Insects and Diseases / A Popular Account of the Way in Which Insects may Spread or Cause some of our Common Diseases

by Rennie Wilbur Doane

The eggs ([Figs. 80], [81]) are elliptical in outline and are provided with a characteristic membranous expansion near the middle.

The larvæ may be found at the proper season and in the localities where they are abundant in almost any kind of standing water, in clear little pools beside running streams, in the overflow from springs, in swamps and marshy lands, in rain-barrels or any other places or vessels where the water is quiet. They do not breed in brackish water. As they feed largely on the algæ or green scum on the surface of the water they are especially apt to be found where this is present. We have already noted that their positions in the water differ from that assumed by other species ([Fig. 82]).

As the breathing-tube is very short the larvæ must come close to the surface to breathe, and when they are feeding we find them lying just under and parallel to the surface of the water with their curious round heads turned entirely upside down as they feed on the particles that are floating on the surface ([Figs. 83], [84]).

The pupæ do not differ very much from the pupæ of other species although the breathing-tubes on the thorax are usually shorter and the creature usually rests with its abdomen closer to the surface, that is, it does not hang down from the surface quite as straight as do other forms ([Fig. 85]).

The adults may be found out of doors or in houses, barns or other outbuildings. They do not seem to like a draft and consequently will be more apt to frequent rooms or places where there is little circulation of air. Although they are usually supposed to fly and bite only in the evening or at night, they may occasionally bite in the daytime. One hungry female took two short meals from my arm while we were trying to get her to pose for a photograph one warm afternoon.

The female passes the winter in the adult condition, hibernating in any convenient place about old trees or logs, in cracks or crevices in doors or out of doors. In the house they hide in the closets, behind the bureau, behind the head of the bed, or underneath it, or in any place where they are not apt to be disturbed. During a warm spell in the winter or if the room is kept warm they may come out for a meal almost any time.

THE YELLOW FEVER MOSQUITO

Ranking next in importance to Anopheles as a disseminator of disease and in fact solely responsible for a more dreaded scourge, is the species of mosquito now known as Stegomyia calopus. While this species is usually restricted to tropical or semi-tropical regions it sometimes makes its appearance in places farther north, especially in summer time, where it may thrive for a time. The adult mosquito ([Fig. 104]) is black, conspicuously marked with white. The legs and abdomen are banded with white and on the thorax is a series of white lines which in well-preserved specimens distinctly resembles a lyre. These mosquitoes are essentially domestic insects, for they are very rarely found except in houses or in their immediate vicinity. Once they enter a room they will scarcely leave it except to lay their eggs in a near-by cistern, water-pot, or some other convenient place.

Their habit of biting in the daytime has gained for them the name of "day mosquitoes" to distinguish them from the night feeders. But they will bite at night as well as by day and many other species are not at all adverse to a daylight meal, if the opportunity offers, so this habit is not distinctive. The recognition of these facts has a distinct bearing in the methods adopted to prevent the spread of yellow fever. There are no striking characters or habits in the larval or pupal stages that would enable us to distinguish without careful examination this species from other similar forms with which it might be associated. For some time it was claimed that this species would breed only in clean water, but it has been found that it is not nearly so particular, some even claiming that it prefers foul water. I have seen them breeding in countless thousands in company with Stegomyia scutellaris and Culex fatigans in the sewer drains in Tahiti in the streets of Papeete. As the larvæ feed largely on bacteria one would expect to find them in exactly such places where the bacteria are of course abundant.