THE HUMAN FLEA AND OTHER SPECIES
THE human flea (_Pulex irritans_) appears to occupy an isolated
position. The genus _Pulex_ which Linnæus established has now been
reduced until it contains one species only. The human flea belongs to
the group with eyes and without combs. In some respects it is the
most specialized of all the _Pulicidæ_. The chigoes (_Sarcopsyllidæ_)
resemble it and are doubtless derived from the _Pulicidæ_. The chief
structural character of this interesting insect is the greatly reduced
thorax. But it can be distinguished from any other known flea by
the fact that the upper segment of the hind leg (hind coxa) bears a
number of hairs on the inner surface of the posterior portion. A more
noteworthy feature in this flea is the presence, in a large proportion
of specimens of both sexes, of a small tooth at the edge of the head.
This small tooth is sometimes absent; but, when present, both its
position and its structure indicate that it corresponds to the fifth
tooth in the head comb of the dog-flea (_Ctenocephalus canis_) (Fig.
7). In the hedgehog-flea (_Ct. erinacei_) the teeth of the combs both
on the head and on the thorax are small in size and few in number.
Occasionally they almost disappear. The conclusion seems justified
that the human flea is descended from an ancestral form with combs. To
discuss whether the combs became useless and were lost when the host
lost the hairy covering of its body would lead into regions of vague
speculation and occupy time unprofitably.
The nearest allies of the human flea, which are found on various
animals, are all inhabitants of the Old World. The indigenous fleas of
America are only distant relatives of _Pulex irritans_. Our knowledge
of the present and former distribution of this species is deplorably
meagre. The many books of travel published in the early part of the
nineteenth century contain hardly any records of fleas. The human flea
is now cosmopolitan. Specimens identical with those from Europe are
found almost everywhere. But it may be doubted whether this was the
case before the great era of travel and steam began in last century.
There is one strange and, indeed, inexplicable fact in connection with
the distribution of this cosmopolitan species of flea. It is absent
from the oases of the Sahara and the Haussa countries immediately
to the south of the great desert. These countries have long been in
communication with places where _Pulex irritans_ is known to abound.
There is no natural barrier. The habits of the natives would encourage
fleas to thrive, and other forms of human vermin are plentiful. There
is, apparently, only one explanation that is forthcoming. It is
suggested that the soil and climate in these regions of Africa are, for
some reason, unsuited to fleas. In other parts of the Dark Continent,
where there are European settlements, the human flea seems to thrive
surprisingly well and to attack Europeans and natives, as well as wild
and domestic animals. In those parts of Asia where there are European
colonies and much intercourse between settlers and Orientals, _Pulex
irritans_ is a well-established and thriving parasite. Unfortunately,
there is no means of knowing whether this was the case among the
native populations before European travellers and traders arrived.
_Pulex irritans_ has, however, recently been found on the natives of
German New Guinea living some 10,000 feet above sea-level and in great
isolation. Seaports are everywhere infested with fleas.
Another problem on which no light has been thrown concerns the
evolution of the human flea. It would be of great interest to know
whether the present species has undergone modifications of form since
it became a parasite of the human race; whether we inherited the
species from our simian ancestors; or whether the flea of one of the
lower mammals became parasitic on mankind. In the Old World this flea
is essentially a parasite of man. It occurs only occasionally on other
mammals. In America it certainly appears to occur more frequently on
mammals, other than man, than it does in the Old World. Human fleas
can propagate in deserted human dwellings. The larvæ find nourishment
in any refuse that has been left behind, and the adult insect can
apparently continue for some time to reproduce itself without a meal of
any sort and certainly without human blood. Travellers in the East and
in Africa have described how on entering huts in deserted villages they
have found their clothing covered with myriads of fleas, sometimes
ravenous, and at others weak from long fasting.
The human flea is a good deal more select in the choice of a host than
some other species. The cat-flea (_Ctenocephalus felis_) has been found
not only on the cat, but also on the dog, tiger, leopard, goat, horse,
rat, hedgehog, kangaroo, deer, guinea-pig, rabbit, and on man. Many of
these were specimens collected in zoological gardens. Although when
hungry and confined in a test-tube the human flea will readily bite a
rat or a guinea-pig, it has been found that human fleas kept with no
other food-supply than rats and guinea-pigs soon die off.
When large numbers of human fleas were wanted for experiments in
Bombay, guinea-pigs were used as traps to attract them. On one occasion
two guinea-pigs placed in a house which had been vacant for some days,
and in which fleas must have been short of food, failed to attract any
of this species; while a man who entered the house shortly afterwards
acted as an admirable trap. Those who have not had experience of the
abundance and voracity of fleas in oriental countries can hardly
believe the numbers of human fleas that may be captured by sending a
bare-legged man into a deserted house and then picking the fleas off
him. In one house 31 _P. irritans_ were taken on a man’s legs in a
few minutes. In another house 84 _P. irritans_, 8 cat-fleas and 1
bird-flea were caught. In a third, 150 _P. irritans_ and 4 cat-fleas
were captured in a short time.
The piercing organs of the human flea are strong and well developed.
This is rare in a flea which, far from having adopted stationary
habits, is a very active insect. It has been suggested, with some show
of probability, that the wide and strongly serrated mandibles were
acquired after man became the host. The naked skin and rough garment
of mankind would render the claws and legs of the flea insufficient to
keep the insect in a steady position when feeding. Natural selection
would, in due course, strengthen the mouth organs.
The division of mankind into different races, many of which are quite
as distinct as the various species of some genus among other animals,
leads one to expect various races among the fleas which are parasitic
on them. If the sand-martin and the house-martin, the rat and the mouse
have distinguishable fleas, one might suppose that the Caucasian and
the Hottentot, the Australian native and the Red Indian would follow
suit. It may be that further study will show that the human flea now
consists of a number of different races. In only one case, however,
does a development of this kind in fact appear. Fleas taken off Mexican
Indians show slight but fairly constant differences from the true
_Pulex irritans_. The specimens are smaller in size, the rostrum is
longer and the clasper of the male is more pointed. If the Mexican
Indians have a special race of human flea it must have developed after
the Indians came to America, or they must have brought it with them
when they came. In the latter case this race of flea may still exist in
the country whence these Indians originally came.
Apart from this apparently constant race, the individual variation in
specimens of the human flea is slight. If a large series of mounted
specimens are examined with the microscope, it will be noticed that the
bristles or spines on the legs are sometimes more or less numerous.
But, with this exception, marked varieties such as are frequently found
among other insects seem to be rare.
Although mankind is the true host of this flea, it has been obtained
in various parts of the world on various mammals and occasionally on
birds. But in England, and probably in other parts of Europe as well,
_Pulex irritans_ is an undoubted parasite of the badger. A good series
of the insect has been got from wild badgers freshly captured near
Reading in Berkshire and Hastings in Sussex. In other parts of the
world it has been obtained from a variety of small carnivora: cats,
dogs, foxes, jackals and polecats. It has also been found on Rodents
(_Gerbillus_) and on Insectivora (_Erinaceus_). In South Africa it has
been taken off a caracal and in North America off a lynx.
Sandy places such as sea-beaches and picnic grounds, where humanity
congregates for pleasure or business, frequently swarm with this
species of flea waiting an opportunity to feed. The larvæ are bred in
the sand and feed on organic refuse.
The genus most closely allied to that which contains the human
flea consists also of a single species only. It is a large flea
(_Pariodontis riggenbachi_) found on porcupines all over Africa and in
India.
Mankind is, occasionally, bitten by a variety of other species
besides _Pulex irritans_. In hot countries the chigoe (_Dermatophilus
penetrans_) is a serious and troublesome pest, particularly to
bare-footed people. In temperate regions there are rat-fleas,
cat-fleas, dog-fleas and bird-fleas which occasionally transfer
themselves to man and feast on his blood. But, on the whole, hunger and
propinquity rather than free inclination seem to actuate these fleas of
which man is only the occasional host. There are besides very numerous
species which have never under any circumstances been known to bite
man. There is no doubt that some persons are more attractive to fleas
than others. The reason for this we do not know. It may depend on the
tenderness of their skin, the quality and taste of their blood, or
their personal smell, or possibly all three combined.
The various forms of rat-flea which are important in carrying plague
from rodents to the human race are dealt with later on. Among the
commonest fowl-fleas which bite man are _Ceratophyllus gallinæ_ and
_C. gallinulæ_. Both species infest the nests of many common passerine
birds besides the domestic fowl. A common parasite of the pigeon is _C.
columbæ_, which also bites man.
[Illustration: Fig. 7. The head of a female dog-flea (above) and a
female cat-flea (below) to illustrate the difference in shape. In the
males the difference is less strongly marked but quite perceptible.
From _Novitates Zoologicæ_, Vol. XII, January, 1905.]
Dog-fleas and cat-fleas frequently transfer themselves to man. It
has been asserted that the flea of the dog and the flea of the cat
are indistinguishable. Several great authorities on fleas, such as
Dr Carlo Tiraboschi in Italy and Mr Carl Baker in the United States,
have maintained that the differences between _Ctenocephalus canis_ and
_Ct. felis_ were unreliable and that they are not distinct species.
Mr Charles Rothschild has, however, shown that the two species are
abundantly distinct. The _males_ of these two insects can be readily
distinguished from each other by differences exhibited in their
respective sexual organs. The _females_ can be distinguished, at a
glance, by the different shape of their respective heads. Fig. 7, which
shows the head of a female dog-flea above and of a female cat-flea
below, illustrates this. It will be seen that _Ct. felis_ has a much
longer and more pointed head than _Ct. canis_. In the _males_ the
difference in the shape of the head is less strongly marked, but is
quite perceptible. There are several minor differences in addition
which serve, but less clearly, to distinguish these two insects. The
first genal spine, or first tooth in the head-comb, is shorter in the
dog-fleas of both sexes than it is in the cat-fleas. The abdominal
stigmata appear to be larger in a dog-flea than in a cat-flea, and
there are differences in the bristles which seem to be constant. Both
species are perceptibly larger than human fleas, and dog-fleas have
always afforded good material for dissection. Very few dogs seem to be
exempt from fleas, and the little pets which are carried in ladies’
arms are often swarming with them.
This account of a despised and detested group of insects would be very
imperfect if it did not mention those educated or performing fleas
which have evoked so much astonishment among people who have watched
them. It will be best to say, at once, that the fleas are not educated
and that the performance can only be attributed to their desire to
escape. It is stated that a performing flea may be broken of the habit
of jumping by being put in a pill-box with glass sides which is made to
revolve like a lottery wheel. A short course of this tread-mill teaches
the flea that the objectionable practice of hopping is useless and
exhausting. It is said that the life of performing fleas averages eight
months, which seems surprising. They are fed every few days, and the
trainers delight in showing the punctures on their arms where the swarm
of pets has been fed.
Performing fleas are first of all securely fastened, and this is
nine-tenths of the secret, and the art of education. A very fine silk
fibre is put round the body and knotted on the back. The flea may
then be cemented to some moveable or immoveable object. It may pull a
coach by being attached to a pole made of a bristle. A little paper
object stuck on its back is termed by courtesy an equestrian or a
ball-dress. The lively imagination of the spectators is of great help.
The strength of a flea is wonderful, and on being placed on a sheet of
blotting-paper, so that the hooks of the feet get a hold, the coach
travels at a fine pace. In the intervals of the performance the coach
is turned over, and the performer with its feet in the air does not get
exhausted with needless struggles. Or else the fleas are fixed head
uppermost, with their legs extended horizontally, to an upright wire
driven into the table. Ladies have fans of tissue paper gummed to their
limbs. Gentlemen are in the same way supplied with swords made out of
fine segments of wire. When two swordsmen are placed opposite each
other and the table is knocked they move their limbs. The swords then
clash by chance, and we have a representation of a duel not much worse
than may be seen in provincial or even London melodrama.
More wonderful are dancing fleas, for there we have a real
representation of a ball-room filled with waltzers. The orchestra
of fleas, all securely fixed with cement, is placed above a little
musical-box. The music proceeds from the box, but the vibrations make
the fleas gesticulate violently over their musical instruments. The
dancers spin round on the ball-room floor. The couples are fastened by
a rigid bar opposite each other, so that they cannot touch or part.
Each is pointed in an opposite direction, and tries to run away. A
rotary motion ensues which, to the spectators if not to the fleas, is
very like waltzing.