Page:Popular Science Monthly Volume 23.djvu/667

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INSECTS AND DISEASE.

swamps and pools drained, so that the mosquito can not so readily find a place suitable to deposit her eggs. And, as the forests and underbrush disappear before the implements of the agriculturist, colonies of mosquitoes, wafted from a distance by winds, are not "obstructed" and "accumulated" by foliage, nor can the insect so readily escape, as before, the numerous fly-catching birds that feed upon it. Even here, however, artificial pools, tanks, and excavations containing water, may constitute mosquito-nurseries from which many millions may be developed in a single summer.

11. Malaria usually keeps near the surface of the earth; it is said to "hug the ground" or "love the ground." When blown by the wind, however, or drifting up ravines, it has been known to rise several thousand feet. Dr. Russell, in his address before the New York Public Health Association, April 13, 1876, stated that, "under ordinary circumstances, a certain altitude affords immunity from malaria, although low elevations of 200 or 300 feet above a miasmatic tract are often more dangerous than the flat lands, the poison seeming to float upward and become intensified." This, he says, has long been noticeable on the heights of Bergen Hill, West Hoboken, and Weehawken, which overlook the Jersey flats. In accordance with the malarial vapor theory, these facts are completely mysterious. The mosquito, on the other hand, is known to hover near the ground (or water) from which it springs, and, being wafted by winds, can readily be understood to be "obstructed" and "accumulated" by forests on the brows of hills, etc.

12. Malaria is most dangerous when the sun is down, and it seems almost inert during the day. Of this there is no doubt, and the various hypotheses on the marsh-vapor theory, that have been alleged in explanation of it, are almost as numerous as they are unsatisfactory. With regard to the mosquito, however, it is well known that it remains, for the most part, during the day, harbored in woods, weeds, or low underbrush, and comes out after sunset and at night to indulge its blood-sucking proclivities.

13. The danger of exposure after sunset is greatly increased by the person exposed sleeping in the night air. Again have the hypotheses based on the marsh-vapor theory been altogether insusceptible of explaining this circumstance satisfactorily. With regard to mosquital inoculation, however, it is undoubtedly true that, while awake, the person exposed will move about, or brush away the insect, while he will submit to be bitten during sleep.

14. In malarial districts, the use of fire, both in-doors, and to those who sleep out, affords a comparative security against malarial disease. Explanations on the marsh-vapor theory are numerous, various, and unsatisfactory. With regard to the mosquito, however, it is well known to be attracted by lamps, lights, and fires, into which it heedlessly flies at the cost of life. In countries where these insects are extremely nu-