260 BACOT : MOSQUITOES AND THE DANGER OF MALARIA.
larvae present ever survived to produce adults might be very
dearly purchased at the expense of the immunity from attack
by enemies, enjoyed by thousands bred or breeding in imper-
manent pools, swamps, or shallow running ditches, which are
untenable for the breeding of frogs, newts, and the larger
aquatic insects.
The oiling of ponds and streams, if indiscreetly carried out,
may also reduce the number of the enemies of the mosquito,
including fish, and then, unless repeated at regular fortnightly
or three-weekly intervals, will allow of entirely unchecked breed-
ing and emergence of the later broods of mosquitoes.
Due consideration of the time of breeding is also very essen-
tial : otherwise, the labour and cost of oiling may be largely,
if not entirely, wasted. This happened, for instance, a few years
since, when some of the larger ponds near Loughton were oiled
in May. This was done at that time in consequence, apparently,
of the annoyance caused by adult mosquitoes, a few of which
may have come from these ponds, but the larger number of which
had certainly been bred in very small ones which had probably
escaped the oiling, because overhung and concealed by bushes,
and from small shallow pools wherein larvae had swarmed in
April, though they were dry by the middle of May.
Nothing, therefore, should be clone that offers any check to the
life or breeding of the foes of the mosquito : on the contrary, it
might pay to make suitable ponds where such do not exist. The
first steps should be to chain swamps (not bogs), and to clear
ditches of grass and give them a better level and quicker flow. In
fens, the planting of osier beds is said to have caused a diminution
of mosquitoes. It is not apparent why this should be the case,
as these plantations are, I believe, regularly flooded ; but it
may be that the osiers afford the enemies of the mosquito cover
and protection from wading birds, such as herons. Next, ponds
should be deepened at their margins and cleared of excessive
weeds, while any small and semi-isolated puddles on their margins
(such as the hoof-holes of cattle and horses) should be filled.
If this is impracticable, such margins ought to be systematically
oiled at three-weekly intervals from March onwards. All small
impermanent pools should be filled or carefully charted and oiled
regularly while they contain water. Water tanks and butts
should be kept stocked with a few small fish or carefully screened