108 WILLIAM F. FISKE— THE BIONOMICS OF GLOSSINA ; 



this is manifested by the growing interest in parasites and in micro-parasites as 

 factors in " natural control " of noxious insects ; in medicine, by the similarly 

 increasing interest in prophylaxis and prevention of disease through the direct or 

 indirect suppression of micro-parasitic organisms which exert the same sort of 

 " natural control " over our own species ; in general economics, through the 

 increasing measure of support continually given to research along the just- 

 mentioned lines in entomology, in medicine, etc. 



In entomology, the writer feels personally convinced, too much importance, 

 relatively, has been devoted to the various types of pathogenic parasitism, and 

 too little to certain other factors in the natural control of injurious insects, of 

 which migration appears to be as important as any. This first became apparent 

 when studying the natural control of the brown-tail moth, an exceedingly noxious 

 insect much given to migratory flights of longer or shorter duration. It was 

 increasingly evident, as the problem of its suppression was studied, that certain 

 natural " reservoirs " existed. These were represented in France by the haw- 

 thorn hedges ; in America by certain types of forest and varieties of trees. 

 In such places the species found all things favourable to increase and from them 

 it spread to infest other generally unfavourable localities — in France, the 

 nurseries from which it was being sent on the young trees to America — in 

 America, to other types of forests, to other species of trees, and even to geo- 

 graphical regions where it might be unable to exist permanently were these 

 reservoirs eliminated. Once the idea was conceived, the general principle 

 involved (of control over increase through migration) was found to be easily 

 applicable to what was known of the bionomics of a wide variety of insects. 

 Nowhere does it appear more closely to fit than to what is known of the 

 bionomics of Glossina in general, and the East African race of Glossina palpalis 

 in particular. It is, however, necessary that the general beliefs that wide bodies 

 of water form natural barriers to migration and that mammalian hosts are 

 preferred by the fly to avian or reptilian be considered as unsupported by 

 sufficiently valid evidence. 



With these obstacles to the validity of the hypothesis of sex migration re- 

 moved, this not only offers an explanation for the otherwise puzzling phenomenon 

 of sex disparity, but it accords well with the remarkably low potential repro- 

 ductivity of the fly — its low " birth rate " — and, better yet, Avith the curious lack 

 of any form of pathogenic parasitism, so far as published records indicate. This 

 seeming absence of parasites is unusual amongst insects, and if actual and not 

 merely apparent and due to their having been overlooked, renders necessary 

 some other form of facultative control ; a lack which may easily and well be 

 supplied by the migratory habits of the female. 



Possessed of this discriminative instinct, the species would be enabled to avoid 

 the dangers contingent upon overpopulation of particularly favourable localities — 

 represented by the lake islands — and to dispose of the superabundant individuals 

 to its own advantage (as a species) by keeping permanently or temporarily 

 populated portions of: the mainland, where, except for the continual influx of 

 immigrants, it would quickly become extinct. It does this without undue 

 sacrifice of its own life-blood to support the parasites which might otherwise be 

 necessary to prevent its increase to such numbers as to destroy its own hosts or 



