104 WILLIAM F. FISKE--THE BIONOMICS OF GLOSSINA ; 



females relatively numerous, 59 to 16 male. It is certain that this spot is far 

 from breeding grounds — perhaps three miles. 



" Referring to my notes of the flies caught at the spot where pupae were 

 found, I find that in every case but one, males were in excess, and were as a 

 rule two to one. Probably the female must be well supplied with blood to 

 nourish her larva ; if there is a scarcity she must go far afield to procure it. 

 The male could exist with less food and need not range so widely. 



" This is not so unimportant as it at first appears, for in clearing it should be our 

 first aim to attack the breeding grounds ; the sex proportion, if it has the 

 significance which I suggest, will aid to find them. My figures are too small 

 my data too few, to prove anything. Better observers will, it is hoped, follow 

 up these suggestions." 



The plain statement that " as a rule when females are to be found in excess the 

 breeding grounds must be looked for at a distance," amended by the addition of 

 the single word favourable before " breeding grounds," expresses in precise terms 

 the present writer's opinion. The circumstance of so large an excess of males 

 on so many isolated islands, at such distance from the shore as to render homing 

 flights of the females extremely improbable, would seem to be sufficient to 

 overthrow the theory that this segregation is merely temporary and due to the 

 females flying further afield merely for the sake of finding food. Gravid flies 

 would hardly cross five or ten miles of open water to a small uninhabited island 

 merely to deposit their larvae. In short, it does not seem at all improbable that 

 the female Glossina should possess a strongly developed and specialised 

 migratory instinct which would induce her to leave the locality where she was 

 bred, and to fly for long distances ; such an instinct would probably be 

 disassociated with any of her ordinary needs as an individual, but would 

 correspond to that inciting the swarming flights of the bees and other social 

 insects. It would probably lie dormant during the period when she was 

 producing larvae, and very possibly during her entire life, unless (and this is 

 wherein it is worthy of being termed " highly specialised ") her species were 

 approaching the limit of its possible abundance in the locality where she was 

 reared. 



There are no data at hand in any manner suggestive of the distances which 

 might be covered, further than that afforded by the distances of the islands 

 mentioned off shore, and this evidence is of negative value only, because it is well 

 within the bounds of reason that the females should leave the island and yet be 

 unable to reach the mainland.* 



Are the implied migratory movements of Glossina ever encountered 

 amongst other solitary insects ? 

 The hypothesis of female migration involves the presumption that the islands 

 and portions of the uninhabited shores of Victoria Nyanza, and perhaps Lakes 

 Edward and Tanganyika, are markedly favourable for Glossina palpalis. It 

 also presumes that the females of this species deliberately leave these 

 particularly favourable regions to migrate to others less favourable. Is this 

 reasonable ? 



* Migrating swarms of butterflies and moths are well known to occur far out at sea. 



