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flight to take them to their food. Whilst then a house on a bluff,
or a line on an exposed area of ground, may cause anopheles to find
their way considerable distances, it does not follow that they will,
without obvious reasons, be always found at such a distance from the
breeding place.
It also cannot be assumed that an anopheles covers the whole
distance in a single flight or in a single night. A ship lying off a har-
bour seems at first sight an excellent test of the power of flight of
mosquitoes, but it is in reality no test of the distance that anopheles
will penetrate an area free from breeding places on land. The ques-
tion of flight and penetration are very important. Very often breed-
ing places exist a few hundred feet from where adult anopheles are to
be found in dwellings; but it does not always follow that these are the
main sources whence they come. Experience has shown us that the
abolition of such breeding places often does not at all influence the
prevalence of the adults. An llustraton of ths was seen by us at a
garden called Mean Glas n the Duars. In this garden the Manager
had greatly interested himself in a systematic attempt at petrolage
of the breeding places around his bungalow and certain of his coolie
lines. From May onwards systematic petrolage was carried out
round the bungalow to a distance of quarter-mile radius. In August
one of us examined the treated area, but after a careful search found
only a few very young and half-grown larv, though just beyond the
limits of the area were drains with numerous full-grown anopheles
larv and nymphs.
Nevertheless in the bungalow adult M. listoni and N. theobaldi
were abundant and could be seen at night and in the very early morn-
ing endeavouring to enter through the screens of wire gauze, with
which the bungalow was in the process of being protected.
After our visit Mr. Wallich extended his operations to a half-
mile radius and when we visited him during the first week in Nov-
ember the absence of larv in the treated areas was very striking,
nevertheless adult anopheles were still to be caught in the bunga
low.