Ronald Ross Nobel Lecture

70

   1 9 0 2  R .RO S S

tozoal organism able to transfer itself from one host to another was then

known to science.*

I therefore now commenced a long series of differential experiments in

order to establish the fact thoroughly. Grey mosquitoes bred from the larva in

captivity were fed (a) on birds with Proteosoma and (b) on birds without Pro-

teosoma, 

and the results compared. The details will be found in my Report-Q.

Out of 245 grey mosquitoes fed on birds with Proteosoma, 178, or 72 per cent,

contained pigmented cells, while out of 249 of them fed on blood containing

other parasites or no parasites, not a single one contained them.

Another experiment was the following. Three sparrows were selected, one

with no parasites, one with a few Proteosoma, and one with many Proteosoma.

They were placed in separate nets, and numbers of grey mosquitoes from the

same breeding bottle were fed simultaneously but separately on them. Ten

mosquitoes fed on each bird were then examined, and the total number of

pigmented cells in all of them were counted. The results from a hasty enumer-

ation made by myself were as follows. No pigmented cells were found in the

ten mosquitoes fed on the sparrow without parasites; 292 in the ten mosqui-

toes fed on the sparrow with a few Proteosoma; and 1009 in the ten fed on the

one with many Proteosoma

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. The preparations were sent to Manson, who

made a more careful enumeration and found 0, 571, and 1084, pigmented cells

in the three sets of mosquitoes separately

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.

The fact then was proved, and the theory that the parasites of malaria de-

velop in mosquitoes was practically established. Meanwhile I had been pro-

ceeding in the fascinating task of watching the progress of that development.

A number of grey mosquitoes would be fed on an infected bird and would be

dissected two, three, four days, and so on, afterward. It was thus found that the

pigmented cells grew rapidly in size until about the eighth day, when they be-

came so large as to be almost visible to the naked eye. At this point they seemed

to become mature; and it could be seen that many of them burst within the

insect; because mosquitoes which had been infected more than eight or nine

days before dissection were found to contain not the mature pigmented cells,

but only their empty capsules. For the moment I could not ascertain what

became of their contents.

This part of the work led to an interesting observation which influenced all

subsequent researches on mosquito-borne disease. It will be remembered that

Manson had always thought that a few days after her meal of blood the female

* The life-history of Pyrosoma in ticks is not even yet known; and the transference of try-

panosomes by flies appears to be merely mechanical.

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71

mosquito laid her eggs and died; at this moment he considered both filariae

and malaria parasites escape into the water from the insect

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. I had accepted

this view, but had frequently observed that the insects do not die immediately

after laying their eggs; and now, as I watched the pigmented cells growing

larger and larger without apparently ripening, even five days after the insect

was fed, it occurred to me that we had been allowing our mosquitoes to die so

early owing to a very simple reason - we had omitted to feed them again! I

therefore fed my infected mosquitoes a second and a third time, and more;

and found that I could easily keep them alive for a month.* This enabled me to

work out the development of the malaria parasites completely; and also help-

ed others subsequently to find a further stage in the development of filariae,

and to ascertain the mode of infection in yellow fever.

I did not succeed, and, indeed, scarcely attempted to find the host of Halteri-

dium. 

Nor was there time to work out the formation and behaviour of the

"vermicules" in the stomach cavity of the mosquito - although this could have

been done very easily; but on one occasion I saw the motile vermicule of

crow’s Halteridium in a brindled mosquito.

Of course all this time anxious efforts had been made to obtain cases of hu-

man malaria for experiment. Early in March I succeeded after much difficulty

in finding an old beggar with a few crescents willing to submit to the dreaded

operations; and I examined 41 grey mosquitoes and 15 dark greenish dappled-

winged mosquitoes which had been fed on him. The first kind were tried

merely as controls, and were of course negative; but, much to my surprise and

disappointment, so were the latter. I attributed the failure to the facts that the

crescents were very scarce in the patient, that the mosquitoes fed very sparing-

ly, and that there was a spell of very cold weather (for Calcutta) at the time.

A few unsatisfactory experiments with grey mosquitoes fed on a child with

mild tertian parasites also failed. In spite of all efforts no other cases could be

procured.

A full list of all these experiments, beginning with my earliest work in 1895,

will be found in my Report written a few weeks later

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.

Recognizing, of course, the inadequacy of my nomenclature for mosqui-

toes and the urgent necessity for employing the correct entomological names

for the various species used by me, and having failed to obtain any literature

on the subject, I now applied for assistance at the Indian Museum in Calcutta ;

but I

 

received a brief reply to the effect that the savants there could give me no

* I refed them on healthy birds ; but Bancroft subsequently found that they could be kept

alive for some time on bananas.

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