N. H. SWELLENGREBEL AND J. M. H. SWELLENGREBEL-DE GRAAF 185 
M. ludlowi 
_A_ 
Table II. 
M. rossii 
A_ 
Crescent rate 
Locality 
( 
Examined 
Infected 
N. I. 
t 
Examined Infected 
N.i. 
( -- \ 
Adults Children 
Panggoeng 
100 
16 
16 
93 3 
3 
17 32 
Pendjalan 
185 
10 • 
5 
243 1 
0-4 
1-7 15 
Mandailing 
4609 
102 
2-2 
- - 
— 
0-9 9 
The rule of the N. I. being high in regions of endemic malaria, is not 
without exceptions. In Soendatar this figure was less than 1 per cent, in 
N. fuliginosus and M . sinensis, and zero in other species, such as M. aconita 
and N. maculatus. In Naras, Schiiffner found 6 per cent, of M. ludlowi 
infected, whereas in other endemic regions we occasionally found as high 
a rate as 35 per cent, for the N. I. of this species. From this we infer the 
existence of still other factors, whose nature is as yet unknown, which influence 
the N. I. 
When comparing the results of determinations of N. I. in different localities, 
much circumspection is necessary, unless a control, in the form of a known 
carrier, enables us to check our findings. The best species for control purposes 
is M. ludlowi, the only mosquito which is invariably found infected in all 
malarious districts. 
In the following tables we have collected the results of several determina¬ 
tions of N. I. and E. I. together with records of blood and spleen examinations. 
In both sets of observations, it may be noted that at least one species of 
mosquito was found to be infected. We have excluded all observations in 
which this was not the case. For a comparison of the results obtained in the 
Malay Archipelago with those from neighbouring countries, we have added 
to these tables the figures of N. I. from Malacca and British North Borneo, 
and the figures of E. I. from Malacca. 
Supposing the N. I. of a given species to have been determined in various 
districts, likewise the nature of the parasitaemia of the inhabitants, is it 
permissible from these data fco draw any conclusions as to the economic 
importance of this species? As a rule we may do so provided the species is 
found infected in every looality, and that it shows a comparatively high degree 
of N. I. But, this condition is seldom fulfilled (excepting M. ludlowi). In the 
majority of cases the determination of N. I. still leaves some doubt as to the 
significance, from the sanitary standpoint, of the mosquito under observation. 
As an instance, in the district of Mandailing, we found that M. ludlowi showed 
the highest N. I.; but other species were likewise infected, M. barbirostris 
especially. What degree of importance is to be attached to this species? To 
find an answer we must look to neighbouring non-malarious regions, where 
M. ludlowi is absent, but where many of the species, which in Mandailing 
showed malarial infection, are present, notwithstanding a low N. I. Our 
experience in other localities has taught us that severe malaria never occurs 
with M. barbirostris as the only vector. From these three data we can infer 
