BRITISH BLOOD-SUCKING FLIES NEMATOCERA—CULICIDAE
for in England and Wales, and there are many records from Scotland and Ireland
; indeed, from Scotland there are far more records of this species than of
A. 7naculipen7tis, the most northerly being from Lochinver, Sutherland.
The life-history of A. claviger is very different from that of A. maculipennis,
largely owing to the fact that the winter is passed in the larval instead of in the
adult stage. Females as well as males die at the approach of winter, and from
the beginning of November until the end of February only larvae are to be
found. Wintering larvae are mostly half-grown, but a proportion of small and
nearly full-grown larvae is usually also present, so that it appears that the growth
of the larva is arrested at whatever stage it has reached by the end of October.
The larvae are resistant to cold and have been found to survive even after
being frozen for many days in blocks of ice. In a mild spring the larvae
complete their development by the end of February, and adults may be hatched
early in March, though the end of this month is the usual time for the first
brood to appear. Shute has observed that adults of the first generation usually
avail themselves of the warmth and food provided by pig-sties, while those of
the succeeding generations live amongst vegetation. The female lays about
200 eggs at a time, and several generations are produced during the summer,
the shortest time needed for a complete life-cycle being about three weeks in
warm weather.
A. claviger breeds in waters of a more or less permanent character, for the
most part in shady situations (in contrast with A. muculipennis, which prefers
more sun) ; especially in weedy pools, ditches, and the weedy margins of lakes
where these are sheltered by trees, also in greenhouse and other tanks. The
larvae have been found in brackish water with a salinity up to one-third that
of sea-water.
A. claviger bites man readily in the open near its breeding places, but
comparatively seldom indoors. A somewhat unusual record is one by C.
Bonne, a Dutch mosquito specialist, who took a female A. claviger biting
indoors by a gas-fire at Mill Hill on i8th October, 1935 (a late date for this
species).
Anopheles algeriensis Theobald
This is a brownish insect very much resembling A. claviger, from which it
may be distinguished most readily by the fact that the thorax is of a more
uniform brown, the sides not appearing obviously darker than the middle part
when the insect is viewed from above ; the small hairs on the thorax are less
numerous, more evenly spread, and somewhat darker than in A. claviger ; also,
there are no tufts of white scales either on the front of the thorax or on the crown
of the head. This last feature provides a distinction from A. plumbeus and A.
maculipennis as well as from A. claviger.
A. algeriensis, so far as our present knowledge goes, has an extremely local
distribution in Britain, having been found in only two districts in Norfolk—
the neighbourhood of Hickling Broad and at Foulden, near Stoke Ferry ; in
these localities it was very abundant in August 1932. It is to be expected that
it will also be found when searched for in other parts of the Broads district,
but it seems worth noting that the surveys recently made of the fauna of Wicken
Fen failed to reveal its presence in that locality in spite of the fact that the
conditions there are very similar to those which obtain at Hickling.
The life-history of A. algeriensis is not yet known in detail, but it has been
suggested that it may differ from our other three species in having only one
generation in the year the adults of which appear in the warmest month. It
would be of great interest if some resident in the Norfolk Broads would put
this suggestion to the test.
Another point in which A. algeriensis differs from our other British
Anopheles is in its attitude of rest ; when settled on a vertical surface the body
is held practically parallel with the surface instead of at a distinct angle.
Females of A. algeriensis were found to bite readily in calm dull weather
both at dusk and dawn, but only in the open or in tents : none could be found
in pig-sties or in buildings. Larvae were found in extensive shallow puddles
among thick sedge on the marshes adjoining Hickling Broad ; none were seen
in the open weedy ditches dissecting the marshes, though these ditches harboured
plenty of A. claviger larvae.
Anopheles plumbeus Stephens
(Plate 2)
The name of this mosquito indicates one of its most obvious distinguishing
features, the leaden-grey colour of much of the upper surface of the thorax.
It is a much darker insect than A. claviger, the general naked-eye appearance
being almost black. The thorax when seen from above appears blackish at
the sides, the broad grey median stripe being clothed with white hairs, which
are longer and denser towards the front, where they form a rather conspicuous
tuft ; there is also a conspicuous white tuft on the crown of the head (not shown
clearly enough in the plate). The wings are more densely scaled and blacker
m appearance than in A. claviger or A. algeriensis, which like the present species
have no dark spots on the wings.
The distribution of A. plumbeus in Britain is conditioned by the presence
of large trees which can form rot-holes. Subject to this limitation it is found
throughout Britain and in Ireland, and has occurred as far north as Skibo
Castle, Sutheriandshire. It is naturally commonest in wooded districts.
A. plumbeus is unique among British Anopheline mosquitoes in breeding
almost exclusively in the water which collects in rot-holes in trees. The larvae
may often be found in large numbers in the larger rot-holes in such trees as
beech elm, sycamore and horse-chestnut, especially when these holes are
capable of holding a fair volume of water for a sufficient time for it to become
dark brown in colour through dissolved tannin ; the smaller holes, and those
m torks and among roots, which only retain water for shorter periods, are less
suitable breeding-places for this species. Exceptionally the larvae may also
be found ,n water-butts or tanks, but never m ponds or ditches like those of the
other species of the genus. The food of the larvae appears to consist largely