THE CHIGOES AND THEIR ALLIES
THE chigoes and their allies belong to a group of fleas sufficiently
remarkable to deserve a somewhat detailed account. The reader may
remember that they form a family to which the name of _Sarcopsyllidæ_
has been given. They are the most completely parasitic of any fleas;
and the South American chigoe (_Dermatophilus penetrans_) enjoys the
distinction of being the first foreign flea ever described. This
pestilent insect, of which the female has the habit of burrowing into
the flesh of the host, soon made itself known to the early travellers
in the tropics of America. Oviedo, the Spaniard and historiographer
of South America, in his _Historia General y Natural de las Indias_
(1551), seems to have been the first European author who mentions
it. After this the chigoe is referred to by writers of various
nationalities in many works which were published during the sixteenth,
seventeenth and eighteenth centuries. It is an insect which appears
under a vast number of different names: chigoe, chigue, chego,
chigger, chique, jigger, pico, sico, migua, nigua, ton, and tschike are
synonymous. Catesby in his _Natural History of Carolina_ (1743) gives a
figure of the insect, which is easily recognisable. Linnæus, in 1758,
described the chigoe as _Pulex penetrans_, and apparently did not know
much of its appearance beyond what he learnt from Catesby’s picture.
This species and the human flea were the only two which the great
Swedish naturalist distinguished by a name; though, under the title
_Pulex irritans_, he includes a number of different species such as the
fleas from the dog, cat, rabbit and fowl. The chigoe remained the only
member of the family known to scientific entomologists until the year
1860. An allied insect was then found on a South American parrot. A
third member of the family was soon after discovered, and is noteworthy
because it was the first species recorded from the Old World. It is now
known to infest the domestic fowl in all warm countries where these
birds have been introduced by man. A fourth species was collected from
a South American bat. Up to the present time some fourteen different
species (belonging to three very distinct genera) have been described,
and there cannot be the slightest doubt that, when collectors in hot
countries turn their attention to the matter, a great many other forms
of this interesting family of fleas will be found.
The chigoes and their allies are of special interest for more than one
reason. The females are to a greater or less degree stationary; they
fix themselves firmly to their hosts and become veritable parasites.
Several of the earlier zoologists believed that the animal was a mite;
and it is somewhat remarkable that Oviedo himself should have so
promptly detected the relationship of the insect he saw with the fleas.
By reason of the parasitic habits of the females, more is known about
their appearance and life than in the case of the more active males.
In some species the males remain, for the present, quite unknown: and
not very much is recorded about the early life-history, eggs, larvæ
and pupæ of either sex. The parasitic habits of the chigoes and other
allied fleas lead one to expect peculiar modifications of form such as
are usually to be observed when an animal passes from an active to a
stationary life. These modifications are the more easily understood as
the various species are not all stationary to the same degree. It is
fairly plain that this family of fleas is a development from the less
specialised and less parasitic family _Pulicidæ_. In fact the gradual
development of the organs from a generalised to a more specialised
stage is strikingly shown in these insects. To follow this in detail
would, however, require a very minute and technical knowledge of their
form.
The chigoe family is so well characterised that a student of fleas
cannot possibly have any doubt whether a flea belongs to this family or
not. Yet there is great diversity in general appearance, as well as in
details of structure. One very peculiar character, namely the enormous
swelling of the abdomen in pregnant females is, moreover, shared with
certain other fleas. The most distinguishing character of the family,
however, is the rostrum. This organ, which it may be well to remind the
reader, consists of the under-lip and the labial palpi, sheaths the
piercing and sucking mouth-parts. In the chigoes and their allies the
rostrum is reduced, not in length, but in stiffness and in number of
segments. In this group there are never more than three segments to the
rostrum, whilst in the main group of fleas, with one or two exceptions
such as the rabbit-flea (_Spilopsyllus cuniculi_), there are never less
than five. There is no indication of a comb on the head, but all the
family, without exception, have a large triangular projection, which
is more or less curved backwards, at each side of the head. These two
organs doubtless discharge the same functions as the combs of other
fleas, and prevent the insect from slipping back as it works its way
through the fur or feathers of the host.
The thorax of a chigoe is exceedingly short. Two reasons for this may
be suggested. In the first place, the jumping power of these fleas is
very small and the muscles in the thorax are consequently reduced.
In the second place, the value to the insect of a contracted thorax
is obvious; for the abdomen of a chigoe fixed on or in the skin of an
animal does not project so much as it would were the thorax of normal
length. The troublesome parasite is, therefore, less likely to be
rubbed off by the host.
In most fleas the piercing organs of the mouth (upper lip and
mandibles) are directed obliquely downwards. In the chigoes they are
directed obliquely forwards. It has been suggested that this forward
movement of the mouth is connected with the stationary life which the
females assume. Fleas which fasten themselves permanently to the skin
of their host, do so in a manner similar to ticks. The mouth-parts are
in a line with the longitudinal axis of the body. This attitude, so far
as we know, is assumed by the females of all the family. The females of
one genus, _Dermatophilus_, actually go head foremost right into the
skin of their host. The shape of the head is also beautifully adapted
to enable the insect to fix itself firmly in a tick-like posture. The
fore-part is remarkably obtuse, and almost has the appearance of being
truncate and abruptly cut off. When the piercing organs have been
thrown forward horizontally it must be a great support to the insect,
which is fixed by them, if it can press its head down firmly against
the skin of the host. The wider the extent of forehead which can be
pressed against the skin of the host the less the strain on the upper
lip and mandibles, which serve as anchors, when the host scratches.
All the chigoe family have eyes; but in one recently discovered species
the eye is very small and devoid of pigment. Like those of other fleas,
the antennæ fit into grooves at the side of the head, and the club,
which is the sensitive part of the organ, consists of eight segments.
In a good many fleas the antennæ are different in the two sexes, but
there is no obvious sexual distinction in this family.
The peculiar development of the mouth-parts is one of the most singular
features in the structure of the chigoes and their relatives. These
important organs are modified in a fashion not to be found in any fleas
outside the family. Here, as in other fleas, the mandibles are piercing
organs which penetrate the skin of the host, the upper lip serving
in conjunction as a sucking tube. In ordinary fleas these organs are
retracted when the insect has done its meal; in the present family they
remain, in the case of the females, apparently permanently fastened in
the skin. The piercing organs are broader and the serrate edges of the
mandibles more solid and heavy in this family than in the case of other
fleas.
The two methods by which fleas keep in touch with their hosts have
already been alluded to. The two main fixing and clinging organs are
the mouth and the claws. Weak mouth-parts accompany strong legs. We
observe, accordingly, two lines of development. The chigoes and their
allies present an extreme case: for the legs are practically useless
for holding on. The bristles, and the claws as well, are exceedingly
thin. In this family the mandibles serve the purpose of claws. The
other line of development is best seen in a genus of fleas from South
America (_Malacopsylla_), where the piercing organs are short and weak,
whilst the claws and bristles of the legs are very strongly developed.
The modification found in the rostrum of the chigoes has already been
referred to, and the explanation of this will now be understood. The
rostrum is a sheath, on either side of the piercing organs, consisting
of an under lip and two labial palpi. When the flea sucks, the labial
palpi are pushed asunder, as the piercing organs are driven in, and
lie flat on the skin of the host. In this family the rostrum is almost
white in colour and soft instead of being horny or chitinised. Where
the rostrum is strongly chitinised or very horny the flea has to use
a certain amount of force to counteract the spring-like action of the
labial palpi. It is conceivable that rigid labial palpi would prove
inconvenient to fleas which remain permanently attached to their host
by their mouth organs.
[Illustration: Fig. 8. Pregnant female of _Dermatophilus cæcata_, a
South American chigoe which burrows into the flesh of the host. The
abdomen swells until it surrounds the head and thorax, which are shown
in the centre. The natural size is about equal to a small pea.]
The swollen abdomen of the female chigoe is a strange and a conspicuous
object, which is not, however, found occurring to the same extent in
all the members of the family. Having burrowed into the flesh of a
man, or other mammal, the pregnant female swells and enlarges until
she reaches the gigantic proportions of a small pea. Itching and
inflammation ensue unless the whole insect be skilfully removed with
a needle. To such an extent does the abdomen swell that the segments
and the horny plates are driven asunder and the connecting membrane
between is exposed to view. In the extreme case of _Dermatophilus
cæcata_ from South America the abdomen swells until it completely
envelopes the head and thorax after the manner shown in Fig. 8. Dr
Enderlein found seventeen specimens of this species in the skin behind
the ears of a rat from Brazil.
The belief that the eggs are laid in the flesh of the victim is
mistaken. The hind segments of the body and their stigmata are always
exposed to the air. The stigma of the eighth abdominal segment is
particularly large. As soon as the eggs have been laid, the body of
the mother dies, withers, and falls away from the skin of the host.
The fact that several females are often found where one has buried
herself, led to the notion that these parasites bred in the wound. The
truth seems to be that other chigoes are attracted to a spot where
inflammation has made it easy to burrow.
Chigoes love warmth and drought. The deserted huts of natives swarm
with them if they are dry. It is always said that newcomers are more
liable to attack than natives; but the explanation of this seems to be
that they do not understand the significance of the slight pricks which
are felt when the chigoe fixes itself. Once the parasite has got under
the skin no pain is felt unless the wound is inflamed by scratching.
The tender flesh under toe-nails is a favourite spot of attack. The
only remedy is a sharp knife and a little antiseptic wash. Pigs and
fowls are sometimes killed by chigoes, and Indians occasionally are
attacked by lockjaw after the parasite has been removed. But this is
not directly attributable to the chigoe. The eggs are laid one by
one; when this operation is completed the mandibles weaken and the
shrivelled body of the insect can be rubbed off. But a painful sore may
be produced if the parasite is forcibly broken off and the mouth-parts
are left in the wound.
The chigoe (_Dermatophilus penetrans_) is a native originally of South
America. It ranges from Mexico to Northern Argentina. Some time after
the middle of the nineteenth century it was, somehow, carried across
the Atlantic and introduced into West Africa. From there it has now
spread across the Dark Continent to the Great Lakes, and has even
reached Madagascar. Such are the modern facilities of transport which
parasites are quick to take advantage of.