Oviposition by African malaria vector mosquitoes. II. Effects of site tone, water type and conspecific immatures on target selection by freshwater Anopheles gambiae Giles, sensu lato.

Females of Anopheles gambiae s. lat., most of which would have been A. gambiae s. str., were collected from houses in coastal Kenya and tested for their oviposition preferences using Petri dishes in large laboratory cages with lighting equivalent to weak moonlight. Significantly more eggs were laid overnight in water over black than over paler tones, and this difference increased as contrast with the surrounding floor was increased. Direct observation revealed that over white targets, females oviposited from a settled posture, whereas over black targets they did so from flight. The influence on this behaviour of target darkness (tone) overrode that of cage size or target size. In tests which yielded markedly fewer eggs in sea water than in tap water, no significant difference was detected when cage floors were either black or white, although a black floor might have resulted in significantly greater discrimination against sea water had more tests been conducted. All further testing was done over black cage floors. Turbid water from a natural development site received more eggs than distilled, tap or swamp water, even though the turbid water appeared paler than the others. The females did not discriminate between rearing water and tap water, or tap water with and without pupae, but the presence of larvae was repellent. Turbid water from a development site thus seemed to possess an arrestant property which overrode selection favouring darker targets, and which was not derived from prior presence of conspecific immatures. It is suggested that for A. gambiae, oviposition from a settled posture is a response to sub-optimal stimuli, possibly indicating conditions under which oviposition would not occur in nature, and hence why cage experiments using white targets have in the past yielded confusing results.

Oviposition by African malaria vector mosquitoes. I. Temporal activity patterns of caged, wild-caught, freshwater Anopheles gambiae Giles sensu lato.

Blood-fed females of the Anopheles gambiae species complex, most of which would have been A. gambiae Giles sensu stricto, were collected from a freshwater locality in coastal Kenya and used for laboratory studies of their temporal patterns of oviposition. Contrary to previous reports these patterns were not found to be constrained to the early hours of the night but varied widely, evidently dependent on the time of blood-feeding and on subsequent temperatures but not on endogenous activity rhythms of any circadian nature other than daytime inhibition. After two days at mean temperatures ranging from 26 X 25 to 28.1 degrees C the combined results of ten nights showed a build-up from dusk to a peak in the hour following midnight, falling more steeply towards dawn. In combination with patterns derived from mosquitoes re-fed at 20-21.00 hours and 05-06.00 hours this indicated oviposition with a mean 44-47 hours after blood feeding. Considered in the context of available relevant field evidence, temporal patterns of oviposition by A. gambiae in the field in the warmer lowlands of Africa would be expected to show an initial, more abrupt peak in the early night (contributed mainly by females which had failed to oviposit on the night when first due), followed by a more diffuse peak. Patterns would be dependent on local conditions of blood-feeding, temperature and distance from daytime resting sites.

Yellow fever and Zika virus epizootics and enzootics in Uganda.

Data of monkey serology are presented which, together with past evidence, support the view that yellow fever (YF) virus circulates in its primary sylvan host populations, i.e., forest monkeys, in an enzootic state in Bwamba County in western Uganda but as series of epizootics in the forest-savanna mosaic zone of central Uganda. Evidence of an epizootic of Zika virus at the Zika Forest near Entebbe is described which occurred in two episodes, the first (in 1969) apparently following the build-up of non-immune monkey populations since a previous epizootic of 1962-63 and the second (in 1970) when Aedes africanus biting densities rose. This was followed only 18 months later by an intensive epizootic of YF virus, contradictory to the hypothesis that Zika virus alone would suppress subsequent epizootics of YF virus in nature, at least when redtail monkeys are involved. Conclusions are finally reviewed in the light of more recent evidence of transovarial flavivirus transmission in mosquitoes, pointing out that phlebotomine sandflies also require fresh attention.

Paederus (Coleoptera: Staphylinidae) in Uganda. I: Outbreaks, clinical effects, extraction and bioassay of the vesicating toxin.

A brief review is made of the extensive, scattered and sometimes conflicting literature concerning outbreaks and effects of the vesicating Staphylinid beetles of the genus Paederus and their toxin. Observations are reported on outbreaks of Paederus sabaeus and Pachypaederus puncticollis which occurred in Uganda in 1961-62 and subsequently. Clinical effects seen in Kampala are described and a case of ocular involvement is reported in detail. It was found that the cuticle of P. sabaeus must be broken and haemolymph released for any reaction to follow, that the mucosa of the mouth is relatively little affected by the toxin, and that Pachypaederus puncticollis possesses similar vesicating properties to the Paederus species. Methods of extraction of toxin from the beetles using various solvents and a bioassay technique based on mean daily scores of reaction on mouse ears are described. Problems of treatment are discussed.