THE EXTERNAL STRUCTURE OF A FLEA
IN comparing the structure of a flea with that of a man, or any other
of the higher animals, it is of the utmost importance to understand
that the one has an internal and the other an external skeleton. In
either case the skeleton serves as an attachment for the muscles by
which the animal moves itself. Everyone is familiar with the external
skeleton of a lobster and can see for himself how the muscles are
attached. The structure of a flea, though so much smaller, is somewhat
similar, except that the skeleton is composed of a horny substance
known as _chitin_ instead of being calcified. The chitinous cuticle
entirely covers the flea, but it varies in hardness and thickness
on different parts of the body. The epidermis, or true skin, lies
immediately beneath. On those parts of the body which are to the rear
of the head the chitin forms a series of plates or shields which
overlap one another somewhat like the tiles of a house. The segmented
structure of a flea is there most clearly seen; this we may suppose is
an inheritance from the segmented worms.
The chitin which forms the external skeleton of a flea is secreted by
an outer layer of cells on the insect’s body. The deposit being thin at
the joints, and thick on the plates, which serve for protection, the
flea is encased in a suit of flexible armour. It is made of a fairly
solid and dense substance, but, owing to the absence of carbonate and
phosphate of lime, is much lighter than the familiar external skeleton
of the lobster. Chitin is a very peculiar and durable substance which
resists boiling in acids or alkalies. It is a structureless substance,
in the sense that it does not consist of cells. Though horny in
appearance it is, of course, in no sense true horn like that of the
nails, hoofs, claws, and horns of vertebrates.
The different species of flea vary considerably in size. Some are
smaller than the familiar human flea. Others are much larger. A very
large flea (_Hystrichopsylla talpæ_) is that found on the mole. The
largest known flea (_Dolichopsyllus stylosus_) is found on small
rodents in the United States where, as we know, all things are on a
bigger scale than in the Old World. It is seven millimetres long.
The colour of the horny integument varies from a pale or light yellow
to a ruddy or dark brown. It is plentifully sprinkled over with
spines, bristles, or hairs, directed backwards so as not to impede
progress. The presence or absence, the arrangement on the body, and
the size of these serve, along with other features, to distinguish
different species. They seem materially to help a flea in those
wriggles to escape with which we are all familiar. The bristles are not
always the same in size and arrangement in the two sexes of the same
species of flea. As a rule the males are more bristly than the females.
These appendages of the flea’s integument are called by various
writers either spines, bristles, or hairs. There is, however, no real
distinction in the structure or nature of the appendages, and it is a
question of degree which name is most appropriate.
In distinguishing species, very little reliance can be placed on the
colour of the flea. An insect newly emerged from the pupa is always
lighter in colour; and the difference between the appearance of an
empty stomach and a stomach gorged with red blood is surprising.
The general external appearance of a typical flea belonging to the main
family _Pulicidæ_ is fairly familiar to most persons. This is well
seen in the figure of the oriental rat-flea. The body is compressed or
flattened from side to side, and this is a feature which is extremely
rare among insects. It doubtless enables the animal to glide with
greater facility through the hairs of its host. Like other insects,
a flea is readily divided into a head, a thorax, and an abdomen. The
head is rounded on the top and front and shows no obvious trace of
segmentation; but what is known of the development of other insects
leads one to think that it must properly be regarded as a number of
segments closely fused together. On the under side of the front part of
the head is a beak or proboscis for piercing and sucking, composed of
the mouth-parts, whose structure is worthy of minute study. It will be
best to examine them in detail in a subsequent chapter.
Some fleas have eyes, others have none. The common mouse-flea
(_Leptopsylla musculi_) is blind. The bat-fleas are also destitute
of eyes. The nocturnal habits of their hosts would render eyes of
little or no use. If eyes are present they are large and placed on
either side of the head. Each is a simple eye or ocellus; the compound
eyes, divided into a great number of hexagonal facets, which are
characteristic of many insects, are never found in fleas.
Nothing is known about the flea’s powers of vision, but there is no
reason to suppose that they are at all acute. The eyes are marked with
pigment. Ocelli appear to be primitive types of insect eye which are,
perhaps, an inheritance from a wormlike ancestor. Presumably all the
fleas of long ago had eyes and those that are now blind have lost their
organs of sight from disuse. In their simplest condition, the eyes
of the lower invertebrates only enable the creature, so far as one
can judge, to distinguish light from darkness. Entomologists believe
that the power of vision of ocelli is probably confined to very near
objects and that this simple form of eye is more useful in dark places
than the compound eyes. There is no reason for believing that fleas
can distinguish colours or can discern any object which is more than a
few inches away. It is enough for their purpose to perceive from which
point light comes upon them and to make all despatch to escape in the
opposite direction.
In blind fleas there is often a spine where the eye should be. In
one species the spine is rudimentary and there is some black pigment
beneath it. It is not impossible that this is the vestige of a once
functional eye. In one genus, however, the eye and the spine are both
present. Of the fleas belonging to this genus one species is South
American and the other European. The latter (_Typhloceras poppei_) is
confined to the long-tailed field-mouse.
The organs by which fleas keep in touch with the outward world, and
with other fleas, are their antennæ. All fleas have antennæ; but unlike
those of a moth, a beetle, or a grasshopper, each fits neatly into a
groove at the side of the head and can be protruded when desired. This
is another adaptation to enable the insect to creep swiftly through a
forest of hairs.
[Illustration: Fig. 2. Showing a type of (_a_) _genal_ and (_b_)
_thoracic_ combs of a flea, on the under part of the head and on
the thorax respectively. Analagous combs are found in several other
parasitic insects and on the abdominal segments of certain fleas.]
The combs which are found on the heads of many fleas are organs of
exceptional interest (Fig. 2). They are toothed and horny appendages,
which are connected with parasitic habits, for somewhat similar combs
are found on several unrelated groups of parasitic insects, as, for
instance, on parasitic beetles (_Platypsyllus_) found on the beaver,
on insects allied to bed-bugs (_Polyctenes_) found on bats, and on
wingless flies (_Nycteribia_) which infest Egyptian and South American
bats. The majority of the _Pulicidæ_ have one or more combs with
comparatively long teeth. These combs reach their maximum development
in the bat-fleas which have no less than eight. Some Australian and
South American fleas (_Stephanocircus_) have a helmet-like comb
extending all round their heads. These combs are by some supposed to
be of service in holding on to the hairs of the host; and, if one may
judge from experiments made on live fleas in cotton wool, they are also
used in moving forwards through the fur.
All the chigoes (_Sarcopsyllidæ_) have a large triangular post-oral
process which is more or less curved and probably prevents the flea
slipping back as it pushes forward. Bat-fleas (_Ceratopsyllidæ_) have
lobes or flaps placed two on each side of the head, which may possibly
serve an identical purpose, but whether they do so is not known.
These combs may be divided into three groups according to the part of
the flea’s body on which they are found. Those found on the head are
called genal combs and take the form shown in Fig. 2. There are also
combs found on the thorax. The fossil flea (_Palæopsylla klebsi_)
described in the previous chapter has one of these thoracic combs. A
certain number of fleas also have combs on the abdominal segments.
There are really two types of toothed organs on fleas to both of which
the name of comb may be given. One is composed of a sheet of chitin
with a number of slits and teeth and resembles a true comb. The other
consists merely of a number of highly chitinised bristles arranged in
a row. They probably both serve the same function. Apart from their
use as organs to assist movement onwards, they may also serve as
_hair-tight_ joints and protect the flea from the inconvenience of
getting the tips of the host’s fur into the joints of its horny armour.
The size of the head compared to the thorax and abdomen varies
considerably in different species. Some fleas have what may be called
by comparison large heads and others very small ones.
A small head is never found in a flea with powerful mouth-parts. The
head being the bearer of piercing and sucking organs, which require
strong muscles, there must be room not only for the organs but for
their extensors and retractors.
There are normally three rows of bristles on a flea’s head which divide
the head into four sections. It is possible that these correspond to
the four segments of the ancestral insect which are now fused together.
The head of a flea is closely applied by the whole of its back
surface to the body and that slender and conspicuous neck which is
characteristic of the Diptera, or flies, is not to be found in any
fleas. For this reason a flea cannot turn its head in any direction
without at the same time following it round with its body.
The thorax of a flea consists of three segments called respectively
the prothorax, mesothorax and metathorax. The chitinous external
skeleton which covers each of these three segments is primarily a
hoop but each hoop is further subdivided into a number of complicated
plates. Attached to the thorax are the three pairs of legs which are
characteristic of all adult insects. The hind pair are very much the
strongest (Fig. 3). They are the organs of hopping. It has often been
pointed out that if men had the leaping powers of some fleas they would
bound with ease backwards and forwards over the cross on the top of
St. Paul’s Cathedral. Each leg consists of four segments beautifully
articulated and plentifully supplied with bristles. At the end comes
the foot with five very short segments. The last segment is provided
with a pair of more or less formidable claws. Fleas use their legs for
leaping, for running, and for clinging to their hosts. They also use
their mouth-parts for the last purpose and it is worthy of note, as we
shall see later on, that in those fleas in which the mouth-parts are
shortest and weakest the legs are most liberally supplied with bristles
and possess the stoutest claws. The legs of a flea are unique in the
insect world owing to the enormous development of the segment nearest
the body called the coxa. Most leaping insects rely for their activity
on the muscles of the lower joints. In a grasshopper it is the third
joint from the body (femur) which is so immensely enlarged. The three
pairs of legs are each attached to a different thoracic segment.
[Illustration: Fig. 3. The hind leg of a flea. The segment or joint
nearest the body is the _coxa_ which is unusually developed. Next come
the small _trochanter_ and the larger _femur_. The _tibia_ which is
long and slender follows. Then come the five _tarsi_ with the sixth and
ultimate segment provided with claws.]
When fleas walk, they are so to speak plantigrades walking on the
sole of the foot, and all the tarsal or foot joints are applied to
the surface of the ground. The claws serve as grips so as to make the
most of any unevenness; and thus the insect drags itself along with
surprising rapidity when it moves through the hairy coat of a mammal.
But on an open surface fleas are not really rapid movers compared with
many other insects.
The two claws on the end of each ultimate foot segment are freely
moveable and are in fact highly modified bristles or setæ.
In all fleas one of the plates of the metathorax (or hindmost thoracic
segment) called the epimeron, is large and prolonged towards the
rear. It invariably bears a stigma. The epimeron is placed laterally
to the first abdominal tergite. The older naturalists jumped to the
conclusion that this was the remains of a wing. The best judges have,
however, formed a decided opinion that no trace of the relic of a
flying organ can be detected on the thorax of a flea. Heymons, a German
entomologist, has also failed to detect any sign in dissections which
he has made of the larvæ and the pupæ.
The epimeron is in fact neither a scale nor a wing but a portion of
the thorax present in all insects. It is of no special service to the
flea except as a portion of the thoracic armature which covers the body.
The larva of a flea has no legs; the adult insect has six. A study
of other embryo insects shows that the ancestors of insects had many
legs. It is an interesting problem why insects lost the legs on their
abdomens, why legs should now invariably be restricted to the thorax,
and why there should never be more than three pairs. In the earliest
known insects which lived on the earth, before winged forms were
evolved, the number of legs was already six. But our knowledge of fleas
is too small to attempt, at present, to trace their exact line of
ancestral descent.
The abdomen of a flea consists of ten segments. The horny plates which
cover the dorsal side are called tergites; those on the ventral side
sternites. In fleas, as in all holometabolous insects, that is those
which pass through a complete metamorphosis, the sternite of the first
abdominal segment is suppressed and has completely disappeared. The
tergite which covers the dorsal part of the first abdominal segment
nearest to the thorax is, however, always present.
The ultimate segments of the male and female flea are modified for
reproductive purposes and of these segments more must be said later.
Having now given a rough outline of the external skeleton of a flea, it
only remains to say something about the muscular system. Attached to
the inside of the chitinous armature are an enormous number of muscles,
whitish and almost transparent. They act as extensors, retractors,
flexors, elevators and depressors. The joints and hinges of the
skeleton allow of considerable, but not perfect, freedom. The muscles
of locomotion are partly in the thorax and partly in the several joints
of the legs. Our knowledge of the muscular system of fleas is very
imperfect. But, as in other insects, the general arrangement of the
muscles is based on the segmented structure of the body.
For the reader who can accurately picture to himself the external
structure of a flea and of the typical insects belonging to other
orders, a few words may be said on the probable ancestry of fleas and
their relationship to other living insects. This vexed and much debated
question is still, as the older naturalists would have said, _tremendum
mysterium_. Very little light has yet been thrown upon it, and the
most divergent views have been expressed by learned and competent
entomologists. A historic survey of the various opinions that have
been held since the days of Linnæus would fill many pages; but a short
summary of the different orders to which fleas have been referred by
different zoologists will suffice.
The older authors, Linnæus, Geoffroy, Cuvier and Duméril, and Gervais
placed them among the _Aptera_ because they were wingless. Kircher
regarded them as _Orthoptera_, an order which includes grasshoppers
and crickets; but he has had few followers. By Fabricius and by
Illiger they were treated as _Hemiptera_ or bugs. Lameere, a Belgian,
has recently expressed a decided view that fleas are really a family
of _Coleoptera_ or beetles. Those who have held the once orthodox
opinion that they belonged to the _Diptera_ or flies are Roesel, Oken,
Straus-Dürkheim, Burmeister, Newman, Walker, von Siebold and Wagner.
The structure of an adult flea, however, differs from that of an
adult fly in the following noteworthy respects: the mouth-parts are
differently constructed, the head of the flea is closely joined to its
thorax, the three divisions of the thorax are not joined and fused, the
flea is wingless, the eyes of fleas are simple ocelli, and there are
differences of lesser importance in the stigmata, which give access to
the tracheal system by which all insects breathe.
The number of those who have regarded fleas as belonging to a distinct
order of insects is considerable: they are Lamarck, De Geer, Latreille,
Kirby and Spence, MacLeay, Leach, Dugès, Bouché, van der Hoeven,
Westwood, Landois, Brauer, Kraepelin, and Taschenberg. Modern opinion
is all but unanimous on this point.
There remains, however, a second question. Even if it be agreed
that there must be a distinct order for _Suctoria, Aphaniptera,
Siphonaptera_, or fleas; where ought that order to be placed? In which
other order of insects must we look for the nearest relations of fleas?
For a time after the acceptance of the fact that insect forms have
been evolved, and not separately created, the ancestors of fleas were
searched for among some species of fly.
Then Kraepelin rejected the view that flies were as closely related
to fleas as most entomologists thought and his followers could only
find points of difference and no points of resemblance. Dahl (1899), a
German, then took up the cudgels for the fly theory. Dahl pointed out
the resemblance between fleas and a group of flies called _Phoridæ_
also parasitic on warm-blooded animals. During the ensuing years the
debate was resumed afresh with much liveliness and sometimes with a
little acrimony.
The fleas were placed by MacLeay and by Balbiani between the _Diptera_
and _Hemiptera_; by Leach between the _Hemiptera_ and _Lepidoptera_;
by Dugès between the _Hymenoptera_ and _Diptera_; by Brauer between
the _Diptera_ and _Coleoptera_. Handlirsch thinks that fleas have
no connection at all with beetles and Gross can find no signs of
relationship with either _Coleoptera_ or _Diptera_.
Embryology and the study of larval forms have thrown so much light on
the ancestry of many animals, that it was hoped that a microscopic
examination of the larvæ of fleas, in various stages of development,
would produce some facts of importance. In this hope entomologists
have, to a great extent, been disappointed. There seems to be much
similarity between the embryos of beetles, moths, flies, wasps and
fleas. Those who have dwelt on the likeness of the larval flea to the
maggot of a fly seem to forget that the resemblance to an embryo beetle
is nearly as strong.
The young larva of the flea is very transparent and the digestive
canal, heart and nervous system are easily recognised. The egg-shell
breaker is an interesting example of the development of a temporary
larval structure and it is the only known instance of such a
structure in an insect. There are no traces of eyes. The antennæ are
three-jointed. They are rather long and slender, being about one-third
as long as the head. The head is well-developed and the larva has no
feet.
The biting mandibles are broad and triangular. Compared with those of
other larvæ they are said to be more like the mandibles of coleopterous
than of dipterous larvæ. The maxillæ, or second pair of jaws, are
somewhat reduced and rudimentary. The absence of eyes and of legs are
points of similarity between the larvæ of fleas and flies. The maggot
of a fly has also two pairs of jaws, and a pair of antennæ.
At the tail end of the larval flea’s abdomen are two small projections
called caudal stylets (Fig. 1). They are strong, recurved, chitinous,
structures which prop up the body of the larva when it creeps and
wriggles. There are similar props in the larvæ of certain beetles and
no exactly similar organs are known in dipterous larvæ. But caudal
stylets are of small taxonomic importance.
In one respect the mature flea is certainly nearer to a beetle than to
a fly: the three joints of the thorax are free as in a beetle and not
fused as in a fly; but when one studies the mouth-parts, the true view
seems to be that the mouth-parts of a flea are equally unlike those
of a fly and those of a beetle. Such being the present state of our
knowledge, one must wait for fresh light to be thrown on the matter by
further researches. It seems unlikely that the immediate future will
produce a solution of the problem.