70 



" A hypothetical explanation for this phenomenon," continues the 

 author, " which has withstood all tests applied to it, is as follows : 



" (a) The body or mass of active flies is continually in movement, 

 and streams of flies are continually passing points along shore . . . I 



" (b) These streams of moving flies are made up of (1) food-hunting 

 flies of both sexes, which compose a variable proportion dependent 

 upon abundance of food and which move rapidly ; and (2) male flies 

 which are not seeking food, but which frequent the routes followed by 

 food-hunting flies, and which move much more slowly 



" (c) The relatively idle and lingering males tend to prolong their 

 sojourns at points where sheltering vegetation is of the most attractive 

 type, and to pass quickly, or not at all, by points where the vegetation 

 is of an unattractive type. It follows that (1) density of active males 

 will be greatest where shelter is most attractive, and least where it is 

 least attractive, and (2) that the percentage of females amongst caught 

 flies will be greatest where shelter is least, and least where shelter is 

 most attractive to the lingering males (because food-hunting flies must, 

 for several reasons, 1 consist principally of females)." 



It is perhaps advisable to point out that the object of the 

 experiments described above under the previous heading (RANGE 

 OF FLIGHT) was to determine the distance which Tsetse-flies are 

 capable of covering by their own unaided efforts. Without here 

 entering into a discussion of the distance to which G. morsitans 

 may accompany big game, such as buffalos, 2 it may be mentioned 

 that male flies in search of females often travel for several miles 

 on the backs of porters or the bellies of domestic cattle. Thus 

 Swynnerton (145), in Portuguese East Africa, twice found G. morsitans 

 " following" man in this way for "rather more than six miles, and many 

 times various less but still long distances. On one occasion, " he adds, 

 " a journey of over six miles was done in three instalments by a marked 

 fly on three successive days." The same writer refers to a record 

 by another observer " in which tsetses were still on his cattle 25 miles 

 outside the fly-belt." Swynnerton also " measured one of these rides 

 [on cattle] by five male brevipalpis in dull weather, first marking each 

 of them with a dab of white paint. The flies had come to us," he 

 writes, " just before and were following, not feeding. The greatest 

 distance travelled was 5 J miles by one fly that stayed to the end. All 

 five flies were still present at 4 miles and 700 yards. Every kind of 

 country was traversed, brevipalpis bush, simple coppice and open grass. 

 The flies deserted us on our getting back into their native bush." It 

 should be added that Swynnerton states that he has met with " no 

 evidence at all that either sex travels appreciably on its own initiative." 



1 In a foot-note Fiske writes : 



" These reasons include the following : 



" 1st. Because the females in any district or region appear to outnumber the 

 males 



" 2nd. Because the females must nourish their young as well as themselves, 

 and most probably require food somewhat more frequently, in nature, than males. 



" 3rd. Because the males are normally active at all times during good weather, 

 and are more apt than the females to encounter host animals without specifically 

 seeking for them." 



2 That big game, especially the buffalo, is mainly responsible for the annual 

 spread of G. morsitans and other species of Glossina each rainy season is stoutly 

 maintained by certain writers, e.g., Swynnerton (145); others, however, such 

 as Christy (29) and Jack (75), while admitting the movement of the fly, are 

 strongly inclined to doubt its alleged habit of migrating with the game (see 

 pp. 100, 104). 



