THE MOUTH-PARTS AND SENSE-ORGANS
WHEN the outward anatomy of a flea was described, in an earlier
chapter, the mouth-parts, which form a sort of beak or proboscis under
the head, were mentioned. These most interesting parts of the insect
must now be dealt with. The reader probably knows that some insects
have mouths for sucking fluids and others mouths for biting solids. A
moth or a fly cannot masticate solids, whilst a beetle or a cricket has
effective biting jaws.
The first naturalist who studied the mouth-parts of a flea, with
such microscopes as were then available, was Leeuwenhoek. He was
a Dutchman who worked at the end of the seventeenth century, and
the minute accuracy of whose observations still often fills modern
naturalists with wonder. Microscopic work was then in its early days,
but Leeuwenhoek clearly made out the two serrated lancets (Fig. 4)
which are called the mandibles. His “Microscopical observations on
the structure of the proboscis of a flea” were published in the
_Transactions of the Royal Society_ in 1706.
The mouth-parts of fleas are differently constructed from those of
all other insects. Around the orifice of the mouth are a number of
appendages which form a complicated apparatus for piercing and
sucking. Their construction and use cannot be described without
employing some technical terms. When the names of the parts have been
mastered, a diagram will make their relative positions clear. It may
be necessary, first, to remind the reader who is not an entomologist
that the real _mouth_ of an insect is the entrance to the alimentary
canal, and that the appendages of the mouth, which act like jaws for
masticating or like tubes for sucking, are really modified limbs. In
fleas the mouth is suctorial. But before sucking up the blood the flea
must first pierce the skin of its host. The paired mouth-parts, then,
are modified limbs which correspond with those appendages on the thorax
of an insect which we call the three pairs of legs.
The primitive insect, of which fleas and all other insects are
descendants, was, it is supposed, composed of a succession of segments
each bearing a pair of jointed appendages. Insects of the present day
never have more than six legs, but the foremost pairs of appendages
have been bent round, reduced in size, and altered in shape so as to
serve as mouth-parts.
Now the mouth-parts of the flea for which only technical names exist
are the maxillæ and maxillary palpi, the labium and labial palpi, the
mandibles and the labrum. The labrum is considered by some authorities
to be the hypopharynx. It will be best to deal with each of these in
turn and then to explain how they act in combination.
_The maxillæ._ These are a pair of horny or chitinous triangular plates
one on either side of the flea’s face. They are placed some distance
away from the orifice of the mouth and to the right and left of it.
They do not serve for piercing or sucking, and appear to have no active
function unless they serve to separate the hairs of the host and
enable the flea to reach the bare skin. In the majority of bat-fleas
(_Ceratopsyllidæ_) the maxillæ are dumb-bell-shaped but in all other
fleas they are more or less triangular. From the fore part of each
springs a palpus. Like other highly chitinised parts of a flea, the
maxillæ are usually dark in colour.
_The maxillary palpi._ These are jointed hairy feelers which project
forwards and were mistaken by the older naturalists for antennæ. They
spring from the base of each of the maxillæ where these latter organs
are joined to the head of the flea. The palpi are sense-organs as the
number of sensitive hairs on their surface indicates. The maxillary
palpi of fleas are always composed of four segments.
_The labium and labial palpi._ These form together what is called
the _rostrum_ of a flea. The labium is a single organ which projects
beneath the aperture of the mouth. It may be described as the lower
lip of the flea. At its end it divides into two comparatively long
branches. These are the labial palpi. The actual piercing organs, which
will be described below, are the mandibles and labrum. They are not so
conspicuous as the rostrum which protects them.
When the piercing organs are at rest they are partly retracted. The
external portion is encased in the tubular rostrum. The tube is formed
by the two labial palpi which are situated at the apex of the short
non-divided labium. The number of segments composing each labial
palpus in fleas varies, so far as we know, from two to seventeen. In
most fleas, however, the labial palpus consists of five segments. This
appears to have been the original state of things in the ancestral
flea; the palpus with more and the palpus with less segments being
derived from the normal five-jointed one. The rostrum of a flea is
not a piercing organ like that of a fly and a bug. The two labial
palpi separate and lie flat, right and left, on the skin when the true
piercing organ is driven into the host. The labial palpi therefore
require to be flexible, and this is attained by increasing the number
of segments or by reducing the amount of chitinisation or horniness.
We shall find in the chigoes and their allies a rostrum which is pale,
weak, soft and scarcely horny. Among other fleas where the rostrum is
prolonged and strongly chitinised we shall find greater segmentation.
The small bristles at the extreme tip of the rostrum seem to be sensory
organs. They are like those at the apex of the maxillary palpus. When a
hungry flea is put on one’s arm, it appears to test the skin with these
bristles before it ventures to make a puncture.
_The mandibles._ These are a pair of sharp lancets with serrated edges.
They make the puncture and are interlocked with the labrum to form a
sucking tube.
_The labrum._ This is the central portion of the mouth-parts and is in
fact a prolongation of the upper lip of the flea. It is a hard, sharp,
awl-like instrument: in shape like a horny trough. Its edges are more
or less toothed. Its apex is pointed and it is as long as the mandibles.
The general appearance and the relative positions of the mouth-parts
are shown in Fig. 4.
[Illustration: Fig. 4. Diagram of the mouth-parts of a flea. The
slender awl-like structure at the top is the _labrum_. Beneath are
the paired _mandibles_ with serrated edges. The four-jointed hairy
_maxillary palpus_ is below, only one being shown. Protruding from the
base of the face is the _labium_ which supports the jointed _labial
palpi_. The flat obtuse triangular structure from which the palpus
springs is the right-hand _maxilla_. The left maxilla is concealed
behind.]
Bearing in mind, then, that the piercing organs are the labrum and the
two mandibles, and that the rostrum (composed of labium and labial
palpi) is merely a sheath, it is easy to form a clear picture of a
flea feeding. Anyone who is bold enough to place a hungry flea on
the bare skin of the arm can readily observe through a powerful lens
what happens. When the flea has chosen a spot to pierce the skin, the
rostrum, with the mandibles and long upper lip or labrum inside it, is
moved a little forward. The flea then lifts its abdomen upwards and
presses the piercing organs down into the skin. In doing this, it uses
its own weight and the strength of the foremost and middle pairs of
legs. The hind pair of legs are lifted up into the air. The head can
soon be seen coming nearer the skin. The rostrum then divides in the
middle. The labial palpi are forced apart as the mandibles and labrum
penetrate into the victim’s flesh. Finally, they are driven entirely
asunder and lie flat on the skin of the host, one to the right and
the other to the left. The flea then satisfies its hunger. A stream
of blood is sucked up, and when the meal is over, there is a forcible
action of the legs and the mandibles and upper lip are withdrawn with a
jerk. Numerous observers have remarked on the habit possessed by fleas
of discharging the contents of their intestines whilst actually engaged
in sucking. In many cases a drop of bright red blood is squirted from
the rectum during the operation of feeding, and this appears to be
a common practice among blood-sucking insects. Its bearing on the
feeding operation of the flea has not been discovered. But its possible
consequences in transmitting diseases from host to host will be seen in
a subsequent chapter on fleas and the transmission of plague.
It is said that the nervous systems and brains of fleas are not so
highly developed as those of many other insects such, for instance,
as ants, bees and other Hymenoptera. Having drawn attention to the
distinction between the external skeleton of a flea and the internal
skeleton of a vertebrate, one may with profit do the same in the case
of their nervous systems. In both cases the nervous system serves to
convey sensations from the sense-organs, and movements to the muscles.
In the vertebrate, as the reader doubtless knows, there is a brain,
a nervous cord running from it down the backbone, and a number of
nerves issuing, from the spinal cord and from the brain, in various
directions. Here the main nervous system runs down the _back_ of the
animal. In a flea, or other insect, the nervous system consists of a
chain of ganglia connected by a nervous cord. A ganglion is a nerve
centre and, in a sense, each is a brain which may be likened to the one
brain of the vertebrate. We have in the cord of ganglia a series of
brains, as it were, running from the head down to the extremity of the
abdomen. Each ganglion is a mass of nerve cells, from each of which a
fibre passes off to unite with the other fibres and make a nerve. The
first ganglion in a flea is placed in the upper part of the head above
the gullet. It may be called the brain since it receives the nerves
of the antennæ and eyes. In the ancestral insect we may suppose that
there was a pair of ganglia in each segment. Since the head of the flea
consists of several fused segments, we may fairly draw the conclusion
that the brain is the result of the fusion of several pairs of ganglia.
The brain of the insect occupies the same position in the body as the
brain of the vertebrate; but the rest of the nervous system lies on the
floor of the body _under_ the digestive canal of the flea, whereas in
the vertebrate it lies along the back and _above_ the digestive canal.
The dorsal spinal cord of the vertebrate is then a ventral nervous cord
in a flea.
The sensory nerves, which transmit sensations from different
sense-organs, and the motor nerves, which send stimuli to the muscles,
take their origin from other ganglia besides the ganglion above the
gullet. In bees and some other insects it has been shown that the
nerves from the palpi and mouth-parts go to the next ganglion which
is beneath the gullet. The same is probably the case with fleas; so
when we speak of the _brain_ of a flea we must remember that it has a
relative rather than an absolute claim to that title. A flea has really
many brains.
In certain blind insects, where the eyes are wanting, parts of the
brain are completely atrophied. Whether this is so in the blind species
of fleas does not seem to have been investigated.
[Illustration: Fig. 5. The antenna of a flea. A, concealed in the
groove. B, protruded from the head. The versatile _basal segments_ and
the terminal _club_, in this case with segments on one side of it,
should be noticed.]
We pass now from the central nervous system to the sense-organs of
the flea. The chief are the eyes, the antennæ and the pygidium. In
regard to the eyes nothing more need be said. The antennæ are probably
far more important organs to a flea than its eyes; but inasmuch as
they are at ordinary times concealed in a groove they are not very
conspicuous (Fig. 5). The first tolerably accurate plate of a flea by a
naturalist will be found in Hooke’s _Micrographia_ (1664). Robert Hooke
(1635-1703) was a somewhat eccentric and irritable man of science who
acted as secretary to the Royal Society. His labours were too varied to
be effective. He nearly discovered the laws of gravity and also studied
fleas. To him belongs the credit of having detected the antennæ groove.
Just as many of the older naturalists thought that the maxillary palpi
were antennæ, so others thought that the antennæ of a flea were its
ears. And when, with the help of their lenses, they saw the antennæ
erected and protruded from their grooves, they imagined that the insect
was cocking its ears and listening after the manner of a horse or ass.
But the antennæ of fleas are much more to them than ears; though it
may be that they are also auditory organs. They are certainly tactile
and olfactory organs as well. In outward structure each antenna
consists of two parts which may be called the stalk and the club. The
club is divided into a number of segments and is plentifully supplied
with hairs. In some species the cuts which divide the different
segments appear to be confined to one side of the club. In others a
sort of central core holds the segments of the club together. The
antennæ, therefore, are undoubtedly exceedingly complex organs. Such
an insect as a flea may well be far more sensitive to movements of the
air, vibrations of the earth, smells, light rays and sound-waves than a
human being. In their origin the antennæ, like the paired mouth-parts,
are modified appendages of the fused segments which compose the head
of the insect. The fact that there are four pairs of appendages on the
insect’s head, viz. (1) antennæ, (2) maxillæ, (3) labial palpi and (4)
mandibles has been put forward by some entomologists as evidence that
the head is formed of four primary segments.
Antennæ apparently enable fleas to find their bearings, to communicate
with one another and to discover the whereabouts of the opposite
sex. But it is especially as organs of smell that they play a most
important part in the flea’s social life. They enable couples to find
one another; and, when the sexes come together, the antennæ of the
male are usually raised and exposed from the groove. Insects generally
have some means of cleansing dirt from their antennæ. Some make use of
their legs, others of their mouth-parts. In fleas there is often a row
of short hairs at the hind margin of the groove which may serve as a
kind of comb for cleaning these delicate organs of sense. But further
observation on this point would be interesting, for no one appears to
have seen the comb in actual use. Female fleas are said usually to
carry their antennæ ensconced in the grooves, whilst the males more
frequently protrude theirs. The antennæ of the males are generally
longer than those of the females.
There are certain noteworthy organs of sense which appear to exist
on the upper surface of a flea’s head and body. They take the form
of small convexities of the body surface, lentil-shaped and each
surrounded at the base by a ring. Somewhat similar sense-organs are
widely spread through the insect world. As to their function, divergent
views are held. Some think that they are for the perception of sounds,
some for the perception of light rays, some for the perception of rays
of which we are unconscious. Since these organs are placed, at times,
in unprominent parts of the body it seems more probable that they are
affected by sound than by light.
The preference which fleas show for certain animals, and the repulsion
which they manifest on being allowed to suck blood from an unaccustomed
host, lead one to believe that they have a sense of taste. This sense
in other insects is apparently seated in certain microscopic pits and
hairs which form the ends of nerves and are distributed round the
mouth. Whether fleas can hear is not, it seems, definitely known.
A large number of fleas possess what is called a frontal tubercle. It
is a notch in the centre of the forehead but nearer to the mouth than
to the antenna. Sometimes the tubercle projects from a groove. This
is most marked in the genus of African fleas _Listropsylla_. The real
nature of this organ is unknown. Some regard it as an organ of sense.
Its homology is also uncertain. To some it suggests the egg-breaker of
the larva and they regard it as a relic of the larval stage. To others
it suggests an eye and they regard it as the remnant of an unpaired
ocellus possessed by the ancestral flea.
An exceedingly remarkable organ of sense, which is found in all fleas,
is called the pygidium. It is a sensory-plate plentifully supplied with
hairs and nerves and always placed on the back of the ninth abdominal
segment. Of all its uses we are still somewhat uncertain but some
observers declare that at the season of love the male flea bestows
caresses on the pygidium of the female.
In many species the male flea is sufficiently different in outward
appearance from the female to be easily distinguished. The male is
usually smaller and the last segments of the abdomen are so shaped
as to give the look of a tail tilted into the air. The frontispiece
represents a male flea and shows this well. The internal organs of
reproduction (testes and ovaries) in the male and female are placed
near the end of the abdomen. The seminal outlet and common oviduct
open to the rear of the sensory plate on the ninth segment of the
abdomen. The external genital armature of the male flea is exceedingly
complicated and quite unlike that of any other insect. When the sexes
are united, the usual position is reversed, and the male is _beneath_
the female.
It is well known to every entomologist that the hinder segments of
insects are often modified for reproductive purposes. In male fleas
it is the eighth and ninth abdominal segments which are altered. In
the females the eighth, and also often a portion of the seventh,
has assumed a peculiar shape. The clasping organs of the male flea
are portions of the ninth segment and form together a kind of claw
reminding one of the pinchers of a lobster. It is used by the male flea
in the breeding season to detain and hold the female.
Every entomologist also knows that the external sexual organs of
insects, of both sexes, are of special importance to the systematist or
classifying naturalist. They often enable him to recognise the species
when other organs do not show sufficiently striking characters. A
minute study of the genitalia of fleas is an absolute necessity to the
systematic entomologist, the more so as fleas do not present nearly
as many, or nearly as varied, external differences as do the species
of most winged insects where colour and pattern of wings are both
important.