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The Cambridge Manuals of Science and Literature
THE FLEA
CAMBRIDGE UNIVERSITY PRESS
London: FETTER LANE, E.C.
C. F. CLAY, MANAGER
[Illustration: LOGO]
Edinburgh: 100, PRINCES STREET
London: H. K. LEWIS, 136, GOWER STREET, W.C.
WILLIAM WESLEY & SON, 28, ESSEX STREET, STRAND
Berlin: A. ASHER AND CO.
Leipzig: F. A. BROCKHAUS
New York: G. P. PUTNAM’S SONS
Bombay and Calcutta: MACMILLAN AND CO., LTD.
_All rights reserved_
[Illustration:
_After a drawing by Dr Jordan_
Oriental rat-flea (_Xenopsylla cheopis_ Rothsch.). Male.]
[Illustration; DECORATED FRONT PAGE:
THE FLEA
BY
HAROLD RUSSELL,
B.A., F.Z.S., M.B.O.U.
With nine illustrations
Cambridge:
at the University Press
1913]
Cambridge
PRINTED BY JOHN CLAY, M.A.
AT THE UNIVERSITY PRESS
_With the exception of the coat of arms at the foot, the design on
the title page is a reproduction of one used by the earliest known
Cambridge printer, John Siberch, 1521_
PREFACE
THE aim of this book is to give in plain language some account of a
small, but noteworthy, group of insects. I have avoided, whenever I
could, using the technical terms of zoology. To avoid doing so entirely
is impossible in a book which describes insects in some detail. No
technical term has, I hope, been used without an explanation.
Over thirty years have elapsed since Taschenberg’s German book, _Die
Flöhe_, appeared. Our knowledge has made enormous strides since then.
More species of flea are now known from the British Islands alone
than were then known from the whole world. So far as I am aware, no
book, devoted to what is known about fleas, has ever been published in
English. The statements about these insects in the general text-books
of entomology are frequently antiquated and inaccurate. But there is
a fairly extensive literature on the _Siphonaptera_ scattered through
scientific periodicals mostly in English, German, Italian, Dutch and
Russian. I have given some references in the Bibliography.
The naturalists now living who have devoted any time to the special
study of fleas may almost be counted on one’s fingers. In England there
are Mr Charles Rothschild and Dr Jordan; in the Shetland Islands, the
Rev. James Waterston; in Germany, Taschenberg of Halle and Dampf of
Königsberg; in Russia, Wagner of Kieff; in Holland, Oudemans of Arnhem;
in Italy, Tiraboschi of Rome; in the United States, Carl Baker and a
few others. I have not mentioned medical men who have investigated
fleas in connection with plague.
There are small collections of fleas in the Natural History Museums at
South Kensington (London), Paris, Berlin, Königsberg, Vienna, Budapest,
S. Petersburg and Washington. Of private collections Mr Charles
Rothschild’s at Tring is by far the best in the world. It contains
something like a hundred thousand specimens and is most admirably kept.
I must express profound and sincere gratitude to Mr Rothschild for
having helped me in numberless ways and advised me in many difficulties.
It is well known that the mere mention of fleas is not only considered
a subject for merriment, but in some people produces, by subjective
suggestion, violent irritation of the skin. The scientific study
of fleas has, however, received a great impetus since it has been
ascertained that they are the active agents in spreading plague.
Rat-fleas are of various kinds, and not all fleas will bite man. A
knowledge of the different species has suddenly become useful. The
humble, but ridiculous, systematist with his glass tubes of alcohol for
collecting fleas, his microscopic distinctions, and Latin nomenclature
has become a benefactor of humanity. Some people seem to be practically
immune to the bites of fleas, but even to such persons their visits are
unwelcome. A famous Frenchwoman once declared: “_Quant à moi ce n’est
pas la morsure, c’est la promenade._”
H. R.
LONDON,
_September, 1913_.
CONTENTS
CHAP. PAGE
Preface v
I. Introductory 1
II. The external structure of a flea 21
III. The mouth-parts and sense-organs 38
IV. The internal organs of a flea 52
V. The Human flea and other species 62
VI. The Chigoes and their allies 74
VII. Fleas and Plague 83
VIII. Rat-fleas and Bat-fleas 97
Appendix A. Systematic view of the order _Siphonaptera_ 108
” B. A list of British fleas and their hosts 110
” C. On collecting and preserving fleas 113
” D. Bibliography 118
Index 122
LIST OF ILLUSTRATIONS
Male Oriental rat-flea _frontispiece_
FIGURE PAGE
1. The larva of a flea 6
2. Types of genal and thoracic combs of a flea 26
3. The hind leg of a flea 30
4. The mouth-parts of a flea 43
5. The antenna of a flea 47
6. The alimentary canal of a flea 53
7. The head of a female dog-flea and a female cat-flea 71
8. Pregnant female of _Dermatophilus cæcata_ 81
CHAPTER I
INTRODUCTORY
FLEAS form a group of insects that have, until recently, been little
studied by zoologists. We call them insects because they are jointed
animals, or Arthropods, with three pairs of legs in the adult
condition. The reader will best understand the position which fleas
occupy in the general classification of animals by remembering that
the arthropods, or jointed animals, are one of a dozen subkingdoms, or
phyla, to which the various members of the great animal kingdom have
been assigned. There is good ground for believing that all the animals
included in each phylum trace their ancestry back to a common primitive
form which lived in more or less remote ages. Besides (1) _Insects_,
the arthropods, or jointed animals, include (2) _Crustaceans_, such as
crabs, lobsters, shrimps, wood-lice, water-fleas and barnacles; (3)
_Myriapods_, such as centipedes and millipedes; and (4) _Arachnids_,
such as spiders, scorpions, mites and ticks. To all these varied forms
of animal life fleas, and other insects, are therefore more or less
nearly related.
The animals belonging to this large and important collection, which
compose the arthropod phylum, have certain common characteristic
features. We find a body made up of a series of more or less completely
similar segments placed one behind the other. In this they resemble
certain worms which are far less highly organised. The body is
elongated, symmetrical on either side, and the mouth and anus are at
opposite ends. There is, however, an important advance on the segmented
worms. Each typical segment carries a pair of appendages which are very
different from the foot-stumps that are found on certain worms. These
appendages of arthropods are divisible into distinct limb-segments,
separated from one another by moveable joints, and acted upon by
special muscles.
The common ancestor of all the various arthropods which are found
living on the earth to-day, was probably composed of a series of
segments each very similar to the last and each bearing a pair of very
similar appendages. In the course of ages, these appendages have been
astoundingly modified in form and in function. So it happens that
we find in the arthropods of the present day pairs of antennæ, of
mandibles and other mouth-parts, of pincers, of legs, of swimming-feet
and of tail pieces which on close examination can all be traced back to
a common structure. The body-segments, also, have been strangely fused
together and modified. All that has been so far said applies equally to
fleas and to other insects.
It is of great interest, when one comes to make a minute study of
the form and external structure of a flea, to try and trace the
modifications that must have taken place in the course of descent from
the ancestral arthropod; but the relationship of fleas to other insects
living at the present day is of more immediate concern. Insects are
highly specialized arthropods and fleas are highly specialized insects.
This means that they have become vastly modified from the primitive
ancestral type and fitted thereby for a life among certain defined and
peculiar surroundings.
It will be unnecessary to remind the reader who knows anything of
zoology or of botany that all classification is now based on descent.
Since naturalists have abandoned a belief in the special creation
of the various species of animals now living on the earth and have
conclusively shown that they have arisen by descent and modification
from other forms, the problem is to reconstruct a vast genealogical
tree. What then were the ancestors of the fleas and to what other
insects, in consequence, do they appear to be related?
It is probable that the ancestors of the fleas were winged insects, and
that the organs of flight were gradually lost, as they became useless,
when a partially parasitic life was adopted. At one time entomologists
regarded fleas as wingless flies and placed them in the order Diptera.
Certain supposed scaly plates on their bodies were regarded as the
atrophied relics of wings. It is, however, more than doubtful whether
this view is correct; and all modern entomologists who have given any
special study to fleas are agreed that they are sufficiently unlike
any other living insects to deserve a place in an order by themselves.
To this order the name _Siphonaptera_ has been given: which means that
the insects comprised in it are provided with sucking mouths and are
destitute of wings. Another name for the order is Aphaniptera, but this
is gradually falling into disuse. Linnæus (1758) only mentions two
species of flea: the human flea which he appropriately named _Pulex
irritans_, and the chigoe of hot countries which he called _Pulex
penetrans_, from the habit which the female has of burrowing under the
skin of her victims. At the time of writing, about 460 species of flea
have been described and named; but some of the names are doubtless
synonymous, and the actual number of separable species that have been
discovered is somewhere about four hundred. The vast majority of these
have been described within the last few years, which shows what can be
done when attention is turned to any neglected group of animals. There
can be no doubt that many undiscovered species still remain, and will
now, in due course, be collected, described and named.
The position which should be assigned to the order Siphonaptera in the
general scheme of insect classification is a question on which the most
learned modern entomologists have disputed with considerable vigour.
Some see the nearest relatives among the beetles, others among the
flies. The majority, as we shall see later on, would place them near
the Diptera: but since no convincing arguments have been produced on
either side it may be wisest to regard the question as still at present
unsolved.
Fleas belong to one of the groups of insects which go through a
complete metamorphosis. Their life-history consequently falls into
four divisions: egg, larva, pupa and imago. If the climate permits,
the female flea lays her eggs all the year round, and from one to five
are dropped at a time. Unlike those of many other parasites they are
never attached to the hairs of the hosts, but appear to be deposited
indiscriminately on the floors of houses or in the nests and sleeping
places of their hosts. The eggs generally hatch in a few days, and a
minute, white, wormlike larva emerges (Fig. 1). The larvæ, of some, and
possibly of all, fleas are provided with a wonderful adaptation in the
shape of an egg-breaker or hatching-spine. This is a thin plate, like
the edge of a knife, where the point of the head comes in contact with
the shell. The movements of the prisoner make a slight split in the
egg-shell, which then bursts asunder. This organ has vanished in later
larval life, and it is probably lost after the first moult. The larva
is legless and has thirteen segments. It grows rapidly, and, as it
grows, moults its skin several times. It is provided with mouth-parts
adapted for biting, and eats any decaying organic refuse. The larvæ may
be reared on the sweepings of an ordinary room or the dirty scurf which
collects at the bottom of old birds’ nests. It is hardly necessary to
add that the mother takes no interest whatever in the larvæ and that
the belief that she feeds them on dried blood is not based on any sound
foundations.
[Illustration: Fig. 1. The larva of a flea. The body consists of
thirteen segments and is legless. On the fore part of the head are the
antennæ and on the upper part of the head is shown the knife-like edge
of the egg-breaker. The mouth-parts are adapted for biting. On the last
segment of the body are the two caudal stylets.]
The larval stage lasts some days, and the animal spins a small cocoon
before pupating. In the course of a few more days, the time probably
depending on the weather, the perfect flea emerges. The larvæ generally
live in places where the perfect insects will have an opportunity of
finding a host as soon as they leave the pupal envelope. The nests
of their hosts where the young are being reared are always favourite
places. It seems possible that the comparative immunity from fleas
which hoofed mammals or Ungulates enjoy may be due to the fact that
the young beast follows its mother from the time of birth instead of
passing its early life helpless in a nest.
Observations made on the development of the dog-flea (_Ctenocephalus
canis_) in India show that eggs laid on October 17 hatched on October
19. The larva spun its cocoon on October 25 and the mature flea emerged
on November 2. In Northern Europe the human flea takes about four weeks
in summer and six weeks in winter to pass through its metamorphosis.
Unlike many parasitic insects, fleas do not constantly pass their
time upon the bodies of their victims. The greater part of their
life is probably spent on the ground, in the house, or nest, of the
mammal or bird which serves them with blood. In this respect there is
considerable difference in the habits of different species of flea.
Some attach themselves to an animal and actually burrow into the skin.
These are the most parasitic species. Some only come to feed and leave
to lay their eggs. Many probably do not suck blood more than once in
their lives.
An animal which harbours fleas and which nourishes the adult insect
with blood is called a _host_. No fleas are more than what is called
temporary parasites; which means that they pass but a portion of their
lives on their hosts and frequently take occasion to hop on and off.
All fleas, apparently, go from host to host. The labours of diligent
collectors have proved that the great majority of mammals and birds
have fleas. As a general rule, it is true to say that certain species
of flea are associated with certain species of host. Thus man is the
true host of _Pulex irritans_; the cat family are the true hosts of
the cat-flea (_Ctenocephalus felis_); and the dog family are the true
hosts of the dog-flea (_Ctenocephalus canis_). But the human flea is
sometimes found on cats and dogs, and cat and dog-fleas occasionally
bite human beings; and cat-fleas are found on dogs and dog-fleas are
found on cats. All fleas, so far as we know, may occasionally pass from
one species of host to another; but they do not, for the most part,
seem to flourish in unaccustomed quarters. Some fleas are more catholic
in their tastes than others. Some seem to be very strictly confined to
one host, and even when starving only suck strange blood under protest.
There is a species of flea that has only (except by accident) been
found on the long-tailed field-mouse and another that has only been
found on the hedgehog. Other fleas are commonly found on two absolutely
distinct animals; a good instance of this is the human flea which, at
all events in certain parts of England, is a regular parasite of the
badger.
As distinguished from true or natural hosts one must separate what
may be termed casual or accidental hosts. All animals which come in
contact with one another, or which live in close proximity, may
exchange fleas. So even bird-fleas may be collected from mammals and
typically mammalian fleas from birds. In this fashion puzzles may
arise which tax the ingenuity of the collector to solve. Bird-fleas
are sometimes found on bats, and this may be obviously attributed to
the bats having inhabited a hole which was tenanted by starlings or an
old loft infested with the fleas of pigeons. All beasts of prey are
sometimes found to harbour the fleas of animals they have devoured.
Rabbits’ fleas are found on wild-cats; hedgehogs’ fleas on foxes; mice
fleas on weasels; and fleas characteristic of small birds on stoats.
So also in the case of mice, rats and voles with holes and runs in the
same hedgerow, the parasites usually peculiar to one are not uncommonly
found on the others. It is sometimes difficult to determine the true
host of a flea.
Much more puzzling to explain are the reasons which confine a flea to
a certain host and which cause closely allied hosts to have different
fleas. The fleas from the house-martin and the sand-martin are quite
different; those from the domestic fowl and the domestic pigeon are
distinct species. The causes which have affected the evolution of the
various forms of flea are too obscure to enable anyone at the present
day to offer any satisfactory explanation.
Speaking generally, the fleas found on birds have points in common,
and they probably form a natural group to themselves. What may be
called true bird-fleas have been collected from almost all European
birds. An unwieldy genus (_Ceratophyllus_) comprises many species of
different flea. Some species are very abundant and infest the nests of
many different birds. Others are extremely rare. One of these rarities
(_C. vagabundus_) is found in the nests of puffins and other sea-birds.
Another has been collected on antarctic petrels. Penguins have a
special genus of flea to themselves. A specimen, unique at one time
(_Ceratophyllus borealis_), in Mr N. C. Rothschild’s collection was
obtained from the gannet. It has now been found on rock-pipits in the
Shetland Islands.
Two very rare fleas (_C. farreni_ and C. _rothschildi_) are found in
the nests of house-martins; yet the nests of these birds are infested
with common species besides. A plague flea (_Xenopsylla_) has been
found on an African swift.
Forty-six different species of flea have been found in the British
Islands, but many of these are extremely scarce.
We know too little about the geographical distribution of fleas to lay
down many accurate generalities. When a great deal more material has
been collected and studied, it may be possible to show that certain
groups are associated with certain regions of the earth or certain
orders of animals. To some extent this is already seen to be the case.
The fleas indigenous to the New World are distantly related to those
of the Old World. Broadly speaking the geographical distribution of the
parasite must follow that of the host. But sometimes the parasite is
impatient of cold and cannot follow the host out of the tropics. The
chigoes and their allies are fleas of hot countries. Different kinds
of bats are found from the tropics to the Arctic circle, but the same
bat-fleas are not found everywhere.
When a flea has a cosmopolitan range it is probable that it has
travelled over the world in company with its host.
Monkeys have no fleas. This is an assertion that is commonly received
with surprise and incredulity. Occasionally a gorilla or a chimpanzee
may get a chigoe in its toe. And monkeys in zoological gardens or
menageries are possibly exposed to the danger of catching an occasional
human flea from the people who crowd round their cages. These are
remote contingencies which may happen to anyone. Healthy wild monkeys
are much too clean and active to harbour fleas. When they search one
another’s fur in a fashion that must be familiar to most persons, they
are clearing their coats of particles of scurf or of similar scraps of
dirt and not of fleas. So, speaking generally, it may be said that no
fleas have been found truly parasitic on monkeys.
Bats have fleas, but not in great abundance. All bat-fleas are rare
on their hosts and extremely difficult to find and collect. The
same species are not found on fruit-bats and the ordinary smaller
insect-eating bats. The geographical distribution of some bat-fleas is
puzzling. For instance, one species (_Ischnopsylla unipectinata_) is
found on the greater horse-shoe bat in Europe; but it is, apparently,
not found on the same bat in the British Islands. In Somaliland and in
India it is found on other bats.
With certain exceptions, Ungulates are remarkably free from fleas. This
great order of mammals includes a variety of hoofed animals: oxen,
sheep, goats, deer, pigs, camels, giraffes and antelopes.
The only true fleas found on these are two species of the genus
_Vermipsylla_, which resemble the chigoes in so far that the pregnant
females burrow into the host and expand there. One species has been
found on camels and horses in Transcaucasia; another on roe-deer in
Northern China. The female of the last is often found ensconced on
the inside of the nostrils of the deer. Of course chigoes may attack
domestic Ungulates of all kinds; but no other members of the family
_Pulicidæ_ or typical fleas except those two above mentioned have been
found on hoofed mammals.
Insectivora such as moles, shrews and hedgehogs are the hosts of a
great variety of species. The same thing may be said of the Rodents,
which include porcupines, squirrels, rats, mice and a vast number of
other small mammals whose geographical distribution includes almost the
whole of the habitable globe. Probably more different species of fleas
have been collected from Insectivora and Rodents than from all the
other orders of mammals grouped together.
The Carnivora, excluding the Pinnepedia, or seals, sea-lions and
walruses, harbour numerous species.
Among the Edentata a very remarkable and highly specialised genus
of fleas is parasitic on armadilloes in South America. This genus
(_Malacopsylla_) consists of two species only, which are confined to
South America and are found on the armadilloes and on carnivorous
animals which probably have preyed on them. The thorax of these fleas
is much reduced and very small in size. Their piercing organs are
slender and weak, but they possess enormous spines on the legs with
which they hold on to their hosts. These two South American fleas (_M.
grossiventris_ and _M. androcli_) will be referred to again later as
striking examples of fleas with strongly developed legs and weakly
constructed mouth-parts. The contrary combination of powerful mouths
and degenerate legs is also found in other groups of fleas, as will be
seen in the chapter on the chigoes.
The Marsupials of Australia and South America have special fleas which
were probably associated with this strange order of pouched mammals
before they became divided into the American and Australian groups.
Fleas have been collected on the spiny ant-eater (_Echidna_) which
belongs to the lowest order of Monotremes or egg-laying mammals.
On almost every form of bird, including the most aquatic kinds, fleas
of various species have been obtained.
Only one instance has been recorded of a flea occurring on a reptile.
A female of one of the species of burrowing chigoes (_Echidnophaga
ambulans_) from Australia was collected by Dr Woodward from the Brown
Snake (_Diemenia superciliosa_). This reptile, which is well known
in Australia, belongs to a sub-family that contains some of the most
deadly poisonous snakes and is allied to the cobras. The Brown Snake is
a terrestrial snake, and one must regard the presence of the flea on
such a host as a rare and chance occurrence. The snake was captured at
Herdman’s Lake, near Perth in West Australia. The same species of flea
has also been obtained from the phalangers (_Trichosurus_) which live
in the tops of the Australian gum-trees; from the little terrestrial
and nocturnal rat kangaroos (_Bettongia_); and from the banded
ant-eater (_Myrmecobius_), another Australian Marsupial. It is possible
that the flea moved from some small mammal which was being devoured by
the snake and managed to fix itself between the scaly plates of the
reptile.
When fleas are hatched in a nest they have no choice but to attach
themselves to the young mammals or birds. But even in that case
they frequently leave their hosts and do not for very long remain
stationary. Moreover, when a host dies and becomes cold the fleas
invariably leave their quarters, which explains how it may happen that
Carnivora get infested with the fleas of their prey. This change of
hosts which is always occurring makes it impossible to draw conclusions
from material collected in zoological gardens where many animals are
herded together. In menageries, too, the normal conditions of breeding
are absent. A German naturalist collected 2036 fleas from theatres,
concert-halls, ball-rooms, schools and barracks in the grand-duchy
of Baden and found that more than fifty per cent. were dog-fleas
(_Ctenocephalus canis_). What the proportion may be in other parts
of Europe we have no materials from which to form a judgment. In
zoological gardens cat-fleas (_Ct. felis_) are generally numerous in
most of the cages.
It is, of course, well known to every zoologist that species are not
fixed or constant and that various forms of mammal or of bird tend to
show geographical variations. When a long series of skins are laid out
on a table and carefully examined it is seldom that those from the west
of any great region cannot be picked out and distinguished from those
obtained in the east. So we also get northern and southern forms of
the same species varying slightly. These variations are perceptible in
many forms of insects, and zoologists now describe these local races as
subspecies and designate them with trinomials. No one, however, knows
enough as yet about all the various forms which are assumed by fleas
to attempt, except in a few instances, to do so in the case of these
animals.
The study of Siphonaptera is still quite in its infancy. We know little
or nothing of the minute geographical variation of fleas. That there
is such a thing can already be seen in a few species. In the meantime
the study of variation must be postponed until collectors have amassed
a more plentiful amount of material; and it is best to treat all forms
which are to all appearance constantly different as being specifically
distinct until more is known about variation.
Any classification of fleas that may now be attempted can only be
tentative. It will be enough for present purposes if the reader will
remember that the Order Siphonaptera can be divided into three groups
or families: (1) the chigoes and their allies, which are the most
parasitic fleas (_Sarcopsyllidæ_); (2) the typical fleas to which
the majority of species belong (_Pulicidæ_); and (3) the bat-fleas
(_Ceratopsyllidæ_), which have certain peculiarities that will be
described in a later chapter.
Of the antiquity of fleas, and of the period in geological history when
the order made its appearance, little can be said. When it was thought
that fleas were confined, as parasites, to warm-blooded mammals and
birds, evolutionists were inclined to say that the parasites could
not have appeared before their hosts. The discovery of a flea on a
reptile opened the vista of possibly enormous antiquity stretching back
to Permian or Carboniferous ages. The fossil record is most meagre.
If we reject as too doubtful the supposed remains of a flea from the
lower Oligocene strata at Aix in Provence, only one undoubted fossil
has been discovered. Nor does it seem certain that fleas are entirely
restricted to preying on vertebrates. Dr Dampf introduced a number
of common bird-fleas (_Ceratophyllus gallinæ_) of both sexes to some
hairy caterpillars. He observed that several of the fleas buried their
heads in the hairy covering of the larvæ and remained some time in the
attitude of sucking blood. While this was going on the victims made
violent demonstrations of annoyance and discomfort. He also observed
that a naked caterpillar was not attacked.
Mr Boden has also recorded how he found in a seed-warehouse some
peas that were being eaten by two species of Lepidopterous larvæ. On
bringing these home and keeping them in a jar, he found among them
some small larvæ which ultimately turned into fleas, probably _Pulex
irritans_. These fleas, being confined without other food, were
observed to prey on the Lepidopterous larvæ and to feed freely on
their juices. The larvæ which were attacked by fleas pined and died.
The fluid from the stomach of the fleas when they were crushed was
transparent and not red like vertebrate blood which often exudes when a
mammalian flea is pinched and cracked open.
A French entomologist has also reported that the numerous fleas which
swarm in the dwelling-houses of Corsica, for want of other nourishment
turn their attention to flies that may be incapable of flight.
The only fossil remains of a flea that have, so far, been discovered
are a single insect in a piece of Baltic amber of Oligocene age. Many
organic remains have been preserved in this fashion, but this is the
first mammalian parasite that has been found. The flea is admirably
protected by its semi-transparent surroundings, and the most minute
details of structure, the arrangement of bristles on the body, and the
number of segments to the labial palpi can be discerned. This unique
object is in the collection of Professor Klebs. The first point to
note is that a flea of this antiquity hardly differs from the existing
insects of the present day. It has been referred to an existing genus
(_Palæopsylla_) of which there are at least four species living.
Three of these are parasites of the mole, and the fourth is found on
shrews. There is good reason to suppose that the host of the fossil
was some insectivorous mammal. The early specialisation of fleas is
strikingly illustrated. This insect is already adapted for life on
some warm-blooded animal. It has a thoracic comb, and its mouth-parts
are in all respects like those of a modern flea. It belongs to a genus
which is still commonly distributed over Europe. When we consider how
remote are the chances that a mammalian flea should first get embedded
in amber and should, subsequently, be detected and described by a
naturalist, we may well understand that the owner of the fossil asked,
though without success, £1200 for it.
The ordinary person regards fleas as a subject for humour of an obvious
and familiar kind. The utilitarian despises a man who can cheerfully
spend his time in collecting fleas. Yet it seems probable that a study
of their forms and habits may be of immediate benefit to the human
race. The discovery that fleas are connected with the spread of plague
is an instance of apparently unprofitable scientific labour proving of
direct advantage to mankind. An accurate knowledge of the structure
and habits of fleas is now seen to be of importance to all who are
engaged in fighting one of the most dreaded infectious diseases. When
plague breaks out men of science now at once turn their attention to
the fleas. This is likely to prove more directly efficacious than the
mediæval custom of marking the house with a red cross and inscribing
the legend, “God have mercy on us.”