PUBLIC RECORD OFFICE
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Reference :---
། ། ། ། Hmmm C.O. 885
24 PUBLIC RECORD OFFICE, LONDON
ALLY WITHOUT PERMISSION OF THE BE REPRODUCED PHOTOGRAPHIC- COPYRIGHT PHOTOGRAPH-NOT TO
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rarely even bitten; the goats were less attractive, and were also occasionally bitten (enough, probably, to have been infected had the flies carried a virus), but would all but invariably dislodge the fly before it engorged. Young pigs were generally phleg- matic, and, side by side with goats and varanus, were fed upon much more and much less freely respectively; but European pigs, run wild on several islands, are favoured as hosts, apparently to the same extent as antelope. Crocodile, varanus, and ante- lope have been observed at close quarters, while quite free and unrestrained, and generally while unaware of the observer's presence, and with a few exceptions (varanus once and antelope on two or three occasions) have been indifferent to the presence of fly.
12. Many flies may feed upon man if he is sleeping or utterly indifferent, but If he is awake and not indifferent, not so never as many as on a preferred host. many will attempt to feed and very few or none will repeat the attempt at all per- sistently except such as are ravenously hungry. Their behaviour when really hungry Then they will return to the attack again is in striking contrast to the ordinary.
and again, and nothing short of capital measures will persuade them to desist. They will also follow man long and persistently, and have even been known to do so after darkness has fallen.
13. Upon three occasions* only have such flies as these formed more than an Under the cir- insignificant proportion of those infesting the localities visited. cumstances ordinarily attending these investigations man is neither sought after by the flies nor acceptable to them as a host, and this applies to natives and Europeans alike. To what extent the behaviour of the natives has been modified by their know- ledge that the fly is the vector of a dangerous disease is not known. At all events, they do not carelessly and needlessly expose themselves to it, although, on the other hand, they do not particularly trouble themselves to avoid it.
THE FOLLOWING HABIT.
14. It is normal for Glossina palpalis to collect in large "following" swarms in the presence of its favourite wild hosts, when they are free and unrestrained, but it will not behave in this manner toward man (as does Glossina morsitans) nor toward domestic animal habituated to the presence of man (so far as observation
any goes), nor, except on rare occasions, toward individuals of its favourite hosts when tethered and struggling at intervals to escape. In consequence, the following swarm is not as easy to study as it is in the case of Glossina morsitans, and for a long time its existence was overlooked. The principal facts concerning it, and the behaviour of the flies toward various animate and inanimate objects, may be briefly reviewed.
15. For some time (two or three days?) after flies have fed they do not care to feed again.
As soon as they are willing, and before they are particularly anxious to do so, they will leave their usual haunts and rove forth in search of a host. These Hights may be very extended, and nearly always follow a definite route, notably the shore of the lake. Any unusual object disposed in the line of flight excites the curiosity of those passing. It may be a conspicuous boulder or stump, a coat thrown on the ground or over a bush, or merely a place where the herbage or shrubbery has been broken or trampled down, but nothing serves more effectively (to be quite explicit) than an opened black umbrella (whether it is stationary or moving makes no difference). This, and the number of flies attracted to it in a given space of time, has thus come to be a convenient standard for use in comparative studies.
16. The flies drawn to an umbrella thus exposed in their line of flight alight upon and investigate it very thoroughly, inside and out, linger about for two or three minutes, and pass on. No matter how long it is displayed the number resting upon it at any one time quickly reaches its maximum. They are continually going away and others coming. This is easily proved by setting a boy to catching them, for he can continue all day without the catch per hour falling off (and the hourly catch will be many times greater than the number of flies resting on the umbrella at any one time).
* Once on an island from which wild hosts had been purposely driven; once on a peninsula where camp had been made and from which wild hosts had been sccidentally driven (and from which the flies eventually drove the camp): once on an island from which wild hosts may, possibly, have been driven by superabundance of fly.
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17. It is not the fly boy, or the person observing, any more than the umbrella, which attracts, for, if the two are separated, as many (and perhaps more) flies come to the umbrella. More than that, the behaviour of most of them toward a man very closely resembles that of all of them toward the inanimate object. The writer has exposed himself for hours in a pathway along which flies, at the rate of twenty per hour, were passing, and purposely refrained from brushing them away, yet was not bitten once. Apparently, all the flies which passed paused to look him over, found This particular occasion was hardly him not to their liking, and passed on.* typical, however, for usually a small percentage would have attempted to bite.
18. In most of the experiments with animals exposed in the line of flight, the behaviour of the passing flies has been much as above described, except that more will feed upon the more favoured hosts. The fact is that an individual of even the most favoured species is not wholly acceptable as a host if its actions are not normal; but invariably on the numerous occasions when a crocodile, varanus, or antelope has been stalked, flushed, or shot under the proper conditions (in an infested locality, on a fair day, not too early in the morning, etc.), it has been found to be accompanied or followed by a swarm of flies, such as no animal under continuous observation-
On many occasions much less man-has ever been known to bring together. varanus, suddenly surprised while basking on the shore, or crawling slowly through the bush or open grassland, has scrambled rapidly away, and eluded the follow- ing swarm. The flies composing it have risen in the air, swept back and forth over the spot, or circled about the intruder, always emitting a peculiar buzzing, not unlike that of angry bees, and never heard at other times. The same phenomenon has been observed when a basking crocodile has suddenly plunged into the water, or a startled Invariably the swarm has quickly scat- antelope bounded into a dense thicket. tered, and it is unusual for even a single fly to accompany the man away from the spot.
19. The swarm possesses characteristics which are peculiar to itself rather than to the individual flies composing it; that is, the behaviour of the flies while swarming is different from their behaviour at other times and in the presence of other hosts. If the swarm be not eluded it will follow the host with remarkable persistency. One of the largest ever seen was on an antelope, which, after being shot, had run for nearly a hundred yards at great speed before falling, but it ran through open forest."
20. The antelope on the islands will not infrequently accept the presence of man without concern, if they have not scented him, nor seen him moving, and the writer has thus been enabled to observe the following swarm on animals quietly grazing only a few feet distant. More flies rested on near-by grass and herbage than on the animal itself, and relatively very few were feeding. On such occasions none have ever approached or annoyed the observer. Having found their host they are content to remain in its vicinity, and are in no haste to feed.
21. But they are, none the less, willing to feed. for (on other occasions) when an antelope has been shot it has been usual for about half the swarm to drop and engorge simultaneously upon its body.
22. On several occasions the writer, while hunting through bush or forest, has noted the number of flies which seemed to be following him, and, having shot his antelope, counted or estimated the number upon it. (The number, of course, varies with the locality. The largest swarm ever seen was estimated at seventy-five, the next largest at fifty). On every occasion the number which had really been following the antelope was more than twenty times greater than the number which had seemed to follow the hunter.
23. On these and other occasions a trained boy has been set to catching flies in the locality where the antelope had been shot. The number in the swarm follow- ing the antelope has always been two or more times greater than the boy could catch in an hour. In several localities where a few flies have been found on antelope, several boys have been unable to catch any in more than an hour.
24. There can be no question that to form a following swarm is the normal Lehaviour of Glossina palpális in the presence of acceptable hosts. It can and will feed upon man, cattle, and the tethered and very uncomfortable varanus or crocodile. but it does not act toward them as toward its real hosts, under their normal
* To a boy with an umbrella who caught them. Others boys, beyond, caught all that came from
the opposite direction, and the count was thus made.