The discovery of the transmission of disease by mosquitoes required the passage of a decade before its essentials were grasped by the public mind; that of the prevention of small-pox required a century. But the dangers of house-flies is rapidly becoming known in consequence of the popular literature, which is growing, describing the details of the lives of these loathly creatures. In this way only can such knowledge be spread—a knowledge which must become general before flies and the maladies they convey can be generally and satisfactorily dealt with. It is of little use to make great discoveries and then to hide them on the musty bookshelves of learned societies. Instead, they should be adapted to practical purposes applied for the good of suffering humanity; and the best way to do this is to bring out well-written, interesting, and easily read books of this kind, so that all who run may read and their readings endure. This book should assist much to accomplish this end. Thus we may look forward confidently to the day when house-flies, and the diseases they carry, are things of the past. The "Book of the Fly" must take its place in the history of the events which are to lead up to the winning of that goal. HALFORD ROSS (_of the John Howard McFadden Researches at the_ _Lister Institute of Preventive Medicine_). ------------------------------------------------------------------------ THE BOOK OF THE FLY CHAPTER I THE HOUSE-FLY, A PRODUCT OF HUMAN INSANITATION With the present day zeal for popularising interest in common things (called nature study) there has arisen the demand for knowledge practically useful and thoroughly up-to-date, yet in a form free from much of the technical terminology and treatment which are essential in the student's more fully developed scientific handbook.