m
28 BRITISH BLOOD-SUCKING FLIES NEMATOCERA—CERATOPOGONIDAE 29
the front part of the body protruding. They are most commonly found in mud
that is rich in confervae, or among mats of filamentous green algae. The
duration of larval life varies with climatic conditions and with the species ;
in the case of the salt marsh species of North America, Dove, Hall and Hull
(1932) concluded that the larvae live for at least six months if not for a whole
year, and that they flourished best at rather low temperatures. For most of
the European species precise data on this subject are not available.
The pupae are provided with small breathing-horns on the thorax, and rows
of spines on the abdomen ; they are capable of very little movement, merely
turning the end of the abdomen about like the chrysalis of a moth ; usually
thev may be found floating passively on the surface of the water; they breathe
air and are incapable of surviving long submersion in water. The duration of
the pupal stage is from three to seven days.
Species of Culicoides are found in most parts of the world, except (apparently)
in Patagonia and New Zealand, and the blood-sucking habit appears to be
universal, birds as well as mammals being attacked. There appear to be no
records at present of attacks by midges on reptiles or amphibia, but an interesting
variation of the blood-sucking habit is found in an Oriental species {C. anophelis)
which obtains its food from the body of a gorged mosquito instead of directly
from a mammalian host.
In some countries the annoyance caused by midges is extremely serious,
and deaths have even been attributed to their bites (as in Belgium, reported
by Goetghebuer, 1919), though it is probable that in such cases the real cause
of death was blood-poisoning by subsequent infection of a scratch.
Considering the abundance and ferocity of many midges, it is perhaps
surprising that there is as yet little evidence incriminating them in the spread
of disease. Evidence has however accumulated in recent years showing that
species of Culicoides are intermediate hosts of certain filarial worms which
pass part of their life in the blood of mammals.
Dyce Sharp showed, in 1928, that C. austeni Carter and C. grahami Ansttn
are hosts of the worm Filariaperstans Manson, in West Africa ; the young stage
of this worm is very common in the blood of man, gorilla and chimpanzee in
the Cameroons and the Congo basin, but it is not known to cause any serious
pathological developments in man. J. J. C. Buckley, in 1933-4, demonstrated
the similar part played in St. Vincent by C. furens Poey in conveying the worm
Filaria ozzardi Manson, a common human blood-parasite in the West Indies
and Central America. In the same year, 1933, J. S. Steward, working in
Herefordshire, succeeded in establishing that the worm Onchocerca cervicalis
Railliet and Henry is transmitted by C. 7i7ibeculosus Mg., and probably by
some other species of Culicoides ; this worm is the principal cause of nuchal
disease (fistulous withers and poll-evil) in horses ; it occurs also in America,
where it has been suggested that it may be transmitted by C. furens Poey.
The irritation and annoyance caused by the attacks of midges is often so
great as to constitute a serious problem. Their numbers are in some countries
sufficient to make outdoor work impossible, and they may seriously interfere
with tourist business at seaside resorts. It has been suggested that their
abundance on the coasts of Florida and other southern states of North America
has been largely the cause of the lack of early development of these areas ; and
also that the backward condition of croft farming in western Scotland may be
partly due to their pernicious activities. Certain it is that they constitute a
major pest in parts of Scotland as well as in the West Indies.
As in the case of mosquitoes, the effect of the bites varies with different
victims as well as with the species of midge ; on some persons the disagreeable
sensations caused by the bites are transitory, the only after-effect being a small
red spot on the skin, but on others (especially women) weals are raised which
may cause great discomfort for two or three days. B. Jobling (1928), who is
evidently a susceptible subject, thus summarises the effects upon himself of the
bites of three species of Culicoides :
" The intense irritation caused by the bite of Culicoides begins at the
moment of piercing, and lasts from one to several days according to the
species. Scratching the skin increases the irritation as well as the size of
the swelling and prolongs their duration. Moreover, by scratching, it is
very easy to tear the skin from the swelHng ; the resulting sore heals much
more slowly than a sore produced by an accidental cut or scratch, being also
liable to bacterial infection. The irritation and swelling caused by the bite
of C. pidicaris is of very short duration and in many cases disappears on
the following day, whereas the bite of C. vexans, and more particularly of
C. obsoletus, produces a more intense irritation, which may last more than
a week, the swelling being very distinct on the second and in many cases
even the third day after the bite."
Jobling further remarks that " if the lesion is moistened and rubbed with a
crystal of sodium carbonate, the irritation ceases in a few seonds, and does not
recur. Furthermore, the swelling also subsides very rapidly. The same result
is obtained even if the application is not made till twenty-four hours have
elapsed."
Owing to the nature of the life-histories of most of the species, and the
incompleteness of our present knowledge, few- practical suggestions as to control
measures can be made at the present time. In Britain the species most susceptible
of control by direct attack would appear to be ¿7. mibeculosus, which could
presumably be dealt with effectively by drainage or chemical treatment of farmyard
manure-heaps, but the more universally annoying species (such as C.
obsoletus and C. impunctatus) are not so susceptible. Suggestions for control of
salt-marsh species (which are the most serious offenders in North America
and the West Indies, but not, so far as our present information goes, in Britain)
have been made by Dove, Hall and Hull (1932) and Myers (1933). These
workers found that control is possible by drainage, as the midges do not breed
in any great numbers in those ditches which have a free flow of tidal water and
whose edges are kept straight. These authors also found that the flight-range
ot the salt-marsh species is less than half a mile, an important consideration
wtien questions of control arise. In the case of the common British species it
IS thought—though without experimental basis—that the flight-range does not
as a rule exceed a few hundred yards from the breeding places. Nevertheless