14 BRITISH BLOOD-SUCKING FLIES N E M A T O C E R A — C U L I C I D A E IS
The life-history of A. dorsalis, so far as known, is similar to that of its close
relative A. caspius, but larvae have not been found in fresh water in this country.
The winter is apparently passed in the egg stage ; Marshall records that of a
batch of eggs obtained in the early summer of 1935, some hatched on immersion
ten days after being laid, others, after having been preserved dry (on moist
filter-paper) for a year.
A. dorsalis is widely distributed in Europe, North Asia and North America,
chiefly in coastal or saline districts. It is one of several species of Aedes which
have recently been shown experimentally to be capable of transmitting the virus
of encephalomyelitis in horses in the United States.
Aedes flavescens Müller
This is one of four species of Aedes which have conspicuous white rings on
the tarsi of the hind legs, these rings being confined to the bases of the segments
and not embracing the joints. From the other three species of this group A.
flavescens is sharply distinguished by the colouring of the body, the thorax
being dark golden-brown and the abdomen mainly covered with straw-yellow
scales, with indications of interrupted median and lateral black stripes ; the
scales on some of the wing-veins are predominantly yellow, on others predominantly
dark ; the first segment of the hind tarsus is yellow, except for the
narrow basal white ring and a still narrower black ring at the tip.
A. flavescens is at present considered one of the most local of the British
mosquitoes, having been obtained so far in only two localities—the Isle of
Sheppey and Wakon-on-Naze. Both discoveries were due to P. G. Shute, who
found the larvae in deep ditches containing slightly brackish water and situated
not far behind the sea wall in open marshy country.
According to the investigations of Wesenberg-Lund in Denmark and Hearle
in Canada (where this species also occurs) A. flavescens has one annual generation
; the eggs are laid in low-lying places amongst reeds or sedge ; they become
submerged during the winter, the larvae hatch in February or March, and the
adults are on the wing towards the end of May. The life-history of the species
in Britain appears to be similar.
Aedes annulipes Meigen
This is in most respects similar to A. cantans, differing (in the female sex)
in the following particulars : pale scales of thorax, abdomen, wings and femora
and tibiae yellow instead of white ; scales of thorax mainly golden brown, with
a broad darker brown median stripe ; transverse pale bands of abdomen and
white rings of tarsi broader.
A. annulipes is a rather local species in Britain, but has been recorded from
many localities in the south-east and midlands of England, the most northerly
being Austwick (N.-W. Yorks). It is very abundant and troublesome in some
localities in the fens and broads.
A. annulipes breeds in open or only partly shaded situations, typically
among reeds or sedge with sallow and other bushes forming " carr," as at
Wicken and Wood Walton fens, and apparently always in fresh water. Adults
hatch late in April (usually somewhat later than A. cantans) and may remain
on the wing until September. Eggs laid during the summer do not hatch (even
if submerged) until early the following year.
Aedes cantans Meigen
(Plate 5)
From the two nearly related species with white rings at the bases of the
hind tarsal segments, A. flavescens and annulipes, A. cantans is distinguished
by its darker colour : the thorax is dark brown, usually with a pair of small
patches of pale scales ; the abdomen is largely blackish above, though usually
with narrow transverse white bands ; and the legs are more extensively dark,
the white tarsal rings being narrower. The wings have scattered pale scales
(variable in quantity) which are white, not yellowish, and the pale scales of the
legs are also white.
A. ca7itans is recorded from rather more localities in England than A.
annulipes, though there are at present no records from Wales, or Ireland, and
the only Scottish record is of females from Dingwall, Cromarty.*
In contrast with A. annulipes, A. cantans is definitely a woodland species,
its breeding places being temporary pools which are well shaded. At Hayling
Island, the removal of bushes and overhanging trees from around a prolific
breeding place of A. cantans resulted in the entire disappearance of the larvae
for a long period. When, however, the bushes were allowed to grow again
the water became reinfested with larvae. Adults are hatched in April and live
on (or are supplemented by fresh hatchings) through the summer until September.
Eggs laid in the summer do not hatch until the following February
or March (even if submerged in the autumn), and even then some remain
unhatched so that if the breeding pools become dried out in the spring, summer
rams may result in a fresh supply of larvae hatching (from the previous season's
eggs) to replace those which had failed to mature through drought.
Aedes vexans Meigen
In general this species is very similar to A. cantans, though the two are not
nearly related, as is clearly shown by the structure of the male genitalia and
larvae. Apart from its usually smaller size A. vexans may be distinguished
from yi. cantans by having the pale transverse bands of the abdomen constricted
in the middle and the scales of the wings all dark ; the white rings of the tarsi
are narrower and less conspicuous than in the other species.
The few records of A. vexans in Britain seem to be of a sporadic
nature and suggest that the species may not be a permanent native with us ;
r e n r H V r it is possible that they may
rerbeen r Continental s yet Deen recognised as occurring in Britain. pecies which have not