42 W. A. LAMBORN. 



There is no evidence of female migration to account for this, and over the greater 

 part of the fly area of Nyasaland migration hardly seems possible, unless it takes place 

 to the east over many miles of open water or to the west over moimtain ranges and 

 many miles of open country, for series captured down a stretch of 150 miles of the 

 area which runs north and south have always given about the same low percentage 

 of females. 



Careful examination of thorn bushes and other low trees failed to reveal the 

 presence of sitting flies, but they were found on the larger trees, baobabs in particular, 

 and a first impression that the females here occurred in greater proportion has been 

 confirmed by a series of captures, which have shown that occasionally they may even 

 exceed the other sex. The females are to be seen deep in the recesses between the 

 buttresses and component parts of the trunk, and in the hollows and fissures of the 

 bark of the older trees, and they often occur high up and far out of reach. The males 

 on the other hand seem to rest as a rule in more obvious positions. During the cooler 

 hours both sexes can still be found, but they are by no means so numerous. 



For the sake of comparing data, captures were effected for six consecutive days, 

 from 7 to 11 a.m., 11 to 3 p.m., and from 3 p.m. to dusk (about 6.30), on each 

 occasion by four boys under supervision. During the morning and evening shifts 

 flies on the collectors and hovering round them were taken. During the mid-day 

 shift the collections on three days included flies taken off baobabs and on the 

 journeys to and from camp ; on the fourth day flies off baobabs alone ; and on 

 the fifth and sixth days flies off certain other trees as well as baobabs. The data are 

 given in the table on page 43. 



The proportion of females captured in the ordinary way during the morning and 

 evening hours works out at 12 per cent, and 10 per cent, respectively, whereas when 

 the flies were taken during the mid-day hours, mostly off trees, a percentage of 43 5 

 was reached, a close approximation to equality. 



It was very obvious that a far greater number of the female flies caught on the 

 trees had recently fed than of those captured otherwise, and there was a marked 

 difference in their behaviour when placed in jars, those off the trees remaining quietly 

 sitting on the side when the hand was placed over the jar, whereas the second series 

 became clamorous at once, running to and fro over the gauze and from time to time 

 thrusting through their probosces. In view of the importance of the question I made 

 some attempt, as will have been seen in the table, to estimate the degree to which 

 the female flies in each case were replete. I must admit however that an estimate 

 of this kind is necessarily liable to serious error, for, other considerations apart, tliere 

 is the question of pregnancy to be taken into account, and many have since produced 

 larvae ; though it is easy enough to distinguish a newly emerged fly, or one that has 

 not fed for some days from one that has fed a day or two before. 



