INTRODUCTION Vil
INTRODUCTION
THE habit, found amongst certain Diptcra, of piercing the skin of man and
other warm blooded animals and sucking blood is a highly specialised one.
The mouth-parts of the more primitive insects such as grasshoppers and cockroaches
are of a simple biting type adapted for chewing vegetable matter
which, after being reduced to a suitable size, is swallowed in a solid state.
Adult Diptera have deserted this mode of feeding on solid matter in favour of
one which allows them to suck up fluids or finely divided substances such as
pollen, and their mouth parts are modified accordingly.
All the three sub-orders of the Diptera contain species w^hich attack man or
other warm blooded animals for the purpose of feeding upon their blood
The sub-order Nematocera comprises about fourteen families of which three are
notorious for the attacks that some of their members make in their attempts
to obtain blood meals. These families are the Culicidae (Mosquitoes, p. i),
the Ceratopogonidae (Midges, p. 25), and the Simuliidae (Black-flies, p. 50),
Another family, Psychodidae (Moth Flies), includes one blood-sucking genus,
Phlebotomus (not British). In the Culicidae and Simuliidae the blood-sucking
habit is general, but in the Ceratopogonidae it is confined to a few genera,
only one of which {Culicoides') is British. In the sub-order Brachycera it is
confined in Britain to the Tabanidae (Horse-flies, p. 67), though the
Rhagionidae include blood-sucking members in America and Australia. Many
other Brachycera, such as Asilidae (Robber-flies) and Empididae, have their
mouth parts in the shape of a formidable beak but these flies prey upon insects
and other arthropods, and do not attack warm blooded animals. The third
sub-order of the Diptera, the Cyclorrhapha, has blood-sucking species in four
families. These are the Muscidae (" Flies "), where the habit is confined
to one sub-family, the Stomoxydinae (Biting Flies, p. 115), the Flippoboscidae
(Louse Flies, p. 118), the Nycteribidae and the Streblidae (Bat Flies, p. 124);
the last mentioned family does not occur, however, in the British Isles. The
last three are relatively small families of truly parasitic insects and, as far as
is known, blood-sucking is universal amongst them.
The blood-sucking habit in the Nematocera and Brachycera (families
Culicidae, Ceratopogonidae, Simuliidae and Tabanidae) is found amongst the
females only. While there are some species that can, and do, produce fertile
eggs without having partaken of a l)lood meal the number of eggs produced
on these occasions is small. Development of the normal number of eggs and
their subsequent deposition apparently takes place only after the females have
taken a satisfactory meal of blood. There is also a consideral)lc amount oi"
evidence showing that the blood meal itself is taken by the female fly only
after she has copulated with a male and thus more or less assured the fcrtihty
of her eggs. The males are probably nectar feeders.
In a generalised insect the mouth parts consist of the labium, a pair of
maxillae, a pair of mandibles and the labrum. The hypopharynx is a tonguelike
projection on the floor of the mouth on which is situated the opening of
the salivary duct. In blood-sucking Diptera the hypopharynx and the labrum
are produced into awl-like structures which together form a tube up which the
blood is sucked when they are inserted into the skin of the victim. Coagulation
of the blood and consequent blocking of the mouth parts is apparently prevented
by the salivary secretion which is poured into the wound.
In the Nematocera and the Brachycera the wound is cut by the mandibles
and the maxillae, both of which are usually furnished with serrate cutting
edges, and are inserted into the wound together with the labium and hypopharynx.
The labium is soft and fleshy and is not inserted into the wound ;
its function is to protect the cutting and sucking parts and to guide and support
them when they are brought into use. The size and delicacy of the various
parts varies ; in the Culicidae they are very elongate and extremely delicate
while in the Tabanidae they are short and quite stout. In the males the mouthparts
are constructed on the same lines but the degree to which they are
developed as cutting appliances is much less than in the females, as would be
expected from their non blood-sucking habits.
In the Cyclorrhapha the mandibles and the maxillae are completely atrophied,
no trace of them can be seen, and the actual cutting of the wound is
performed by the labium. In the blood-sucking Cyclorrhapha the labium is
in the form of a horny stylet and it is actually inserted into the wound. The
tip of the labium is rasp like and it is grooved above to accommodate the
elongated labrum and hypopharynx.
The males of the cyclorrhaphous blood-sucking species take blood as well
as the females. It is possible amongst them to trace the development of the
blood-sucking habh and the correlated evolution of piercing mouth parts.
Starting with species having normal mouth parts, which are attracted to
wounds and suck up the blood oozing from them, the chain leads through
species (not found in the British Isles) which are able to scratch the skin with
their labium till the blood oozes and can be sucked up, to the species (mentioned
in this book) which actually bite and penetrate the skin in their quest for blood.
Blood-sucking flies take more than one blood meal in the course of their
lives, and are thus liable to transmit disease from one person or animal to
another. The subject is a complex one that cannot be entered into here but
it gives these insects a tremendous importance in the Tropics and some other
parts of the globe. Certain diseases, such as malaria and sleeping sickness,
cannot be transmitted naturally in any other way than through the agency of
a blood-sucking fly, a mosquito in the case of malaria and the Tse-tse fly in
the case of sleeping sickness. So far as is known they are of relatively little
importance in this respect in the British Isles but notes on their status will
be found in the sections of this book dealing with the separate families.
In the introduction to the first edition of this work it was stated that of the
eight (or nine) groups of Diptera, of which some members have developed the
blood-suckmg habit, seven occur in Britain, and that of the total of about 2 700
R A F S I /