Studies on West African Forest Mosquitos

Extract

The present paper is the second of two dealing with field studies carried out near Lagos, Nigeria. The first paper covered four of the most abundant species encountered and the second is concerned with the remaining forty-eight. These are listed and the data relating to the thirty-eight least abundant, with the exception of two which were taken as males only, are given in tabular form. The other ten are dealt with at greater length. Males of twenty-six species were taken and in the case of three of these species they were so abundant as to create the impression that a swarming activity was involved. In the case of the ten more abundant species seasonal distribution curves are given based on average monthly catches and an attempt is made to relate these to variations in rainfall. The biting curves which are next discussed show a greater variety of types than those described in the first paper. In general, they are characterised by a fairly sharp peak associated with morning or evening twilight, but some have a more or less pronounced peak at both ends of the cycle and so correspond to the “eo-crepuscular” type of Haddow (1945). Anopheles paludis, like Taeniorhynchus africanus (described in the first paper) has an ill-defined period of maximum activity during the middle of the night. The necessity of sub-dividing the figures in order to obtain a picture of the vertical distribution of the various species led in one or two cases to anomalous results, but in general a clear and fairly convincing picture was obtained. None of the species dealt with in the present paper was sufficiently abundant to afford reliable data on hourly variations in vertical distribution. In the general discussion, which embodies such inferences as it has seemed permissible to draw from the data, attention is drawn to the fact that the so-called “biting cycle” does not, in all likelihood, depend exclusively on variations in the urge to bite, but is more probably an expression of cyclical changes taking place in the general level of activity of the mosquito. The possible relationship of such changes to changes of a similar kind in the physical environment is discussed, and it is pointed out that intrinsic rhythms in the mosquito, perhaps related to the breeding cycle, may also be involved. Some data are presented which, while partly explained by the presence of a sibling species, may also indicate an interrelationship between the breeding cycle and vertical distribution.