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191. In addition to the "Piku" a number of local fish of other species
have been found to prey readily upon mosquito larvæ. Among these may be
mentioned Polyacanthus cupanus,* a small fish present in Gowalia tank, Anabas
scandens,* the climbing perch, which can be obtained in large numbers from tanks
and pools on the island and from the mainland, and a species of cat-fish which
is found in pools in the bed of streams near Thána. Of these fish Anabas
scandens, known locally as "Kazari ", is the most valuable. It is larger
than the Haplochilus and, as usually obtained, is from three to six inches or
more in length. It is stoutly built, with a large head and well-developed
pectoral fins. In colour it varies from yellowish brown to greenish black but
there are usually patches of a reddish tinge upon its back. Its eyes are very
large and brilliant, coloured bright yellow, with well-marked pupils. It is
exceedingly active, readily jumping out of the water after flies and mosquitoes,
and it devours mosquito larvæ and pupæ voraciously. About two dozen of
these fish placed in a masonry tank four feet square containing about four feet
of water, kept the tank absolutely free of mosquito larvæ, although many
thousands were added to it every day. Anabas scandens has also been tried in
wells ; and when present in sufficient number has been successful in freeing the
water from mosquito larvæ ; but in wells containing much weed and floating
leaves or rubbish it is not successful as the larvæ have cover. There is little
question, however, that in wells kept free of floating matter the presence of
a sufficient number of these fish would prevent the breeding of mosquitoes.
192. The species of cat-fish mentioned, which has been identified as Ma-
crones Corsula* var. Trachacanthus, is also a useful fish to employ as an anti-
mosquito agent. Its presence has been noticed in a certain number of wells in
the north of Bombay. The little fish from Gowalia tank, Polyacanthus cupanus,
is not such an active destroyer of mosquito larvæ as either of the other fish, and
as it seems to be exceedingly delicate it is not likely to be of so much use in
wells as Anabas scandens or the cat-fish. The two latter species also appear
to be better able to resist a change of habitat than " Piku," which does not
always thrive in captivity.
193. In conclusion it may be stated that, as a result of the experiments and
observations that have been made, there appears to be no reason why fish of the
species mentioned should not be successfully employed in protecting many of
the small permanent collections of water in Bombay such as wells, fountains,
and masonry tanks, from the breeding of anopheles and other mosquitoes,
provided that the conditions indicated below are adhered to.
These conditions are :—
(1) The number of fish employed must be sufficient for the task and
proportionate to the amount of water to be protected.
(2) The water must be kept free from weed, leaves, flowers and floating
matter which might afford cover to mosquito larvæ.
Of course it must be understood that fish cannot he employed in wells which
are too foul or stagnant to allow of there being a sufficient supply of oxygen
for them. Wells in this condition must be protected in some other manner.
194. Plants inimical to mosquitoes.— Wolffia arizha, the rootless-
duckweed, may be placed among the natural enemies of mosquitoes.
There are three varieties of this minute water plant present in Bombay
which differ from one another in the size and shape of the grains. The
commonest form is to be seen in the Mint Tank and a number of other
collections of water mostly in the north of the island. In this form the plant
consists of tiny round fleshy masses about the size of a pin's head and of a
brilliant green colour, In another variety the individual grains are cat-shaped
instead of round and rather smaller in size. In a third variety the grains are
much larger and more flattened and resemble small single leaflets of lemna or
ordinary duckweed without the roots. "Where the rootless-duckweed is present
in large amount it forms a thick scum on the surface of the water and by its
mechanical action destroys mosquito larvæ just as a film of oil might do.
Mosquitoes will lay their eggs on water containing the weed and young larvæ
hatch out and live for a time, because while small they can pass their breathing
syphons to the surface between the particles of weed, but larger larvæ die of
*These fish were identified for me by Mr. N. B. Kinnear of the Bombay Natural History
Society.