8
SUPPLEMENT TO THE ANNUAL REPORT OF THE
3. Irrigation canals .—The irrigation canals are full of water throughout the
year except for two months (June and July) when the water is shut off for annual
repairs. During the present survey, irrigation was going on. In the 'main
channel where the current was swift, no larvæ could be found. In the Botlagu-
dur branch channel, the conditions ought to have been the same. But the sides
of this channel are not well cut and jambo grass has been allowed to grow wild in
several places, so that the current in the middle was very sluggish, and anopheles
larvæ of all kinds particularly fuliginosu's and culicifacies were found breeding
along the sides. The subsidiary distributing channels were the worst in this
respect. In several places, these were so overgrown with vegetation of all kinds,
that one could hardly suspect the existence of a water channel. They were full
of anopheles larvæ A. rossi predominated, but culicifacies and fuliginosus were
also found.
4. Tanks.—In the whole area there are only two big tanks, the Mopad Reser-
voir and the Kattakindepalle tank. In both of them on their shallow sides a fair
number of anopheles larvæ was caught. They were mostly rossi, but a few culici-
facies were bred out from the collection from the Kattakindepalle tank.
5. Pools.—Pools and borrow pits were innumerable all over the area.
Earth has been dug up in the most indiscriminate way, particularly near Mopad,
to form the reservoir and canal bunds. Huge pits of all sizes and shapes have
been left behind. Though dry during March, they would undoubtedly form
excellent breeding places during the rains. There was water in some of these
borrow pits that were along the canals and along the Pamur Chundi road. They
had. a constant supply of water by seepage from minor irrigation canals and by
overflow from paddy fields. They were overgrown with rank vegetation and
yielded a good collection of anopheles larvæ— mainly rossi culicifacies and fuligi
-nosus. In addition there were a large number of natural pits and pools in the
uncultivated land beyond the fields, where, during the rains, mosquito larvæ could
breed abundantly.
6. Paddy fields.—Paddy cultivation was going on at the time of the survey
and the fields were all full of water. Larvæ could not be caught in any appre-
ciable numbers in them. The few that were obtained were only rossi. It is
probable that these fields are not an important source of anopheles.
. 7. Wells .—A.ll the wells 14 in number were examined for anopheles larvæ,
but none were found. The explanation for this is not obvious. It was observed
that well water was not good for larvæ caught elsewhere.
Discussion.—A practical conclusion required is an estimate of the relative
importance of breeding places. This is rendered difficult by our not being able to
incriminate any one carrier, and by the fact that breeding places usually yielded
more than one species of carrier—culicifacies on the whole was the commonest,
and further by the survey not having been done at the most important.time.
What appeared to be most important breeding places in March were those in the
bed of the Maneru., the drainage streams and the pit pools. Now although their
importance in September and October can certainly be determined only by a
direct inspection in those months, yet we think that there is very little doubt that
the conditions then will be similar, for, although it is likely that due to the first
rains and the rise in the level of the sub-soil water there will be some extension
in size and number of the breeding places of all three sorts in September, yet the
autumnal periodicity of malaria is probably due less to the increase in breeding
facilities than to the atmospheric conditions becoming more suitable to adult
anopheline life. This explanation is in harmony with the seasonal dependence of
malaria on rainfall along with its independence of total rainfall. The favouring
conditions are probably the onset of cooler (or rather less hot) temperatures and
greater humidity—with the rainy season. Evidence in favour of this view is the
scarceness of adult carrier anophelines in March despite their abundance as larvae
in innumerable breeding places.. As noted, even for larvæ the conditions appear-
-ed to be becoming unfavourable—too hot. As regards the bed of the Maneru in its
non-flowing state it is an ideal breeding place for anophelines, in which state it