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.

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.

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.

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.

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.