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 done 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 drain 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 smalt 
impermanent pools should he 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 
