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THE "NATURAL Balance" BETWEEN GLOSSINA PALPALIS AND ITS HOSTS.

48. If food and shelter are in any locality good the adult flies are content to If good breeding grounds are remain, but cannot always reproduce successfully. also present the locality is wholly favourable to increase, and unless this is checked in some manner the hosts would eventually suffer, or be forced to leave the locality. If they went to other localities where there were no breeding grounds, the fly would perforce starve or follow, and if it followed its numbers would rapidly decrease. Then the hosts could return and the whole series of events be repeated. It was, at one period, thought that the equilibrium between parasite and host was thus kept stable.

*

49. Continued study convinced that the balance is much more delicate in its adjustment, for, given opportunity, it is the fly which goes, not the host. The females seek breeding grounds, and hosts are frequently more common at breeding grounds than elsewhere, so that there is always a tendency for flies to concentrate in the neighbourhood. But soon as they are willing to feed they begin to rove, and a dispersive movement begins. If a strange inanimate object is encountered by the roving flies or an animal of a species not particularly attractive as a host, or an animal favoured as a host which behaves strangely or impatiently (as have ante lope where fly was very common, or cattle assailed by Tabanids, or crocodile or varanus tethered by a cord), this dispersive movement is only slightly interfered with, but if a host in all respects acceptable is encountered it is checked, a follow- ing swarm of flies collects, and, provided shelter is also acceptable, reconcentration begins.

50. The dispersive movement from good breeding grounds is extensive in pro- ption to scarcity of preferred hosts. In some localities the average duration of food-hunting flights may be only fifteen or thirty minutes; in others, two or more days. In the one instance, dispersion is probably overbalanced by the concentrative movement to the breeding places-in the other, it certainly overbalances any such concentrative movement. It is also extensive in proportion to intolerance of the individual hosts of a favoured species which chance to be encountered. The annoy- ance displayed by the cattle under attack of Tabanids was sufficient to prevent most It was the same Glossinæ from feeding, and to start them on their roving anew.

It would happen in any with the young bullock which objected even to Glossina. locality where Glossina had increased to an extent to cause manifest annoyance to hosts formerly complacent. In this manner, it is believed, one of the most delicate of "natural balances" between the biting fly and its vertebrate host is established, which often works out to their mutual advantage.†

51. This theory was in part suggested, and latterly in part confirmed, by inves- When a large number of tigations into the curious phenomena of sex disparity. flies is captured it is unusual for the sexes to be evenly represented. The percentage of females ranges from as low as 1 or 2 to as high as 80, and a high or low percentage is as much a characteristic of a given locality or island as few or many flies. By a series of experiments it was conclusively demonstrated that these varia- The tions in sex ratio are primarily due to a difference in habit between the sexes. males are normally active during good weather and easily caught, whether or not willing or desiring to feed. The females are normally inactive, and may only be caught readily when willing or anxious to feed. In consequence the percentage of females amongst caught flies may indicate:--

(a) The percentage of the fly population which is willing to feed and would

do so if a complacent host of a preferred species is encountered..

(b) The average duration of the food-hunting flights, in point of time, and

thus to the relative tendency to dispersion.

(c) The relative as well as actual abundance of acceptable hosts.

52. This applies to a general, not too extensive, region, but not to specific locali- ties within this region. Experiment has shown conclusively that the active flies are constantly roving about, but that the satiated males are much less inclined to wander

* It may be prevented from migrating on islands.

+ The large and increasing numbers of marsh buck on the islands certainly owe their existence to their willingness to serve as hosts to Glossina palpalis, if, as believed, the species is the one wild reservoir" of Trypanosoma gambiense native to the region applies to other antelope in con- nexion with other species of Glossina and of Trypanosomes.

...

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than food-hunting individuals of either sex. Apparently such males will never leave good shelter to traverse localities where shelter is repulsive (such as papyrus- fringed shore) or where it is quite lacking, and their movements are checked, though not inhibited, by short reaches of unattractive shelter. Food-hunting flies, on the In consequence other hand, quite freely traverse uninviting shelter or open shore. the sex ratio amongst flies caught at any particular point becomes, in addition to the above,

(d) An index to the proportion of hungry roving flies, with females

pre- dominating (of which both sexes may be caught), satiated, resident flies (of which only males may be caught),

(e) To the relative attractiveness of shelter at the precise point where the

catch is made.

53. This seems to be rather a lot of significance to attach to what might, at first glance, seem like a trivial bionomic phenomenon, but in the course of the inves- tigations many hundreds of records have been made which have conformed to the conditions thus implied with a consistency which would be surprising if it were not so logical. Latterly the work was much more carefully done than at first, and it has invariably worked out as follows when catches of fly are made at frequent intervals along a reach of varied shore a few miles in length:

(a) Wherever shelter is most attractive the maximum number of males and minimum percentage of females for that limited region will be found. The percentage of females may vary from 1 or less to as high as 50, dependent upon the abundance or scarcity of hosts in that region. (b) Wherever the shelter is least attractive the minimum number of males and maximum percentage of females for that region will be found.

(c) Wherever it is positively repulsive, such as outside papyrus or saw- grass, the maximum percentage of females ever encountered is approxi- mated irrespective of abundance of food or percentage elsewhere. (This has invariably been above 60 per cent. in catches of ten or more flies; generally above 70 per cent., occasionally 80 to 84 per cent.)

(d) Whenever the percentage of females is high in the first instance (a), the number of males will be relatively high in the second and third, (b) and (c). That is, a larger number of flies is forced to cross obstacles which would ordinarily impede dispersive movement, in regions where

food is scarce.

54. By stationing boys at various spots in a locality (each with his black umbrella, to which most of the passing flies are attracted, and of which he catches nearly all), the routes followed by the roving flies have been mapped with some exactitude, and by placing several boys along a route thus indicated it is easy to ascertain the direction from which they come. Ordinarily they follow the foreshore of the lake, edge of woodland, and open glades or trails through it, or pass from opening to opening in dense bush or forest. They seem always to follow something, if it is nothing more than the edge of the water where open grass land comes down to it, and they appear not to strike boldly across either open water or open grass unless shelter is plainly visible on the farther side. Other things quite equal they will follow the sun,* passing to the eastern side of a small island in the morning and western in the afternoon.

55. They turn on their course rather frequently. Sometimes they are naturally led in loops and circles by intersecting game trails, etc. A part will always turn when repulsive shelter is reached, and the longer it continues the more will turn. But no reach of repulsive shelter or open shore found on the islands was long enough to stop them all. "Six hundred yards of sheltered shore was quite freely traversed. and continuous belts of papyrus two miles in length were also traversed on the land if not on the water side. Under favourable conditions, such as might be supplied by indefinite reaches of fairly attractive and uninterrupted shelter--and it is difficult to estimate the limit to such flights-ten, fifteen, twenty or more miles may not un- reasonably be expected.

56. That which seems most frequently to interfere with the proper working of the mechanism which actuates the natural balance is insularity, whether geo- graphic or biotic. If an island is too well supplied with breeding grounds the fly may increase to the point of driving away its hosts, and actual starvation may ensue, accompanied, it may possibly be, by racial degeneracy. One such island is known.

This point was not investigated thoroughly with respect to movements on large islands.