Observations on the Mosquito, Aëdes (Stegomyia) aegypti (L.), in East Africa. II.—The Biting Cycle in a Domestic Population on the Kenya Coast

Abstract

Since the sharply defined biting cycle shown by outdoor populations, in Uganda, of Aëdes aegypti (L.), which closely resembles oviposition and general activity rhythms, could not be reconciled with the ill-defined patterns found in domestic populations, on the Kenya coast, in the absence of knowledge of the composition of the population involved there, 24-hour baited catches were made at Ganda on the Kenya coast, and the abdominal colour pattern of each individual was noted. In the series there was gradation from the darkest to the palest.Of 427 females taken biting, 82·1 per cent, were of var. queenslandensis (Theo.) in the generally accepted sense, 14·5 per cent, of the type form and 0·9 per cent, of subsp. formosus (Wlk.). The material of var. queenslandensis was further divided into two groups in which the lateral spots on the abdominal tergites were of silver and matt white scales, respectively.Very similar biting cycles, with the main peak in the hour before sunset, occurred both inside a hut and within an adjacent compound, although catches were not concurrent and were made under different seasonal conditions.The distribution of the various colour forms in the total catch was independent of the hour of capture.There is a resemblance between these cycles and those reported in comparable situations in Nigeria, where, with the exception of the paler forms, the population of A. aegypti is similar to that at Ganda. It is concluded that the main biting peak is shortly before sunset. Subsidiary peaks, when present, occur with increasing intensity as the day progresses.The relationship of biting activity to other aspects of general activity is discussed, and it is concluded that several aspects require further investigation.The following hypothesis is offered to account for the various patterns of biting cycle observed in different situations. It is suggested that A. aegypti possesses an endogenous multi-peaked diel rhythm of activity. Activity impulses occur in a cycle of waves of increasing intensity from dawn onwards, reaching a maximum at the end of the daylight period, and a biting peak will result when such an impulse coincides with environmental conditions within the range between upper and lower thresholds of activity.