A REVIEW WITH HYPOTHETICAL CONCLUSIONS. 103 



from such an island is more likely to occur than of males to the island, because 

 this latter would seem to infer good feeding grounds and poor breeding grounds. 

 An island in a lake is a small target to hit from the mainland as compared with 

 the mainland from an island ; and, other things being equal, the chances seem 

 much greater of males leaving an island, not to return, than of congregating on 

 an island, as would of necessity be the case if it were the male that migrated. 



The only analogies to such migrating tendencies on the part of the male, in 

 the writer's knowledge, are certain little understood flights of male butterflies. 

 What induces these is not known, and what end they serve is problematical. 

 Probably they are undertaken for the " good of the species," and if we knew 

 more about them we could understand. 



There are many more available instances of female migration, notably amongst 

 the parasitic Hymenoptera. These, of course, are associated with a partial 

 segregation of the sexes, and the males remain in the locality where the females 

 were reared. The females are fertilised as they reach maturity, and with full 

 spermothecae become practically bisexed. They can then wander as far afield 

 as they like and a single individual is capable of establishing a colony, or 

 populating a continent. It is much better " for the good of the species " that 

 the males remain where the females are the most likely to issue, than that the 

 females should remain there and the males fly afield. Males alone cannot 

 establish permanent colonies, except by continual migrations. 



Another point to be considered is the physiological condition of the female for 

 the first two weeks following eclosion. The male is sexually functional from the 

 first, but the female is not. During the first two weeks she has no responsi- 

 bilities whatever, except being careful for the future. Afterwards, when the 

 rhythmic round of gestation and parturition begins, it is very different, but 

 until then she is even more care-free than the males. It is during this period, 

 while she is in full possession of all her powers, except of reproduction, that she 

 may be presumed to fly far afield. 



These various considerations, none of them furnishing overwhelming argument, 

 but all together tending to support the hypothesis, would be sufficient in 

 themselves to convince the writer of its reasonableness, if nothing more. On top 

 of them, however, is a statement by Dr. Bagshawe, contained in a note appended 

 to an account of his second series of flight experiments,'" which is worth quoting 

 verbatim. 



" It is worth noting that while of 1,521 flies caught at certain points near 

 Foweira, only 478 were female while of 129 caught at another point 66 were of 

 that sex. These figures and others in my possession lead me to suspect that as a 

 rule where female files are found in excess the breeding grounds are at a distance. 

 Thus at Harubule (Lake Ruisamba) females were on every occasion in excess : 

 of 1,420 flies caught in a series of flight experiments 827 were female. When, 

 however, I found the breeding ground some hundreds of yards from the scene of 

 previous captures, I caught very quickly 61 male and 45 female flies. At a 

 bridge over the Upanga river, much used by caravans, flies were scarce, but 



* ' Observations Relating to the Transmission of Sleeping Sickness in Uganda,' by Dr. 

 Aubrey D. P. Hodges, S. S. Bull., 1909. 



