

74 



In a foot-note to the foregoing passage the author adds a series of 

 directions, which are here reproduced for the guidance of future 

 investigators. 



" Proper attention to the following suggestions will serve to eliminate 

 various possibilities of error : 



" (1) Boys should be trained at least one month ; all new boys in 

 a corps without one or two experts as teachers would require longer 

 training. 



" (2) Nets must have a standard-sized ring and a standard length of 

 handle ; an 8 in. ring and an 18 in. handle were used. 



" (3) Catches should not be made before 8.30 or 9 a.m., nor later 

 than 2.30 or 3 p.m. 



" (4) Boys will make more even catches if provided each with a 

 dark-coloured umbrella, upon which the flies will settle, and from which 

 they are easily caught. 



" (5) Unless density is very low less than 1*0 it is better not to 

 spend more than 2 to 3 ' boy-hours ' at the same point on the same 

 day ; density is easily reduced, temporarily, by catching. 



" (6) Boys should be stationed along the routes most likely to be 

 followed by moving flies, and always, if there is shadow, at the "edge 

 of it. 



" (7) Estimates of density must be based on catch of males, on 

 account of the variability of female activity. 



" (8) Good results cannot be secured on cool, cloudy or windy days." 



With reference to the proportion of the sexes in G. morsitans taken 

 by him in Nyasaland, Dr. Lamborn (83) says : " My experience in 

 this matter is the same as that of other observers, namely, that when 

 the flies are bred out from pupae, the sexes emerge in almost equal 

 proportions ; and when the flies are captured, the males far outnumber 

 the females .... There are very definite reasons why the female 

 flies should to some extent shun the society of the males. As I have 

 before remarked, and have since repeatedly observed, coitus takes 

 place as the result of capture without preliminary courtship . . . 

 In captivity even females in an advanced state of pregnancy are not 

 secure from the violence of the males, and as abortion is so frequent with 

 captive females, it seems possible that this may conduce to it . . . . 

 These facts undoubtedly indicate the necessity for the pregnant 

 female to seclude herself, when once fertilised, from the further atten- 

 tions of the males ; hence the unequal proportion of the sexes among 

 captured flies." 



Further statements on this subject, in connection with G. morsitans 

 in Nyasaland, are contained in a subsequent paper by the same author 

 (84). In studying the proportion of sexes among captured flies, it 

 was observed that male flies become attached to possible hosts in order 

 to secure females which come to feed. When the possible hosts rest, 

 the females are less attracted and most of the males fry off. Pairing 

 is attempted by the male whatever may be the stage of development 

 of the female. Collectors can readily obtain flies by catching them 

 off each other, and these captures always show a small proportion of 

 females. If flies which have settled near the possible host are included, 

 the results show a much higher percentage of females. The number 

 of active G. morsitans decreases during the hottest part of the day. 

 Examination of possible sheltering places during this period showed 

 that large numbers of females are to be found in recesses in the trunk 

 and bark of large trees, especially baobabs, while the males occur in 

 more exposed positions. The proportion of females taken in the 



