Killers of the insect world: they once conquered whole armies, science now has them on the run

There are many diseases that cannot spread unless carried by insects. They are among the most ancient afflictions of mankind, and have played their part in shaping his history. Malaria has influenced the rise and fall of civilizations, epidemics of plague and of yellow fever have again and again decimated populations in the old and the new worlds, while outbreaks of louse-borne typhus have often determined the outcome of military campaigns. Sleeping sickness and a less well-known .disease, onchocerciasis, have held back progress in Africa.

These and a score of other diseases carried by flying and crawling insects have enfeebled whole sections of the human race, depopulated fertile food-producing tracts, and held down man's levels of living particularly in the tropics but also in temperate climates. Despite the strides that have been made in our own day towards the control of many of these scourges, there is scarcely one which does not still represent an actual or potential danger to large numbers of human beings.

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