CO885-(21-23) — Page 545

PUBLIC RECORD OFFICE

Reference :-

TLC.O. 885

22 PUBLIC RECORD OFFICE, LONDON

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The females are presumed to fly for long distances across open water or tree- less plains away from favourable breed ing grounds.

Reasonable explanations for absence of specific pathogenic parasites or other natural enemies is provided.

Reasonable explanation of absence of fly from localities apparently favourable is provided.

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There is no evidence that either sex ever does this, except through analogy (and one very ephemeral and unconvinc- ing argument in addition).

There is no explanation possible except by pure analogy.

There is no explanation for this phenomenon.

NOTE. The lack of specific information at once suggests specific lines of research these will not be recapitulated.

The questions arising as a result of lack of information are multiplied because of the conflicting character of much that is at hand. This is well illustrated by that which has a bearing on feeding habits.

The tentative conclusion arrived at by the writer that East African palpalis is more largely dependent on avian or reptilian than on mammalian blood ŝuggests that morsitans may exhibit a somewhat analogous partiality. Various minor pecu- liarities in feeding habits are often associated with the character of the favoured host. It is these which are so confusing.

Most observers agree that Glossina morsitans is attracted to moving objects; that more will settle on a man or animal when walking than when sitting or standing. This might indicate that the most favoured hosts were large diurnal vertebrates which were attacked while passing the haunts of the fly.

Other observers have stated categorically that the fly is not attracted to moving objects, but that it attacks as freely, or more freely, a man while standing or sitting than while walking. Inferentially, such flies would be accustomed to seek their prey while in repose-as nocturnal animals in diurnal retreats.

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In certain localities it has been noted that the fly crawls under the clothing- In other localities this has not been noticed, up the trouser's leg to the knee." and one commentator has definitely stated that it is a local characteristic. It is somehow suggestive that such flies are habituated to an avian host. They must have learned the trick in some manner, and none other than through crawling between the feathers or under the wings of birds comes to mind.

There are some very remarkable observations on nocturnal feeding. According to one, a large swarm of flies made a vicious attack on a pony and rider, late at night, in a locality which was apparently fly-free during the day. This, in connection with diurnal quiescence, would seem to imply habitual attack on night-roving hosts. On the other hand, after a year's continual residence in the midst of morsitans Kinghorn and Yorke state that certain small animals, being nocturnal,

are there- fore not subjected to" its bite, and, but for the few perfectly explicit statements to the contrary, these conclusions would be supported by the evidence.

Several observations are recorded indicative of a gregarious tendency, and a habit of hunting in swarms. This might imply that gregarious and nomadic animals were preyed upon, baboons for example. But since there are but few comments on what seems to be a fairly striking phenomenon, when it is actually encountered, it must be presumed to be unusual.

In some localities the fly is not freely, or not at all, attracted to larger vertebrates, whether standing or moving. In one such, its presence would have escaped notice but for mere chance. Surely these flies must have had an abundant source of food quite different from the large diurnal vertebrates on which they are supposed to feed.

All the preceding conflicting observations have been supported by more than one independent observer. Passing over those not so well supported, there comes finally the curious controversy which has arisen over the relations between morsitàns and game animals. Opinions seem to be about equally divided, and partisans on both sides are equally emphatic in their convictions. To say that either party was right would almost be the equivalent of saying that half the recorded observations upon morsitans were unreliable.

The truth not improbably lies half-way between; both sides may be right, and vet both wrong, each in thinking that the other must be so. No other basis for judgment exists than the more or less casual observations made throughout the length and breadth of the entire geographical range of morsitans. As an attempt has been made to show, these observations are frequently so contradictory as to leave none but

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the middle way open. The only safe presumption is that in certain geographical regions the local race favours certain hosts; in other regions, other hosts. The fine points of difference between local distribution of vertebrates in relation to suitable breeding grounds for the fly may easily account for this, and a change in habitat, or the elimination of these particularly favoured hosts, might react most disadvan- tageously upon the fly. It is easily possible that, under certain circumstances, dis- appearance of antelope, or even of a few species of antelope, might result in dis- appearance of the fly from certain regions or localities. In another locality the antelope may have no such intimate connection with the fly, and their rôle might be taken by wild swine* or birds or buffalo or baboons, and so on to the end of the list.

The conflict of evidence concerning seasonal movements and breeding grounds is almost as marked. Some observers state that the fly extends its range notably during the rains, but the opinion is not universally held. It is contended by some that its numbers diminish with the coming of the rains; by others that they increase. Some very good notes have been made on breeding places, but they lose a good part of their value through not being intimately enough associated with notes on annual movements of the fly in respect to these particular places.

It is certain that in some regions, at least, the fly extends its range during the rains, and wherever this holds there is an opportunity to differentiate (as with palpalis) between permanent and temporary haunts. But with palpalis it is reasonably sure that the fly finds conditions made the more favourable for continued existence and rapid increase in the permanent haunts, and there is nothing but analogy to support such contention with regard to morsitans. It is easily possible that this species should breed to so much better advantage during the rains in its temporary haunts as to keep constantly populated its permanent haunts, which might offer but indifferent opportunities at any time. This is not probable, it is Whether merely possible; there is no evidence at hand to controvert the statement. any particular type of locality is likely to prove a permanent reservoir for morsitans, as is the case with palpalis, remains to be seen. It is conceivable that two quite different types may be essential for morsitans—one as a dry season residence, the other as a wet season breeding ground.

Without knowing what are really favourable and what unfavourable breeding grounds it is impossible to guess whether such specialised migratory or colonizing flights as are attributed to palpalis are also characteristic of morsitans. They may be neither necessary nor desirable for the good of the species. If neither, then by the terms of their definition they would not be made. On the other hand, the character- istic is not illogically associated with several other characteristics possessed by palpalis and morsitans in common, and if proven to be actually characteristic of palpalis, may be expected of morsitans. Even though the flights themselves should not be observed, it would not preclude the existence of the specialised instinct which lies at the back of them, and which may remain dormant for many generations, as is known to take place in other insects.

If morsitans, possessed of such instincts, follows the lead of a vast number of diverse insects similarly incited from time to time to undertake unusual and extended flights, it would probably result in scattered individuals wandering into open country and elevated regions far beyond the recognised limit of the fly-belt. It would be very difficult to prove this directly. It would be indicated by the occurrence of sporadic cases of trypanosomiasis amongst cattle pastured outside of the fly regions; perhaps it has been thus indicated. If it were actually true, then as herds of cattle increased outside the fly-belt but within the migratory range of the fly, the danger of infection would increase proportionately. If the parasitic protozoa are likely to be transmitted, mechanically, by other biting flies, then the danger of losses to stock would increase disproportionately to the increase of stock. The point seems worthy of especial investigation on this score alone.

LINES OF BIONOMICAL RESEARCH SUGGESTED BY REVIEW OF LITERATURE, It seems so desirable to secure a knowledge of the migratory habits of Glossina- of any species of Glossina-that the writer is inclined to the opinion that any It appears certain that Glossina morsitans feels to some considerable extent on wild swine owing to the frequency with which it is infected with Trypanosoma simine, of which no other natural reservoir is recorded. It is inferentially certain that the receptivity of the flies to this trypanosome is notably greater than to Trypanosoma rhodesiense, or else they feed more freely on wild swine than on antelope,

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