PUBLIC RECORD OFFICE
Reference :-
C.O.885
18 PUBLIC RECORD OFFICE, LONDON
ALLY WITHOUT PERMISSION OF THE BE REPRODUCED PHOTOGRAPHIC- COPYRIGHT PHOTOGRAPH-NOT TO
20
there can be no doubt, however, that, wherever flies have been seen in considerable numbers, some pool or stream has existed near at hand.
Possibly the natural range and certainly the "following" range is increased in the wet season when the undergrowth is more luxuriant. The latter is also wider in thick jungle or forest and in dull weather, and narrower in the open and in bright sunlight.
I have elsewhere recorded (Note I.) that, on approaching a fly-area, flies were rarely met with more than 50 yards from the water, but that, on leaving such an area, a few may follow for longer distances up to 300 yards. This latter and also somewhat greater distances have been recorded during the present investigations for following "flies," the "following," in the case of these longer distances, being generally through scrub or some sort of continuous or slightly intermittent shade. An artificial shade, such as an umbrella, a wide-brimmed hat or a pail of water carried on the head, will sometimes be taken advantage of by a fly, especially when gorged, and it might possibly remain in its retreat, if undisturbed, over a distance of several miles. All such cases are, however, exceptional, and only serve to account for the occasional observation or capture of flies in unusual or unexpected places.
The "following "-range is, no doubt, also influenced to a great extent by the conditions of food-supply. For instance, at uncleared forc-shores where there is frequent human traffic through wide belts of jungle, conditions which obtain at many native markets on the shores of Lake Victoria, the range is apparently wider and the flies exceptionally numerous, as was noticed and recorded by Dr. Wiggins, after he had examined many of these markets in Usoga. This is so, no doubt, because the food-supply is constant, plentiful and easily obtained, and since the flies can follow inland for long distances in the shade, their opportunities for feeding are at a maximum. A like increase in numbers and in range would prob- ably be found at habitual drinking-places of herds of cattle or game, at any rate I have noticed frequently that swarms of flies haunt the habitual shore-resorts of hippopotami and crocodiles; but here, strikingly enough, the range is not increased, for, in the case of water animals and those whose excursions on land are almost entirely at night, the opportunities for feeding would occur only at the water-side. In the case of man or land animals, however, where there is frequent or regular traffic to and from the water by day, the roads or tracks, and sometimes the bush generally, might become infested by "following" flics; for gorged flics, having fed at some distance inland, would often retire to the nearest shade to digest their meal and might even feed again before returning to the water, but this would not occur to any great extent except in damp weather and where the scrub or some sort of shade was practically continuous for some distance inland from the water-side.
I think it highly probable that all flies encountered at a greater distance from water than about 80 yards are "following" flies; that is to say, they either are following or have followed persons or animals that have passed through their natural range. Where flies are exceptionally numerous and the conditions favourable, as in the wet season in a country thickly covered with scrub, and especially where there exists also, perhaps, a diffused condition of traffic, owing to the absence of open roads and a multiplicity of native paths through the bush, as was the case in South Usoga at the time of Dr. Wiggins's investigations there, it is quite possible that flies might be met with in such numbers as to make it appear that the long distances from water at which they are found constitute their natural range. And so, indeed, they do, to all intents and purposes, so long as such conditions are left untouched. But the point to which I wish to draw attention here is that wide ranges such as these can be abolished, just in the same manner and just as readily as the narrower ones, by dealing with the natural range alone. Take, for instance, a native market 300 yards from the lake shore, the canoe landing for which is in a fly area from which the shore fringe of jungle reaches inland for half a mile. The natural range from shore is probably really the average one of 10-30 yards, but, owing to the special conditions prevailing, many flies are met with, not only outside this, but also in and beyond the market itself. Clear the landing place and 100 yards beyond it on each side, along the foreshore to a width of, say, to be safe, twice the average natural range, which would amount to, at the most, 60 yards. Flies would then disappear not only from the cleared foreshore, but also from its hinterland of scrub, which includes the market, since they cannot remain away from water. Moreover, people coming to the market by canoe would no longer
21
encounter flies at the landing and, therefore, they would no longer be constantly introducing them into the surrounding bush to replace those which had returned to the uncleared part of their natural range. So long as only the cleared landing was used as an approach from the shore the market would, in fact, be now fly-free. The above clearing would be rendered more complete and perhaps safer by the addition of an open track, 20 to 30 yards wide, cut from the centre of the cleared foreshore to the market. Supposing that it were possible, instead of clearing the fly-infested landing, to close this and substitute a fly-free landing near by, the same results would ensue.
Dr. Wiggins's experiences in South Usoga were not only exceptional as regards the experience of the other observers, but were probably quite unusual in the locality itself. The seasonal conditions at the time were most abnormal with excessive rains, extensive floods, and an unprecedentedly high lake-level, so that he describes large areas as being covered with water, which was only hidden by the grass, and in this flooded country, the fly was more or less scattered through the bush over a wide range from the lake, embracing practically the whole of small peninsulas such as Naniumba's, which is several miles across. On several occasions, too, he found these inland flies were more plentiful than at the corresponding lake-shore. This perhaps suggests the possibility of the fly having been actually driven inland from its natural haunts by the high lake-level and, since this has taken several months to subside, there may be reason to hope that some, at least, of the breeding-grounds along this shore have been destroyed and that, in the ensuing dry season, not only the range but the numbers of the fly may be found to have materially decreased.* The natives themselves said that the range had been increased by the heavy rains, and Dr. Wiggins was inclined to agree with them.
Apart from these abnormal conditions the fly is naturally more widely spread
in this part of Usoga, owing to the indented character of the coast, the wide and thick jungle-fringe which borders it, and the numerous small streams which flow through this, many of which present patches of open water. The observations made here should be repeated under normal conditions in order to get a clear idea of the actual state of affairs.
It may be taken as a rule that the fly range is narrow where the forest or bush- fringe is narrow, but the reverse does not always hold good, since shade is not the only factor concerned. It shows, however, that the fly will not follow so far over clear or comparatively open spaces as through bush, forest, or high grass and other undergrowth.
It will be understood, from all that has been said, that the range, taken as a whole and including the "following" range, certainly varies much in different localities and under different conditions. I think it will be found to be practically sound, however, in applying any general preventive measures, to take as a standard the average
"natural' range and to adapt afterwards such a measure to local necessities.
I have mentioned that Dr. Wiggins discovered a small epidemic of sleeping sickness to the south-west of Mount Elgon. He describes it as being situated in the angle formed by the two rivers Marakisi and Rumba (or Chao), both of which enter the Mpologoma eventually, and between the Wehala and the Urororo (Tororo) Hills. This would place it roughly in a line between Mount Elgon and the mouth of the Sio River, about 20 miles north of Lake Victoria and about 10 miles from the mountain. The epidemic was said to be quite circumscribed and apparently completely isolated and to be, so to speak, burning itself out. It had commenced, according to local account, about five years previously, having been introduced from the people living to the west, many of whom were said to have died; and it was evident, from the already large mortality, that it had been of considerable duration. The name of the principal local chief, whose villages were examined, was Kapeto, and the rate of infection was still very high, being, in one village, estimated at about 80 per cent.
No connection could be traced between the fly-areas of this and of the Lake Victoria epidemic, though the epidemics themselves are undoubtedly related, and the smaller probably, though not certainly, of subsequent origin. Nearly all the
Not much is yet known as to the seasonal prevalence or variation of G. palpalis, but it is evident that a season of this kind would, by increasing its range, aid the spread of an epidemic.
!