67
index of about 40 per cent. As it was considered advisable to examine both
the stomach and salivary glands of the mosquitoes, in order to determine
the presence or absence of both the zygote and sporozoit stage of the malaria
parasite, it was necessary to keep them for a time until their stomachs
became empty.
155. A total of 4,235 dissections of mosquitoes were carried out during
the investigation, about 2,050 of which were for the purpose of examining the
salivary glands and the remainder for the examination of the stomach or
midgut. Of the mosquitoes examined the great majority were specimens of
Nysso-myzomyia rossi and N. stephensi.
The examination of 925 stomachs and 806 salivary glands of Nysso-
onyzomyia rossi gave entirely negative results ; whereas the examination of
adult Neocellia stephensi showed among 1,228 stomachs, 91 or 7.4 per
156. A few feeding experiments carried out with these two species of
anopheles gave similar results. Of 51 rossi fed upon cases of malaria harbouring
crescent infections none showed zygote development, whereas among 24
siepheoisi two developed zygotes in the midgut.
During the examination of mosquitoes at certain seasons of the year the
infections met with among N. stephensi were very numerous. In July 1910,
10 per cent. harboured sporozoits and in August, 18 per cent. were found to
contain zygotes ; but among large numbers of rossi caught in the same houses
at the same time, none showed a sign of infection.
These observations ara most important as they appear to prove conclusively
that Nysso-onyzomyia rossi is not concerned in the spread of malaria in Bombay.
157. The only mosquitoes that were found infected with malaria during
the investigation belonged to the species N. stephensi. It may be remarked
that laboratory experiments conducted by Stephens and Christophers with
N. stephensi as long ago as 1901 showed that malarial parasites could complete
their development in the stomach and pass to the salivary glands of this
mosquito. This observation was not confirmed by the finding of naturally
infected insects until 1908 when Major Liston, I. M. S., investigated the question
in Bombay. But although the dangerous nature of N, stephensi was not
definitely proved until 1908 there is not the least doubt that it had been spreading
malaria for many years in Bombay City, and it is a mistake to suppose it has
only recently become a carrier of malaria.
15S. In addition to N. stephensi three other species among the anopheles
mosquitoes of Bombay have been shown to be definitely concerned in the
spread of malaria in certain parts of; India.
They are—
(1) M. culicifacies.
(2) M. listoni.
(3) N. fuliginosus.
Investigation has not as yet shown the two remaining Bombay anopheles
to be natural carriers of infection, and although both N. jamesi and M.
barbirostris have been infected experimentally in the laboratory there are strong
reasons for supposing that the latter at all events is generally innocous. As it
was impossible to catch any great number of adult N. jamesi, M. barbirostris,
31. culicifacies and 31. listoni during the investigation, only a very few could be
dissected, and the negative results obtained are inconclusive, but there is strong
presumptive evidence that M. barbirostris is not a common carrier of: malaria in
Bombay, because the parts of the island where it is most numerous are almost
free of malarial infection.
159. The observations upon N. stephensi which have been carried out
regularly since July 1909 bring to light certain interesting facts regarding the
seasonal prevalence of malarial infection among mosquitoes. This is of some
importance as there is a direct relation between the existence of malaria-infected
mosquitoes and the occurrence of fresh infections in man.