The results of a series of catches of biting insects at human bait on platforms in the canopy of the forest at Mongiro, Bwamba, Uganda, are reported. The catches were designed to investigate differences in the numbers taken, due to platforms, season and weather, but mainly to study the short-term changes in the activity of biting insects near the time of sunset. Each catch lasted from 3 hours before to 3 hours after sunset, and was divided into 10-minute periods from 30 minutes before to 1 hour after sunset. The main analysis is of catches made on five platforms on each of 100 evenings which covered the end of the dry and beginning and end of the wet season, 1948, and the end of the dry season, 1949.
A list of the species of biting insects encountered is given. Eight species (Anopheles (M.) gambiae, Taeniorhynchus (C.) fuscopennatus, Taeniorhynchus (M.) africanus and uniformis, Aëdes (M.) nigerrimus, Aëdes (S.) apicoargenteus and africanus, and Chrysops centurionis) were found to be abundant and data relating to them has been subject to more or less detailed analysis. The remaining 27 species or groups of species are discussed shortly.
Six of the eight abundant species show statistically significant differences between the catches on the five separate platforms. Arrangement of the five platforms in order of the total numbers taken, for each of these six species, gives results which are practically identical in three species (Anopheles (M.) gambiae, T. (M.) africanus and uniformis) and in two others (Aëdes (S.) apicoargenteus and africanus) ; however, the arrangements in these two species groups differ widely from each other. The arrangement for C. centurionis differs again.
These ordinal arrangements are compared with the obvious characteristics of the platforms and no consistent relationship is found. It is pointed out that the first three species are derived, largely or even entirely, from breeding places outside the forest, in or associated with the adjacent extensive areas of open swamp, while the last three are likely to be completely confined to the forest for breeding ; both the latter two mosquito species breed in tree holes, and dense shade is characteristic of the breeding places of species of Chrysops closely allied to C. centurionis. It is considered most probable that these platform differences in the numbers yielded are due to local differences in the densities of the adult populations which in turn are related to the abundance and proximity of breeding places.
Seasonal variations in the numbers of the abundant species are also studied and are considered to be almost certainly related to the amount and distribution of rainfall affecting the profusion of breeding places at least as far as the mosquito species are concerned. The tree-hole breeding species, however, suffered a reduction in numbers before the end of the wet season, an effect also noted by Mattingly in West Africa.
The amount of the reduction of the numbers of A. (S.) africanus in the dry season is of importance in assessing the possibilities of the survival of yellow fever virus in that mosquito as a host. The estimated reduction in the dry season of 1949 was to less than one twenty-third of the population existing at the end of the previous wet season (November, 1948).
C. centurionis occurred during all the catch series but was extraordinarily abundant in late March and early April, 1948. It is considered probable that this time of increase represents a yearly period into which hatching from the pupa is compressed. This conclusion is supported by the finding that during this period the biting population was composed of two groups one of which tended to bite before, and the other after, sunset.
Evidence is produced to show that the biting activity of Anopheles (M.) gambiae was depressed by wind, that of Aëdes (S.) africanus by wind and rain but that that of C. centurionis was little affected by either of these factors.
Studies of the biting-activity changes in the nine ten-minute periods extending from 30 minutes before to one hour after, sunset are presented. Each of the species which were known to be mainly active by night and which were sufficiently abundant for study, show activity increasing about sunset, rising to a maximum shortly after, and thereafter declining again to moderate levels. This initial peak is very clearly marked in the cases of Anopheles (M.) gambiae, T. (C.) fuscopennatus, T. (M.) africanus, Aedes (S.) africanus and C. centurionis, less so, but still distinct, in T. (M.) uniformis and A. (M.) nigerrimus. Its position differs in different species being earliest in the case of C. centurionis (18.10–18.20 hours) and latest in that of T. (M.) africanus (18.40–18.50 hours). However, in any given species the peak period of biting activity was subject to only slight movement in time in relation to sunset; in analyses of the data by different platforms, by season, and by types of weather it was the exception to find the time of peak activity to be shifted more than one ten-minute period away from the position which it occupied over the whole series. Further, in studies of the curves of biting activity on individual nights, three consecutive ten-minute periods centred on the period which showed the peak over the whole series, included the peak in 21 of the 23 A. (M.) gambiae cases available, in all five of the T. (M.) africanus, in eight of the 11 A. (S.) africanus and in 15 of the 20 C. centurionis, cases. The regularity of the occurrence of the peak period of activity in any given species is considered to indicate that it is largely controlled by changes in light intensity about the time of sunset.
These findings are discussed in relation to the general problem of the mechanism controlling the rhythm of activity in insects biting by night and a hypothesis, which is based on the conception that different population groups may have different biting habits, is advanced to account for the different types of cycle commonly encountered.