In further support of the mosquital origin of malarial disease, we may remark the general admission on the part of medical authorities that sickly seasons and localities are usually accompanied with an extraordinary number of winged insects, many of them being bloodsucking diptera. Lind, in his "Essay on the Health of Semen" (p. 58), referring to an army of Christians, half of whom were lost by fever while passing through Hungary, tells us, "The air swarmed with insects—a sure sign of its malignancy" (p. 60); and, in referring to the climate of Guinea and of the East and West Indies as being fatal to Europeans, he adds, "More especially when molested with heat within-doors, and the plague of mosquitoes, they have ventured to sleep in the open night-air" (p. 71). And again, describing a journey from the interior of Guinea to Senegal, he says (p. 94): "The earth had its white ants, the air its wild bees, its sand-flies, and its mosquitoes. These insects, though not the most tremendous, were perhaps their (the travelers') most distressing enemies." On page 77 he remarks: "The greatest plague was the mosquitoes and sand-flies, whose incessant buzzing and painful stings were more insupportable than any symptom of the fever." After landing on the Canaries the health of the men immediately improved, but they were here no longer "tortured with swarms of blood-sucking gnats and flies" (pp. 83, 85).
Mosquitoes are not generally troublesome in England, yet in the ague-fens of Lincolnshire and the swamps of Essex the use of mosquito-nets is as necessary and common as in India or any other tropical climate ("Chambers's Cyclopædia," article "Gnat"). The prevalence of the mosquito-plague in the fever-districts of Italy is also well known.
Furthermore, in certain districts where the so-called "malarial poison" is supposed to be lodged in trees and bushy plants near the ground, it has been observed that those persons are particularly prone to fever who cut down or disturb these malaria-laden plants, which is extremely suggestive of the mosquitoes being disturbed from their reposing haunts, just as one might get stung by stirring up a bee-tree or a hornet's nest. La Roche, in his well-known work (p. 282), says: "Malaria is collected by plants, particularly those of a dense and entangling foliage, so as to be disengaged on cutting them down or rooting them up, thus exciting fever in the laborers who might otherwise have escaped, as proved by the circumstance that in all these situations, while the workmen are in the erect posture and engaged at their work, they escape the fever, but are attacked if they sit, and more particularly if they lie down on the ground—and that whether they sleep or not."
Here it may be observed that the circumstances stated to conduce to the production of fever (viz., sitting or lying still) are exactly those which would favor being bitten by mosquitoes, the insect having less chance of inflicting its inoculating wound while the men when in motion are "engaged at their work."