactory estimate of the rate of fat usage. Flies in this study had longer wings and higher TRDW than those from refuges in an earlier study, but had lower levels of fat and haematin.     

Responses of tsetse to ox sebum: a video study in the field

The behaviour of tsetse (mainly Glossina pallidipes Austen) around odour-baited targets, with or without a coating of ox sebum, was recorded in the field using video. The addition of sebum increased the total time a fly was in contact with the target, as well as the time spent flying around and landing on it. When carbon dioxide was released as part of the attractant odour plume, the presence of sebum on the target increased the number of landings made by each fly, but did not significantly affect the duration of each contact. When carbon dioxide was absent from the odour plume, sebum did not affect the number of landings made by flies but the duration of each contact with the target did increase. Evidence for an interactive effect of sebum and carbon dioxide was obtained. In addition, the presence of sebum on the target increased the percentage of landed flies which walked on its surface; such behaviour may represent an 'inspection' of the artificial host. The potential tsetse control application of the current findings are discussed.     

Age-grading adult insects: a review of techniques

Abstract.The range of techniques available for age-grading adult insects is reviewed, with particular emphasis on species of medical and veterinary importance. Chronological and physiological age-grading are discussed, considering the relative merits of each approach. The techniques described include those based on changes to the reproductive system, those relying on somatic changes and those dependent on cuticular degradation. The advantages and problems of each technique and the use of a number of techniques in combination are discussed.     

Reciprocal regulation of fat content and flight activity in male tsetse flies (Glossina palpalis)

Abstract. To disentangle cause and effect in previously observed relationships between fat content and flight activity in male tsetse ( Glossina spp.), three groups of flies were fed at different intervals to raise their fat content to different levels before their flight activity was recorded. The greater the mean daily blood intake, the higher the fat content and the greater the subsequent spontaneous flight activity, thereby using up almost all of the fat reserves before the next blood meal. It is proposed that although male flies would benefit from maximum food intake to permit maximum flight associated with mate-seeking, they do not in fact feed as often as possible either in the field or the laboratory. This is explicable if energy acquisition is constrained by an additional mortality risk associated with feeding.     

Copulatory behaviour of the tsetse flies Glossina morsitans and G.austeni

ABSTRACT. The copulatory behaviour of Glossina morsitans morsitans West. and G.austeni Newst. was analysed by filming. After a male and female engaged genitalia, the male performed a repertoire of five actions for 3–4 min: (1) 'rubbing' his metathoracic, tibiotarsal joint against the region of genital contact; (2) 'stroking' or hitting the female's head and thorax with his meso- and metathoracic legs; (3) 'wing flick' by moving his mesothoracic legs in a rowing motion whilst at the same time vibrating the wings as in normal flight; (4) 'wing vibration' with the wings vibrated in the closed position; (5) 'wings out' in which the wings are moved out to the flying position without any observable vibration. Each action was repeated many times, to give variable individual sequences, but declined in frequency exponentially over the first 3–4 min in copulo. The two species differed in the frequency of acts. Shortly before separation, a few hours later, actions 1 and 2 reappeared. Receptive females exhibited little overt behaviour except in the maintenance of a passive stance. Refractory females rejected a mating attempt by flexing the abdomen ventrally, vibrating their wings in the closed position, and pushing the male with meso- and metathoracic legs. The significance and possible functions of male behaviour are discussed in relation to mating in Glossina and other Diptera.