383 
H. L. Duke 
suitable for food is left unprotected, and the following morning some wing 
fragments and a few wandering loiterers are all that remain to mark the 
tragedy. 
In view of these sources of error there is nothing in the above experiments 
inconsistent with the generally accepted belief that, with this trypanosome, 
a fly is infective when its glands are invaded, and not before. Ibe peiiod that 
elapsed after the first infecting feed before the flies became infective, in each 
of the five positive experiments of Table V was 35, 25, 45, 29 and d 5 da} s 
respectively. Miss Robertson found in the course of her investigations into 
the development of trypanosome strains in G. palpalis at Mpumu, that in 
one case the glands became infected on the 12th day, and in laie cases on the 
16th day, gland infections being increasingly frequent after the 20th day. 
She states that a fly is capable, apparently, of conveying infection when only 
the proximal part of the gland, close to the duct, shows parasites (Robertson, 
1913). It is thus easily possible to overlook a gland infection when only 
fragments of the distal end of the viscus are inspected. Under the coverslip, 
a positive gland may often be seen almost completely to erupt) itself of ti \ - 
panosomes through a hole in its side caused by pressure. 
Miss Robertson also refers to the rather rare cases where a fly may show 
a considerable number of flagellates in the gut as late as the 56th day of the 
experiment, without the salivary glands being infected. She found that, under 
ordinary feeding conditions (§,s contrasted with certain starvation experiments) 
the strains of trypanosomes employed produced 3 per cent, of infected flies. 
Kleine and Fisher, in experiments with palpalis and T. gambiense at Rutschugi, 
a region free from Sleeping Sickness, found that 8 of their 881 experimental 
flies became infective, i.e. 0*9 per cent.; and that of the flies that lived foi 
more than 12 days, slightly more than 1*7 per cent, contained flagellates; 
the cycle lasted 28-31 days. In a sleeping sickness region 2-5 per cent, to 
6 per cent, of their flies became infective, and the cycle lasted 20—25 days 
(Kleine, 1913). 
Bruce and his co-workers at Mpumu gave 27 days as the shortest time in 
which a fly became infective with T. gambiense , the longest cycle taking 
53 days, and the average 36 days (Bruce, 1910 a). 
In Tables V, VI, with both the human and wild-fly strains, the percentage 
of infected flies is lower than that obtained at Mpumu. This is curious, as 
one would expect that conditions at the Entebbe Laboratory a few feet abo\ e 
lake level would be more favourable than at 600 feet higher on the top of the 
Kyagwe hill. As a rule, in the present experiments, the flies were only fed 
on the nourishing animal every other day, while those at Mpumu were fed 
daily except on Sundays. Again at Mpumu, in all save Miss Robertson s 
experiments, cocks were employed to a considerable extent in nourishing the 
flies during the development of the cycle. In individual experiments at 
Mpumu the percentage of positive flies reached as high as 20 per cent. (Duke, 
1913 b). 
