BRITISH BLOOD-SUCKING FLIES NEMATOCERA—C ULICIDAE
of the remains of insects which fall into the water and are drowned. Larvae
have been found in all stages of development in every month of the year, and
it is in the larval stage that the winter is passed ; adults occur from April to
October, and there are probably several overlapping broods during the summer.
A.plumbeus bites persistently in the shade near its breeding-places during
the day, and not infrequently enters houses and bites the occupants at dusk.
The bite may be painful. It has been found experimentally that this species
can efficiently carry the parasite of benign tertian malaria.
Genus AËDES Meigen
This genus includes about half the known species of British mosquitoes.
These may usually be distinguished without much difficulty from the other
species by the fact that the abdomen of the female is more or less pointed,
with a pair of small processes (cerci) projecting distinctly at the tip, but this
feature is not very noticeable in two species {A. cinereus and A. geniculatus).
Another characteristic more absolutely diagnostic of female A'êdes in Britain
is the form of the claws : on at least the front and middle legs of all the species
the two claws have each a small tooth in the middle (readily seen under the
low power of the microscope), this tooth being absent in all the species of other
genera.
Aëdes also exhibits peculiarities in its life-history distinguishing it from
most other genera of mosquitoes. The eggs are laid singly either on dry
ground or at least above water-level, and are capable of surviving long periods
of dessication ; in the case of some species occurring in desert or semi-desert
regions they can certainly remain viable for several years, and then hatch
within a day or so of being washed into puddles by heavy rain. Nearly all
Aëdes larvae are found in temporary collections of water, their occurrence in
permanent pools or lakes being unusual. Most of the species living in temperate
regions have only one annual brood which is hatched in the spring or
early summer, but the adults may live for several months ; in no case, however,
do the adults of either sex survive the winter.
The genus Aëdes includes among its several hundreds of species one which
is of immense importance to humanity owing to its role in the spread of yellow
fever. This species, Aëdes aegypti (formerly known as Stegomyia fas data,
Aëdes calopus, or Aëdes argenteus) occurs almost throughout the tropical and
subtropical parts of the world, but is not a native of Britain. On one occasion
two larvae were found in a tree-hole in Epping Forest and mosquitoes hatched
from them in a laboratory. The find was duly recorded in print, but it is fairly
certain that the occurrence of these specimens was in some way accidental—
rumour even suggests that they may have been "planted " by students in order
to provide a " surprise packet " for an enthusiastic investigator.
Aëdes cinereus Meigen
This is the dullest-coloured and least ornamented of all the British Culicine
mosquitoes, but for this very reason should not be difficult of recognition ; it
is the only species occurring with us which has at the same time no pale rings
on the leers, no transverse pale bands on the abdomen, and no markings on the
thorax. The thorax is reddish-brown, the abdomen dark brown above, with
a stripe of yellowish scales running the whole length on each side. It is rather
smaller than most of our other Culicines, the average length of the wing being
about 4-4-5 mm.
A. cinereus is a common species in many parts of Britain, especially in the
more low-lying districts ; it has been recorded from many counties in the south
and east of England, as well as from Glamorganshire and several localities in
the lowlands of Scotland. In the Broads district of Norfolk and Suffolk it is
particularly abundant.
The most characteristic breeding places of A. cinereus are riverside marshes
and flooded meadows, but the larvae are sometimes found also in woodland
pools. The adults first appear in June and may remain on the wing until
September ; eggs are laid during this period, but do not hatch until the following
April, the larvae taking about two months to develop. There is thus of necessity
only one generation in the year.
Where it is abundant A. cinereus is often a very troublesome insect.
Dr. Gunnar Olin, in Sweden in the summer of 1938, found specimens of
A. cinereus infected with tularaemia (an infectious disease of rodents, which
can also attack man).
Aedes geniculatus Olivier
(Plate 10)
The silvery-white knees, to which the specific name of this species alludes,
provide one of its most distinctive features ; this, taken in conjunction with
the pattern of black and white scales on the thorax, well represented in the
accompanying plate, will serve to distinguish it from several other woodland
species of the genus Aedes with which it might be confused. Another striking
characteristic is the blue-black colour of the upper surface of the abdomen,
which has conspicuous silvery-white spots along the sides, but no transverse
white bands such as are to be seen in most of the other species. The legs are
blue-black, with the hind femora extensively white.
A. genictdatus is common in most parts of England where there are suitable
large trees, but at present there are no records of its occurrence in Wales,
Scotland or Ireland ; this may be in part due to the fact that few persons have
attempted to find it, but it seems certain that it does not extend nearly so far
north as its common associate Anopheles plumbeus.
This is the only species of Aedes as yet found in this country which breeds
exclusively in the water which collects in holes in trees. As in the case of
A^iopheles plumbetis the larvae are found chiefly in those rot-holes which
contain brown tanninised water, but they also occur in smaller holes and forks
between trunks which contain clear rain-water. The larvae may be found
(though usually in small numbers) throughout the winter, but are most numerous
m the early summer months. Adults occur from April to September, being