CHAPTER XII
THE STABLE-FLY (Stomoxys)
The common names for common insects in English are confusing. Not only are the same insects frequently known by different names on different sides of the Atlantic, but in many cases quite different insects—insects even belonging to different genera—are connoted by the same common name. In this respect matters are different in Germany: partly, perhaps, because the Germans on the whole are more scientifically inclined than we are, but partly, I suspect, because the German language lends itself more easily to express in one word—however long—the characteristics of any given insect.
The genus Stomoxys is generally called in Great Britain the ‘stable-fly,’ but there are other ‘stable-flies.’ One of the commonest species of the genus is S. calcitrans, a two-winged muscid fly, not at all unlike the common domestic fly, Musca domestica; but there are one or two points which readily distinguish it from the commoner insect. To begin with: it has a hard, firm, chitinous, piercing proboscis, which when at rest stretches forward in front of the head, and when in action is pressed down at right angles to the longitudinal axis of the body; then, again, when resting, its wings diverge; those of the house-fly approximate. Like other flies, the Stomoxys varies somewhat in length, between 5·5-7 mm. The thorax has on its back four longitudinal, dark stripes, broken by a transverse suture; and, as the accompanying figure shows, the third of the great, long veins which traverse the wing is much more slightly bent than is the case in Musca domestica. Further, whereas the hinder edge of the eye in the house-fly is straight that of the stable-fly is concave, and the antennae bear hairs on the upper side only and not above and below as they do in the domestic fly.
As a biting-fly and a blood-sucking fly, the habits of Stomoxys naturally differ from those of Musca domestica; but, like the latter, its distribution is almost world-wide. It is found in all temperate and tropical countries, and extends as far north as Lapland. But it is perhaps most abundant (or shall we say it has been most observed?) in temperate climates and during the summer months.
In any farm or country house large numbers of Stomoxys calcitrans are found in and about the cowsheds and stables, and in warm weather the same is true wherever cattle are grazing in the field. Later in the year, at the beginning of autumn, they are frequently found indoors, and in some ‘fly counts’ they have furnished quite 50 per cent. of the flies of a country house, the remaining 50 per cent. being made up of many other species and genera. When resting on a vertical surface Stomoxys generally has its head pointing upwards, whereas, as a rule, the house-fly rests upside down. The adult fly feeds upon any decaying matter; but whenever it can, it sucks the blood of vertebrates, and at times is a real nuisance to animals as well as human beings. So voracious are they that should a well-fed one be injured, the others immediately attack it and suck up every drop of blood which it had secured for its own food.
It has often been disputed whether a meal of blood is essential to the female mosquito before oviposition, but it seems perfectly clear that the female Stomoxys can produce fertilised eggs without having had a meal of blood.
The female lays a number of white, banana-shaped eggs a few inches below the surface of any decaying organic matter; fermenting grass from the lawn, decaying garden stuff, stable manure—each forms a favourable nidus. The eggs are laid in a heap like those of the house-fly, each heap containing from fifty to seventy. The egg is 1 mm. in length and has a grooved side, through the thicker end of which the larva escapes when the egg-shell splits.
The issuing larva is almost transparent. It not only has no head, but the anterior end dwindles almost to a point. When fully grown it attains a length of 11 mm., and the larval stage usually lasts from two to three weeks; but development may be retarded by adverse circumstances up to eleven or twelve weeks, and in such cases the full-grown larvae are often stunted in size. In these circumstances the pupae they produce are markedly smaller than those which have followed a more normal course of development. As is true of the egg and of the larva, the pupa resembles the pupa of the house-fly, being barrel-shaped and of a chestnut-brown colour; it is 5 to 5·5 mm. in length. The pupa stage lasts from nine to thirteen days, but this period is prolonged by cold.
On emerging from the pupa-case the insect has to push its way to the surface of the rotting vegetation in which it has been produced. This it does partly by the alternate inflation and deflation of the so-called ‘frontal sac,’ and by actively pushing forward the body by means of its legs. Once on the surface the insect begins to clean itself, pumps air into its body, forces it along the tracheae in the wings, which expand and ultimately harden. In the processes of unfolding they are aided by the hind legs. For a time the insect is immobile, gradually stiffening; but when the integument has hardened it flies off to explore the outer world. Under normal conditions the whole life-cycle varies from twenty-seven to thirty-seven days.
The chief interest of Stomoxys to the public, rests upon the fact that it is a very potent carrier of disease. There are certain forms of Trypanosoma which, under experimental conditions, are undoubtedly transferred by this species. But opinion is still unsettled as to whether the transference of these protozoa occurs in nature. The Surra diseases of horses and camels is, according to some authorities, transferred by Stomoxys, and so is the Surra disease of cattle; and there are others, all fully set forth in Mr. Hindle’s work on ‘Flies and Disease.’
Certain thread-worms—for instance, Filaria labiato-papillosa—which occur in the peritoneal cavity, and sometimes in the eyes of cattle and deer in India, are undoubtedly conveyed by Stomoxys calcitrans. The superficial vessels of the cattle swarm with the larvae of these thread-worms, which readily pass through the proboscis of the insect into its stomach. They then wriggle through the walls of the stomach and make their way into the thoracic muscles; here they undergo a ‘rest-cure,’ and after a time they are readily transferred to a new and possibly uninfected host.
But by far the worst infection which is attributed to this fly is acute epidemic poliomyelitis, or infantile paralysis. That this disease occurs in epidemics has been known—especially in Scandinavia—for some time; and eight years ago it attracted serious attention in North America and in our country. In 1907 there were many local outbreaks in the United States and Canada, and it is thought that the infection was first introduced from Scandinavia along the Atlantic coast, and later, inland, as far as the State of Minnesota, by the numerous Scandinavian immigrants that settle there.
The disease is one of those which are apparently due to a protozoon too small to be visible under the highest power of the microscope, and so small as to be able to pass through a Berkefeld filter. It can readily be artificially transmitted to monkeys. It is thought that the disease is by no means transmitted only by means of the biting Stomoxys, and that it may be directly transmitted from one person to another without the aid of any intermediate host. But there seems little doubt that it can be, and is, transmitted by Stomoxys, and therefore it is of the highest importance to reduce the number of these insects.
The most efficient way of controlling this pest is to destroy or put out of action its breeding-places. All decaying vegetable matter should be either removed or burnt or buried, or covered with some agent which will prevent the larvae living. In fact, the methods that have been advocated for the common house-fly are applicable to Stomoxys. If stable manure were carefully removed, from May to October, at least every seven days, the number of flies would be materially reduced. Where this is impracticable, manure-heaps should be covered with some insecticide, so as to destroy the eggs and larvae. Experiments are still being made with the view of finding a substance capable of killing the eggs, larvae, and pupae, which will be at once cheap and unharmful to the fertilising value of the manure. The American experts recommend borax or colemanite (crude calcium borate), calcined, powdered, and applied by a flour-dredger. The proportions which seem most effective are 0·62 lb. of borax and 0·75 lb. of colemanite to 10 cubic feet, or 8 bushels of manure. Two or three gallons of water should then be sprinkled over the manure-heap.