The fact that certain arthropods are poisonous, or may affect the health of man as direct parasites has always received attention in the medical literature. We come now to the more modern aspect of our subject,—the consideration of insects and other arthropods as transmitters and disseminators of disease.
The simplest way in which arthropods may function in this capacity is as simple carriers of pathogenic organisms. It is conceivable that any insect which has access to, and comes in contact with such organisms and then passes to the food, or drink, or to the body of man, may in a wholly accidental and incidental manner convey infection. That this occurs is abundantly proved by the work of recent years. We shall consider as typical the case against the house-fly, which has attracted so much attention, both popular and scientific. The excellent general treatises of Hewitt (1910), Howard (1911), and Graham-Smith (1913), and the flood of bulletins and popular literature render it unnecessary to consider the topic in any great detail.
The House-fly As a Carrier of Disease
Up to the past decade the house-fly has usually been regarded as a mere pest. Repeatedly, however, it had been suggested that it might disseminate disease. We have seen that as far back as the sixteenth century, Mercurialis suggested that it was the agent in the spread of bubonic plague, and in 1658, Kircher reiterated this view. In 1871, Leidy expressed the opinion that flies were probably a means of communicating contagious diseases to a greater degree than was generally suspected. From what he had observed regarding gangrene in hospitals, he thought flies should be carefully excluded from wounds. In the same year, the editor of the London Lancet, referring to the belief that they play a useful rôle in purifying the air said, "Far from looking upon them as dipterous angels dancing attendance on Hygeia, regard them rather in the light of winged sponges spreading hither and thither to carry out the foul behests of Contagion."
These suggestions attracted little attention from medical men, for it is only within very recent years that the charges have been supported by direct evidence. Before considering this evidence, it is necessary that we define what is meant by "house-fly" and that we then consider the life-history of the insect.
There are many flies which are occasionally to be found in houses, but according to various counts, from 95 per cent to 99 per cent of these in warm weather in the Eastern United States belong to the one species Musca domestica ([fig. 108]). This is the dominant house-fly the world over and is the one which merits the name. It has been well characterized by Schiner (1864), whose description has been freely translated by Hewitt, as follows:
"Frons of male occupying a fourth part of the breadth of the head. Frontal stripe of female narrow in front, so broad behind that it entirely fills up the width of the frons. The dorsal region of the thorax dusty grey in color with four equally broad longitudinal stripes. Scutellum gray with black sides. The light regions of the abdomen yellowish, transparent, the darkest parts at least at the base of the ventral side yellow. The last segment and a dorsal line blackish brown. Seen from behind and against the light, the whole abdomen shimmering yellow, and only on each side of the dorsal line on each segment a dull transverse band. The lower part of the face silky yellow, shot with blackish brown. Median stripe velvety black. Antennæ brown. Palpi black. Legs blackish brown. Wings tinged with pale gray with yellowish base. The female has a broad velvety back, often reddishly shimmering frontal stripe, which is not broader at the anterior end than at the bases of the antennæ, but become so very much broader above that the light dustiness of the sides is entirely obliterated. The abdomen gradually becoming darker. The shimmering areas on the separate segments generally brownish. All the other parts are the same as in the male."
The other species of flies found in houses in the Eastern United States which are frequently mistaken for the house or typhoid fly may readily be distinguished by the characters of the following key:
a. Apical cell (Rs) of the wide wing open, i.e., the bounding veins parallel or divergent ([fig. 100]). Their larvæ are flattened, the intermediate body segments each fringed with fleshy, more or less spinose, processes. Fannia
b. Male with the sides of the second and third abdominal segments translucent yellowish. The larva with three pairs of nearly equal spiniferous appendages on each segment, arranged in a longitudinal series and in addition two pairs of series of smaller processes ([fig. 100]) F. canicularis