CO885-(21-23) — Page 544

PUBLIC RECORD OFFICE

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Reference :-

C.O. 885

22 PUBLIC RECORD OFFICE, LONDON

ALLY WITHOUT PERMISSION OF THE BE REPRODUCED PHOTOGRAPHIC- COPYRIGHT PHOTOGRAPH-NOT TO

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repeated-which gave rise to these questions, and answers to them were sought-for a long time in vain. No, conceivable hypothesis fitted with all the recorded and unchallenged statements.

Finally, however, it was discovered that, if only two of these generally accepted statements as to the habits of the fly were considered as based on insufficient evidence, an hypothetical bionomic scheme might be devised which would supply an answer to all three questions, and which would also be in accord with everything else recorded of the East African (not necessarily the West African) race of Glossina palpalis.

The two bionomic characteristics which had to be challenged were:-

(1) That the fly does not freely cross a wide expanse of open country or large

bodies of water.

(2) That it favours a diet of mammalian blood above one of reptilian or avian. Elsewhere, and at considerable length, the writer has set forth this hypothetical scheme of bionomics, and has supported it by recorded observations and analogy in a manner measurably satisfactory to himself. It leads to the following tentative conclusions:-

Glossina palpalis does freely traverse, under certain circumstances, wide expanses of water, and, inferentially, of open country.

It finds by far the most favourable conditions for rapid reproduction in a type of locality represented by certain islands in the East African lakes and unhabited portions of their shores, where its diet must consist principally of avian or reptilian blood or both.

The great disparity of the sexes and preponderance of males in such localities As in many other is due to the migration of the females to the mainland or inland. insects, these particular flights appear to be governed by a specialised colonising or emigratory instinct, which is presumably incited (as in the swarming of bees) by local congestion. The trend is thus, somewhat paradoxically, but perfectly logically, from more favourable to less favourable breeding grounds. In the latter the sexes are even or females predominate.

The fly, through such automatic control over its abundance, is able to avoid the disastrous results of overpopulating a too favourable locality. The absence of the much sought and never found specialised pathogenic parasite is thus easily accounted

for.

Somewhat conversely, through constant influx of immigrants from more favour- able localities, consisting of females, young, ready fertilised (and perhaps ready infected with pathogenic trypanosomes), the fly is able to maintain itself permanently in considerable abundance in localities from which it would quickly disappear were this flow of immigration to cease. Its absence from other, similarly, and actually, as well as apparently, favourable localities, may be due to nothing more mysterious than the greater distance of these from the true breeding grounds.

These conclusions, tentative and unproven, appear, at the same time, to be incapable of being disproven, without new data being accumulated. If they should be established as sound, they would certainly have a bearing on the big general problem.

Such localities as certain islands and portions of the unhabited shores of Victoria Nyanza would thus be proven to he veritable reservoirs of fly from which immense swarms passed continuously to the mainland.

Clearing measures, as undertaken by the Germans on a large scale, would very largely depend for their efficiency on the relation of the cleared areas to the fly reservoirs. No amount of clearing on shore would eliminate the danger as long as the flies continued to breed on the islands, and a reservoir of pathogenic trypanosomes existed there, but clearing of the islands might conceivably result in disappearance of fly from the shore.

In the past an attempt has been made to differentiate between unfavourable and favourable localities by existence or non-existence of the fly, and by comparison between them to determine what surroundings were conducive and what prejudicial When the differenti- to it. No constant points of difference have been discerned.

ation shall have been made along the proper lines it is not unreasonable to expect that constant differences will become apparent. Their character, not improbably, will be suggestive of new methods for the control of the fly, which may be cheaper, or more efficient, or more generally applicable, than anv yet proposed.

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THE BIONOMICS OF GLOSSINA MORSITANS.

Knowledge of the bionomical characteristics of one geographical race of Glossina

is principally of value outside that geographical area as a basis for comparison with known or suspected characteristics of other species or races. At the present time an attempt is being made to review everything recorded of Glossina morsitans for the purposes of instituting a comparison with the bionomical history of Glossina palpalis

as set forth.

Two serious obstacles have been encountered: one, the insufficiency of data concerning the bionomics of Glossina morsitans; the other, insufficiency of data con- cerning the bionomics of Glossina palpalis; or, in other words, the hypothetical character of the standard of comparison.

There are, however, certain points of resemblance and certain points of difference, which may, in part, be presented in tabular form.

Points of Resemblance between Glossina palpalis (East African race) and Glossina morsitans.

Similar process of reproduction. Sexes produced in equal numbers. Blood diet necessary for existence.

Range frequently extended during rains, and restricted during dry season. Marked apparent disparity in sexes locally.

Localities apparently favourable to fly not always infested.

No efficient pathogenic parasites or other specific impediment to increase known

Points of Difference.

Glossina palpalis. Restricted in local distribution to neighbourhood of water.

Distinctly characteristic of equa-

torial fauna.

Glossina morsitans. Generally considered to be independent of water. At all events it is much less dependent than palpalis

Geographic range both north and south of equatorial regions.

Points in more or less doubt.

Glossina palpalis. Appears to find most favourable breeding grounds where diet of avian or reptilian blood only is available.

No evidence to support so broad a statement concerning nocturnal feeding. Available evidence. supports conclu- sion that sex disparity is an actual and not merely an apparent local pheno-

menon.

There is a definite connection between certain types of localities and predomi- nance of males.

Specialised migratory habits are assumed as an explanation of sex dis- parity, and are supported in various ways by other recorded observations.

The males appear to remain at breed- ing grounds to meet the females as they emerge the females to migrate.

There is evidence to support contention that favourable breeding grounds are indicated by predominance of males.

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Glossina morsitans.

Appears not to be found except associ- ated with mammalian hosts.

Under certain circumstances may feed freely at night.

No evidence available to support such contention.

There is no evidence for or against any such connection.

Except by analogy no evidence for or against specialised migratory habits exist.

There is no positive evidence to sup- port such conclusion. On the contrary, it is stated that the male flies, even though they have no desire to feed, are attracted to moving animals for the purpose of meeting the females. This would indicate radical difference in assembling habits and migratory

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habits as well.

There is no known method of differ- entiation.

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