32
The experiment also suggested that different species behave
differently in respect of the distances they travel. Most of the drains
at Mean Glas just beyond the area of operations contained larv of
M. listoni and N. theobaldi; but one stagnant drain contained con-
siderable numbers of the larv of M. barbirostris and A. aitkeni.
Only very few specimens, however, of the adults of these two
latter species were caught, two of the one and three of the other being
seen in the course of ten days.
In many instances we found anopheles mosquitoes in dwellings,
although it was impossible to find any breeding places except at a
great distance; whether in any particular instance this is the result
of flight or of a condition to which we shall now refer is often very
difficult to determine.
RESIDUAL ANOPHELES.
Apart from an allowance for flight and penetration there is
another condition which requires mention. When extensive breeding
places dry up adult anopheles are very frequently found, often
in considerable numbers, for weeks or even months afterwards.
The condition in which adults are present in the absence of
breeding places we shall call one of residual anopheles. This is a
very common condition, and is important, since not to recognise it
may lead to considerable error in estimating the nature, means and
extent of prophylactic measures required.
Where a number of breeding 'places have dried up it is easy to
explain residual anopheles; but residual anopheles may be found
where the only explanation of their presence seems to be that under
certain conditions very suitable for the adults these make use of the
most temporary means of breeding to maintain their numbers.
A very paradoxical condition, which we can only explain on this
hypothesis, was found by us in certain gardens in the Dina-Torsa
district situated on peculiar, porous, alluvial slopes, uninterrupted for
very long distances by permanent streams or ravines. In this area
the ground is so porous that the heaviest rain only serves to create
very temporary freshets which cease to flow and are more or less
completely absorbed within a few hours. Many streams indeed,