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20 BRITISH BLOOD-SUCKING FLIES NEMATOCERA—CULICIDAE 21
differences between the two in the structure of the genitaha ; (i) the distinctions
are of the same order as those between Anopheles maadipe7inis atroparvus and
A. 7)1. messeae, which have hitherto been treated as races or varieties and not
as species. On the other hand, Marshall points out that : {a) Weyer has shown
that molestus, but not pipieiis, will interbreed with the tropical C. fatigans
Wied. ; (¿) close similarity or even identity in genitalic structure is no proof
of identity of species, as is demonstrated, for example, by Theobaldia morsitans
and T. ftimipemiis ; (¿r) different strains of C. molestus will not interbreed,
and these strains are perhaps comparable with the races {atroparvus, messeae,
etc.) of A. maculipennis.
The distinctions between C. pipie7is and C. 7nolestus are most readily
appreciated in the larval stage and in the male sex, the larva (in all four growthstages)
having a shorter siphon and shorter anal papillae and the male shorter
palpi in C. 7nolestus than in C. pipietis. The females of the two cannot always
be distinguished with absolute certainty, the most reliable distinction, according
to Marshall, being in the colour of the scales on the under surface of the
abdomen ; in C. pipiens these scales are mostly cream-coloured, but there is
always a small patch of black scales in the middle, and usually another on
each posterior corner of each segment, whereas in C. 7nolestus the scales are
either all cream-coloured or with small median black spots but no lateral black
spots. Marshall also states that when adults of C.pipie7ts and C. molestus are
viewed side by side, the former species is distinguishable by the darker brown
colour of the abdominal tergites and the more obvious pale spots at the tips
of the femora and tibiae. Much more important than the very small morphological
differences between the two species are the differences in habits and
life-history as described below.
Culex pipiens Linnaeus
This species is abundant throughout the British Isles, and is probably to
be found in every house in country districts, often in very large numbers, but
is very much less in evidence in towns. It is one of the three common British
mosquitoes of which the females hibernate, passing the winter in a torpid
state in cool but more or less frost-free cellars, out-houses, caves, mine-adits
or similar places. They emerge from hibernation in April and commence egglaying
in May. Two or more generations are passed through during the
summer, and by the beginning of September the species has reached its peak
in numbers and in favourable years is very abundant. The mean autumn
temperature, however, is not sufficient to allow breeding to continue in nature
—though in the laboratory with somewhat higher temperature and increased
illumination it has been found possible to induce the insects to continue breeding
through the winter. During September females begin to enter sheltered
places for hibernation ; males and larvae do not survive much if at all beyond
the end of October.
The characteristic breeding-places of C. pipieTis are rain-water butts, tubs
on allotment gardens, and puddles and ditches about farms, also pond-margins
and sometimes wells ; though normally living in fresh and fairly clear water
the larvae can accommodate themselves to salinity up to half that of sea-water
and to a considerable degree of pollution.
It is commonly stated that the female of this species requires a feed of
blood before she can lay fertile eggs. This blood-meal is generally obtained
from birds (including poultry), sometimes from rodents or even from frogs,
but rarely if ever from man. The man-ignoring propensity of C. pipieTis may
be regarded as definitely established by numerous laboratory experiments
which have shown that it is a matter of great difficulty or even impossibility to
induce females of this species to feed on a human subject, and that in spite
of the vast numbers which are to be found in houses in this country no proof
of their biting is forthcoming ; in all cases where complaints of mosquitoes
biting indoors have been investigated the offenders have been shown to be
either 1 he allied C. molestus or some other species such as Theobaldia annulata.
As an alternative or supplement to the blood-meal, C. pipiens sometimes
takes honey from flowers, and Myers has recorded a remarkable case of its
feeding on milk in separating-pans, sitting on the creamy surface and piercing
this to obtain the skim-milk beneath. Whether milk would be an adequate
alternative to blood as an egg-producing meal is a point which has not been
investigated. Food taken by the female in autumn, whether in the form of
blood or not, is stored in the fat-body for consumption during hibernation.
C. pipie7ts is unable to mate in small confined spaces ; mating takes place
in the open during the swarming of the males. Autumn mated females store
the sperms in their spermathecae for use in the spring when, after the first
blood-meal, development of the eggs takes place.
Culex molestus Forskál
In striking contrast with C. pipieTis, C. molestus is only very sporadically
distributed m Britain ; apart from a number of occurrences in London its
presence has only been established in two seaside places—Hull and Hayling
Island, and suspected in another seaside town—Harwich. The fact that it
has only been found in or near sea-ports suggests that it may not be a permanent
native but is introduced sporadically by shipping. It is abundant
m southern Europe, but in France and Germany, as in Britain, it is much less
widely distributed and occurs chiefly in towns. It is thus even more strictly
domestic than C. piptens, but it does not hibernate so completely
C 7nolestus has been the subject of detailed laboratory researches during
he last few years in this country as well as on the continent. It has been shown
o be capable of mating in small confined spaces (even in jars) and of producing
iertile and viable eggs without any food having been taken by the adult, also
in com f t T ? life-history, even for several successive generations,
but o 7 t ; " temperatures than C. pipíen^
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