69
Infected anopheles were not met with before the middle of July either in
1909 or 1910; in the dry weather of 1909-1910 156 stomachs and 83 sali-
vary glands of N. stephensi examined consecutively gave negative results; in
February and Match 1911, 238 stomachs showed only one infection ; and the
same number of salivary glands were examined without the finding of sporozoits.
160. The relation of Anopheline infection to weather was noted in 1910.
In July of that year a sudden break occurred in the rains, only 179 inches of
rain falling during a period of three weeks, and there being only three days
during this period on which the rainfall was 0.25 inches or more. The weather
during this period was bright and sunny, the temperature rose slightly and the
atmospheric humidity remained very high. At this time a large number of N.
stephensi became infected with malaria and the infections reached maturity
very rapidly as shown by the fact that 11.4 per cent. of those examined showed
zygotes in their stomachs and 10 per cent. had sporozoits • in their salivary
glands. A little later, during the first half of August, the occurrence of a
period of almost unbroken rain appeared to reduce the proportion of sporozoit-
carrying anopheles considerably and though 18.6 per cent. of those examined
had zygotes in their midgut only 5 out of 158 or 3.1 per cent. showed sporozoits
in their salivary glands.
Section 3.
161. MOSQUITO LARVÆ.—Since the commencement of the enquiry
about 20,000 collections of water of all kinds in different parts of the island,
have been examined for mosquito larvæ, including about 5,000 Avails, nearly
5,000 cisterns, 2,386 small masonry tanks in gardens and compounds and
215 large tanks, 551 public and private fountains, 3,500 tubs, barrels and
buckets, many hundreds of large and small pools and many minor collections of
water in tubs, troughs, tin-pots and hollows in old machinery, etc. In a large
proportion of theses potential breeding places larvæ of different species of
(Julex have been met with and N.-M. rossi has been found breeding in vary
many of them.
162. Method of examining breeding places for Mosquito Larvæ.—Early
in the enquiry it became apparent that many of the collections of water
used as breeding place by the mosquitoes of Bombay were of such a
nature that it was not possible to examine them properly by means of ordinary
dippers. In Bombay wells form common breeding place of anopheles
mosquitoes and in the vast majority of cases the surface of the water can only
be reached with difficulty ; and although it is sometimes possible to dip larvæ
out of a well by means of a bucket and rope, this is by no means a certain
method of examination. On some occasions as many as twenty buckets of
water have been removed from a well known to contain larvæ without one
being found in this way.
163. To meet the difficulty of examining wells a special form of apparatus
was devised early in the course of the investigation, a brief description of which
is as follows. A locp about 15 to 18 inches in diameter is made of very thick
brass or galvanized iron wire, and upon this frame a shallow net of very thin
mull-mull or muslin is stretched. The net is suspended by three short brass
chains about 2 feet long, which are attached to the frame below and to a small
ring above. To the ring a rope is tied and the net can then be lowered to any
required depth. When a well has to be examined the net is lowered to about a
foot beneath the surface of the water, the weight of the frame causing it to sink.
It is then gently moved below the surface and pulled up at a point some distanc s
from where it first entered the water. On examining the net larvæ, if plentiful,
are easily seen and can be secured by washing them off into bowl of clean
water. If larvæ are very small or scanty in number they are not easily seen
on mere inspection of the net but by inverting it into a shallow white enamelled
vessel of water they can be washed off and easily detected. It has been found
useful to adopt this washing of the inverted net as a routine method, and it has
been found that a large-sized white enamelled frying pan is a useful vessel in
which to invert the net and wash its surface. The value of the net method for
examining wells is shown by the fact that in certain areas double the number
were found to bo infested with larvæ of Neocellia stephensi, than had been
supposed, when examination had been made by means of a bucket.
B 136-18