A REVIEW WITH HYPOTHETICAL CONCLUSIONS. Ill 



(3) The disparity between sexes in the catch seems to be a fair index of the 

 disparity prevailing in the locality. 



(4) The superabundant individuals reared undermost favourable conditions are 

 disposed of by migration of the females, and the disparity between the sexes is 

 thus to be explained. 



(5) These migratory flights of the females are quite distinctly different from 

 the ordinary goings and comings in which both sexes participate. They probably 

 take place during the period following eclosion and fertilisation and preceding 

 full sexual maturity. The general tendency is to pass from more to less favourable 

 breeding grounds. 



(6) There is no evidence as to the extent of migratory range. Wide extents 

 of water or of open country probably offer no obstacle to flights of this 

 character. 



(7) Through possession of such habits the species would theoretically be 

 enabled to dispense with any other element of " facultative control " over 

 increase. The failure to discover efficient parasites, pathogenic micro-parasites, 

 specialised predatory foes, or other natural enemies capable of exercising such 

 control, might thus be explained. 



(8) In the absence of efficient facultative control, it must be assumed that, in 

 general, localities where males predominate are favourable and where females 

 predominate unfavourable to increase ; that the species maintains itself in the 

 latter only through immigration, and that it would become locally extinct were 

 this to cease. 



(9) When localities actually favourable to increase are compared with localities 

 actually unfavourable it is not improbable that points of difference will be 

 discerned, which may be of value for economic application. Failure in the past 

 to discover constant points of difference between given localities that are fly-free 

 and fly-infested is not improbably due to the fact that both may be equally 

 unfavourable, but that certain localities are fly-free simply owing to their distance 

 from really favourable localities. 



(10) The extent to which these tentative conclusions regarding the bionomics 

 of G. palpalis in a certain geographical region will apply to the same species in 

 other regions, or to other species, is wholly problematical. Different species will 

 probably differ bionomically in detail, but the broader general principles of most 

 favoured hosts, of most favourable type of breeding ground, of automatic 

 control over increase through emigration, etc., if substantiated for one species, 

 will probably apply to all nearly related or perhaps all species in the genus. 



31343 B 



