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The development and survival of immature stages of Bactrocera invadens Drew, Tsuruta and White (Dipt.: Tephritidae), a new invasive fruit fly pest in Africa, was studied in the laboratory at five constant temperatures of 15°C, 20°C, 25°C, 30°C and 35°C and photoperiod of L12:D12. The developmental time of eggs was 5.71 days at 15°C, decreasing to 1.24 days at 35°C. Larval development periods decreased from 35.95 days at 15°C to 6.64 days at 35°C. Pupal development at 15°C took 34.08 days while no adults emerged at 35°C, this being the most lethal temperature. The longest total development period occurred at 15°C (75.74 days) and was shortest at 30°C (17.76 days). The linear model provided a reliable fit of development rates vs. temperature for the immature stages. Lower developmental thresholds that were estimated from linear regression equations for the egg, larva and pupal stages were 8.8, 9.4 and 8.7, respectively. Total degree‐day (DD) accumulation was estimated at 376 DD for development from egg to adult emergence. The highest adult survival given as the mean of emergence from a cohort of 50 eggs occurred at 20–30°C. At the egg stage, survivorship was highest at 20–30°C and at the larva and pupa stages, it was at 25°C. The practical implication of the findings is discussed in relation to mass rearing of B. invadens and understanding its biology and ecology.

The aposematic savanna butterfly Danaus chtysippw seems to be an exception to the rule that Batesian models and Miillerian mimics are not polymorphic. Throughout Asia and in much of Africa the species is in fact monomorphic and polytypic, as expected. It is, however, polymorphic for 3 4 colour genes over a large area of central and eastern Africa, where deviant sex ratios and non-Mendelian segregations also occur. AU-female broods are widespread and frequent, often outnumbering bisexual progenies and giving rise to heavily female-biased populations. Full and partial sex linkage, which is not controlled from the X or Y chromosomes, and male-biased broods also occur. Genetical analysis for the BC-autosome carrying colour genes suggests there are two, probably mitochondrial, cytotypes (microbe-induced early male death syndrome is considered unlikely) and an autosomal, incompatibility (I) gene, two alleles of which are male-specific killers. F2 and backcross matings by females heterozygous at the I-locus give progenies which are either thelygenic, all males dying at or soon after hatching, or bisexual but showing full or partial sex linkage. Male death is attributed to nuclearcytoplasmic incompatibility (NCI). Females achieve reversion from a thelygenic to a bisexual line by mating with males of compatible (maternal) cytotype. A second NCI system causes

Summary The relationship between negative cervical cytology reports and risk of adenocarcinoma of the cervix was evaluated in a case-control study of 113 cases and 452 controls. All cases and controls had received at least two negative cytology reports. There was no significant difference between the cases and controls in the number of negative cytology reports or in history of cervical abnormality; while a test for trend in the time since last negative cytology report was significant (P<0.001), the estimated benefit was very modest. Although the estimates of relative protection were higher in women aged less than 35 years than in women aged (Brinton et al., 1987; Parazzini et al., 1988).Case-control studies have become an established method of evaluating screening programmes. The degree to which adenocarcinoma of the cervix can be prevented by cervical cytology screening has not been well defined. While some published case-control studies have included adenocarcinoma cases in their series (Clarke and Anderson, 1979;La Vecchia et al., 1984;Brinton et al., 1987;Olesen, 1988;Celentano et al., 1989;Shy et al., 1989;Cohen, 1993), the cases have been few in number, constituting only a small minority of all cases. One study of 40 patients with adenocarcinomai found no significant difference in self-reported screening history between cases and controls (Brinton et al., 1987). No validation of the screening history was undertaken, and it appears that the time interval since the last negative Papanicolaou smear report was not specifically sought. Three studies have commented that Papanicolaou smear screening appears to be less effective for the prevention of adenocarcinoma than for squamous carcinoma but have not provided separate analyses for adenocarcinoma (Clarke and Anderson, 1979;Olesen, 1988;Shy et al., 1989).We undertook a case-control study to evaluate the duration of low risk for adenocarcinoma of the cervix after negative cervical cytology results. Cases were diagnosed mainly in the 1980s when screening was well established in Australia. Adenocarcinoma of the cervix constituted 13% of all cervical malignancies at the time of the study (Giles et al., 1992

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Samples of'the polymorphic buttcrfly Dunnus c.hys$qus arc analysed from six well separated sitrs in East Africa. Morph-ratio clines are described for four diallelic genes A, B, C: and I., each o f which influences the visual phenotype. Each of the four clines has a diircrcnt orientation, consistent with an hypothesis that the polymorphisrri originated from hybridization between a ii~irnbcr ofpolytypic denies which have at various times undergone range expansion. Allopatric suhsprciation in isolated Pleistocene rcfugia is postulated. Thc phenotype of each geographical race is shared with one of the moiphs within the hybrid zonr; other sympatrically miiintained polymorphic forms arc normally confined to the hybrid zoiic. Wright's isolation-by-distance model best explains the prcsent disrribu~iori o f p i c frequencies. Morph-ratios difler significantly bctwecn the sexes and are sornctiinrs associated with hctcroxygote excess; garnctic and genotypic disequilihria are general throughout the region and suggest the clines are maintained by strong natural selection. Seasonal cycling of phenotype frequency is believrd to result from extrnsive misgrgratory rriovenients rat her than natural selection. Female-biased sex-ratio, which is also seasonal, and Haldaric rule cffects, result from hybrid breakdown when genetically distinct drmcs meet and interhreed. Oscillating sex-ratios and frequency ofcolour genes are functionally (:orrespondciice to: I>r 1). A. S. Smith. 0024-4082/97/05005 I + 28 $25 00/O/zj0(10073 51 0 1997 The Linnean Sotircy of 1,mdon 52 L). A. S. SMI'I'H B'TAL.linked by negative feedback. The polymorphism owcs its origin t o allnpatric evolution but is now mairitaincd sympatrically. 0 l9!17 ' I ' h 1.innean So< irly 1~i'I.i~ndnn ADI)I'FIONAL KEY WOl

Subspecies chrysippus, dorippus and alcippus of the butterfly Danaus chrysippus differ at three biallelic colour gene loci. They have partially vicariant distributions, but their ranges overlap over a substantial part of central and East Africa, where hybridism is commonplace. We now report that the West African subspecies alcippus differs from other subspecies, not only in nuclear genotype but also in mitochondrial haplotype in both allopatry and sympatry. The maintenance of concordant nuclear and cytoplasmic genetic differences in sympatry, and in the face of hybridisation, is prima facie evidence for sexual isolation. Other evidence that suggests alcippus may be isolated from chrysippus and dorippus include differences in sex ratio (SR), heterozygote deficiency at one site and deduced differences in patterns of migration. We suggest that, within the hybrid zone, differential infection of subspecies by a male-killing Spiroplasma bacterium causes SR differences that restrict female choice, triggering rounds of heterotypic mating and consequent heterozygote excess that is largely confined to females. The absence of these phenomena from hybrid populations that test negative for Spiroplasma supports the hypothesis. The incomplete sexual isolation and partial vicariance of alcippus suggests that it is a nascent species.

SUMMARYThe polymorphism of Danaus chrysippus is anomalous since it is a distasteful butterfly. In a field study of the B locus polymorphism in Ghana, significant changes in the morph frequencies were recorded over eighteen months at Cape Coast. There were also significant differences between localities. There was some evidence that brown morphs are favoured by wet conditions, and that orange forms have higher survival rates as caterpillars. There were no differences between forms in larval foodplants, sex ratio, body size, wing damage, mating success, or in the effects of weather on adult activity. There was however, strong assortative mating between forms. Crosses with Kenyan females gave aberrant segregations for the sex chromosomes and the C locus. The results are discussed with regard to the hypothesis that polymorphism arose as a result of the overlapping of previously isolated geographical races.