THE YELLOW-FEVER MOSQUITO (_Stegomyia calopus_)
... et nova febrium
Terris incubuit cohors.
(HORACE.)
Like other branches of human activity disease has its romantic and its
unromantic side. Nobody can regard mumps or measles as romantic. On the
other hand, yellow fever calls up all the romance of slave-trading,
pirates and the Spanish Main, buccaneers, maroonings and other grisly
horrors, whose sole redeeming feature was a touch of romance. Lovers
of pirate stories—and who are not?—will always remember their graphic
description of Yellow Jack in the West Indies.
We have probably always had disease with us since the creation of the
world—that act of ‘_impardonnable imprudence_,’ as Anatole France calls
it; but the first description of yellow fever only dates back to 1647,
when an outbreak occurred in the Barbados. Then, as now, it devastated
the shipping of the port, and was soon introduced by ships into St.
Christopher and, later, into Guadeloupe. The following year it was in
Cuba, and in 1655 in Jamaica, and it gradually spread throughout the
whole of the West Indies until a century or more later it reached the
Island of St. Thomas.
One of the peculiarities of the disease is that it frequently
disappears from a given locality for long periods of time. For
instance, it was absent in Barbados after the first outbreak until
1690, and when it recurred it was at first not recognised as being the
same disease which devastated the islands forty-three years before. In
the eighteenth century there was another break of fifty-four years, and
similar breaks can be recorded in most of the West Indian islands.
Besides the West Indies, it is at present endemic in Brazil and
on the west coast of Africa, and is common in Mexico. Whether the
disease arose primarily in Africa and is part of the toll the American
continent has had to pay for the slave-trade, or whether it was brought
to the west coast of Africa from the other side of the Atlantic, is not
certain. It apparently appeared as a regular disease in Brazil in the
year 1849, and from that time onwards, with the exception of one year,
has been a permanent trouble at Rio. From time to time the disease has
been carried to neighbouring parts of America, especially to the Gulf,
Central America, and the northern coast of South America. It has been
introduced more than once into Monte Video and Buenos Ayres, and has
even penetrated up the Parana as far as Asunçion. Every few years it
extends into the Southern States and has even reached Philadelphia and
Boston. With the exception of an outbreak in Leghorn in 1804, European
epidemics have been confined to Portugal, Spain, and the Balearic
Islands.
It will have been noticed that most of these outbreaks occur on the
coast and then pass up the rivers. It is thus most probable that the
disease is one which is brought mainly by ships. It is obviously a
disease which must be guarded against by our troops fighting near the
coast in West Africa, as well as such troops as are left in the West
Indies. But, above all, it must be guarded against in relation to our
shipping fleet and our Navy, operating off the South American coasts.
The danger, now the Panama Canal is open, of introducing the disease
from America to Asia is a danger that should carefully be considered.
Yellow fever is a disease which requires a winter temperature of at
least 68° F., for it is a mosquito-borne disease, and the yellow-fever
mosquito flourishes best at about this temperature. It can be
introduced into a new locality by the arrival of an infected mosquito,
or by the arrival of an infected human being. In the former case the
disease breaks out within a few days; in the latter at least ten or
twelve days elapse before new cases arise, for, as we shall see later,
the organism, whatever it is, that causes the fever is not capable of
passing from the mosquito until it has been in its body for ten or
twelve days.
[Illustration: FIG. 26.—_Stegomyia fasciata._ Female, lateral view
(magnified.) Note hump-backed outline, and also the position of the
posterior pair of legs.]
Thirty-six years ago Finlay of Havana suggested that the virus of
yellow fever was inoculated by mosquitos; but it was not until the
publication of the discoveries by Sir Ronald Ross and others, that
malaria is transferred by _Anopheles_, that a thorough investigation
of yellow fever was made. In the last year of the last century
an American Commission, consisting of Drs. Walter Reed, Carroll,
Agramonte, and Lazear, investigated the whole subject, and, taking
extraordinary risks, were able to prove that the infection was not
conveyed by contact or through the air, or from bedding or clothes
soiled by the dejecta of yellow-fever patients, but by a mosquito of
the genus _Stegomyia_. Whatever the virus is, it is invisible, even
under the highest powers of the microscope. It can be filtered through
a Berkefeld filter. It is destroyed by heating to 55° C. If the blood
of a yellow fever patient, during the first three days, be inoculated
into a healthy man he gets yellow fever, and it is only during the
first three days that the blood is infective. On the other hand, the
mosquito is incapable of transferring the disease until the unknown
organism has been in its own body for at least ten or twelve days.
The mosquito in question belongs to the species _Stegomyia calopus_
(Blanchard), or, as it is more often called in English textbooks,
_Stegomyia fasciata_ (Fabricius). The genus _Stegomyia_ differs from
other _Culicidae_ in having a dark grey or black colour, whilst the
_Culicidae_ are as a rule browner. _Stegomyia_ also has silver-white
spots and silver glistening scales, especially on the back of the legs
and on the abdomen. The grown-up mosquito is comparatively small, and
very elegant. Its length is some 3 to 4 mm., but if the mouth parts be
added is some 6 to 6½ mm. long. As is usual, the male is smaller and
feebler than the female. When settled—as, for instance, whilst sucking
the blood of its host—it rests upon its first four legs only, the two
hindmost being stretched out abaft like pennants waving in the air; but
in general it has the hump-backed appearance of _Culex_ and not the
straight outline of _Anopheles_. The colour is greyish black, modified
by numerous white spots and rings. There is a white rim round the eyes,
and a very characteristic lyre-like pattern on the dorsal surface of
the thorax. The structure of the mouth parts is much the same as that
of any other _Culicidae_. The antennae have fourteen joints, the last
two of which in the male are longer than the others. As is again usual,
the antennae of the male have long brush-like hairs, organs by means of
which they find the female. The legs are banded alternately with white
and black rings. It is this character, indeed, which has given this
mosquito the name of the ‘tiger-gnat.’ The wings are very iridescent.
[Illustration: FIG. 27.—_Stegomyia fasciata._ Above, the larvae;
below, the eggs. Both natural size.]
The pupa of _Stegomyia_ is darker and blacker than that of _Culex_,
and, seen from the side, the head and the thorax are somewhat more
triangular than the same parts in _Culex_. As the pupa grows older it
grows darker. The length of the larva is 4 to 6 mm., somewhat larger
than that of the gnat. But, like that, it has a respiratory-tube
stretching out from the last segment of the abdomen, almost at right
angles to the rest of the body. This respiratory-tube is much shorter
than that of _Culex_, but is long enough to enable the larva to hang
obliquely down into the water. The eggs are very large. They are
covered by a mass of small ‘cells’ containing air, and they never tend
to form a conglomerate mass like those of _Culex_, but are laid singly,
and remain isolated until the larvae hatch. After floating a certain
time they usually sink to the bottom of the water. Their length may
be about a millimetre, and their colour is almost black. When the egg
hatches, the anterior third of the shell splits off and the larva at
once emerges.
[Illustration: FIG. 28.—Larva of _Stegomyia fasciata_ breathing on the
surface of the water. Highly magnified.]
As is so often the case with mosquitos, it is the female alone
which bites. The male nourishes itself on plant-juices, saps, and
so on—especially they like sugary secretions—and in the absence of
blood the female is reduced to a similar diet. Hence _Stegomyia_ is
comparatively common in dwellings where sweetstuffs are—bakeries,
sugar-refineries, and so on. These mosquitos are, like the cockroach,
the fly, and the bed-bug, inhabitants of human dwellings. They are
indeed domesticated, and are always to be found in the neighbourhood of
human houses or buildings or ships, and are very rarely indeed found
far away from the sphere of man’s activities.
[Illustration: FIG. 29.—Egg of _Stegomyia fasciata_ (highly
magnified). Notice the air-‘cells.’]
They are very apt to bite one in the neck, creeping along the darker
parts of the clothing until an unprotected region of the body is
reached. Unless one has very thick socks they frequently bite the
ankle, and they are as tireless in their pesterings as ever Mrs.
Pardiggle was—no sooner are they driven away than they return to the
attack. The bite is painful, and in many people raises a considerable
swelling.
The _Stegomyia_ bite not only during the night, but also during the
day. According to R. O. and O. Neumann—in Brazil, at any rate—they are
capable of biting not only during the twilight, but at any times. The
bite lasts twenty to thirty seconds, after which the mosquito rests
a bit, waving its third pair of legs in the sun. After this rest she
flies away to some sheltered spot, and whilst blood is being digested
the mosquito takes nothing but water—a very proper dietetic measure.
After three or four days the female is ready for another meal.
In the absence of man these mosquitos will suck blood from other
animals, and in confinement they are generally fed on rats or canaries,
and they will even suck up a drop of blood presented on a piece of
cotton-wool.
If the female mosquito has been fertilised before the sucking of blood
she will commence egg-laying two or three days later, and two or three
days later again the larva will emerge. The larval stage lasts from
nine to twelve days, and the pupa stage three to four, so that the
whole metamorphosis takes from sixteen to twenty-two days. Hence,
during warm weather, many generations succeed each other, but one must
have a temperature of at least 20° to 27° C. Below that temperature
the processes tend to slow down, and under a temperature near
freezing-point the regular development is definitely interrupted. But
the interruption is only a suspense, and living activities are resumed
should the temperature rise again.
It is a disputed point whether these mosquitos must have a meal of
blood before they can lay eggs, and on this point the evidence is not
yet sufficient to make a dogmatic statement. These mosquitos are very
indifferent where their eggs are laid. The smallest collection of water
in an empty sardine-tin, a broken tumbler, a puddle in the street, a
gutter-pipe, is good enough for _Stegomyia calopus_. She will even lay
her eggs on moist cotton-wool.
Although _Stegomyia_ bites freely during the day-time, it, as a rule,
avoids the light and seeks some dark shelter. Contrary to the habits
of _Anopheles_, it prefers a light ground to rest upon. The larvae
live on algae, vegetable-matter, or plant-detritus, or, in captivity,
on white bread or Indian corn. They can remain for a considerable time
without food, and this without materially diminishing the rate of their
development. _Stegomyia_ breeds well in ships, and is occasionally
found in one part only of the ship—such as the engine-room or cook’s
galley, where the conditions seem to be most favourable to its
development. Thus it comes about that at times certain quarters of a
ship provide the greatest percentage of yellow-fever cases.