The naturalists now living who have devoted any time to the special study of fleas may almost be counted on one’s fingers. In England there are Mr Charles Rothschild and Dr Jordan; in the Shetland Islands, the Rev. James Waterston; in Germany, Taschenberg of Halle and Dampf of Königsberg; in Russia, Wagner of Kieff; in Holland, Oudemans of Arnhem; in Italy, Tiraboschi of Rome; in the United States, Carl Baker and a few others. I have not mentioned medical men who have investigated fleas in connection with plague. There are small collections of fleas in the Natural History Museums at South Kensington (London), Paris, Berlin, Königsberg, Vienna, Budapest, S. Petersburg and Washington. Of private collections Mr Charles Rothschild’s at Tring is by far the best in the world. It contains something like a hundred thousand specimens and is most admirably kept. I must express profound and sincere gratitude to Mr Rothschild for having helped me in numberless ways and advised me in many difficulties. It is well known that the mere mention of fleas is not only considered a subject for merriment, but in some people produces, by subjective suggestion, violent irritation of the skin. The scientific study of fleas has, however, received a great impetus since it has been ascertained that they are the active agents in spreading plague. Rat-fleas are of various kinds, and not all fleas will bite man. A knowledge of the different species has suddenly become useful. The humble, but ridiculous, systematist with his glass tubes of alcohol for collecting fleas, his microscopic distinctions, and Latin nomenclature has become a benefactor of humanity. Some people seem to be practically immune to the bites of fleas, but even to such persons their visits are unwelcome. A famous Frenchwoman once declared: “_Quant à moi ce n’est pas la morsure, c’est la promenade._” H. R. LONDON, _September, 1913_. CONTENTS CHAP. PAGE Preface v I. Introductory 1 II. The external structure of a flea 21 III. The mouth-parts and sense-organs 38 IV. The internal organs of a flea 52 V. The Human flea and other species 62 VI. The Chigoes and their allies 74 VII. Fleas and Plague 83 VIII. Rat-fleas and Bat-fleas 97 Appendix A. Systematic view of the order _Siphonaptera_ 108 ” B. A list of British fleas and their hosts 110 ” C. On collecting and preserving fleas 113 ” D. Bibliography 118 Index 122 LIST OF ILLUSTRATIONS