Male Oriental rat-flea _frontispiece_ FIGURE PAGE 1. The larva of a flea 6 2. Types of genal and thoracic combs of a flea 26 3. The hind leg of a flea 30 4. The mouth-parts of a flea 43 5. The antenna of a flea 47 6. The alimentary canal of a flea 53 7. The head of a female dog-flea and a female cat-flea 71 8. Pregnant female of _Dermatophilus cæcata_ 81 CHAPTER I INTRODUCTORY FLEAS form a group of insects that have, until recently, been little studied by zoologists. We call them insects because they are jointed animals, or Arthropods, with three pairs of legs in the adult condition. The reader will best understand the position which fleas occupy in the general classification of animals by remembering that the arthropods, or jointed animals, are one of a dozen subkingdoms, or phyla, to which the various members of the great animal kingdom have been assigned. There is good ground for believing that all the animals included in each phylum trace their ancestry back to a common primitive form which lived in more or less remote ages. Besides (1) _Insects_, the arthropods, or jointed animals, include (2) _Crustaceans_, such as crabs, lobsters, shrimps, wood-lice, water-fleas and barnacles; (3) _Myriapods_, such as centipedes and millipedes; and (4) _Arachnids_, such as spiders, scorpions, mites and ticks. To all these varied forms of animal life fleas, and other insects, are therefore more or less nearly related. The animals belonging to this large and important collection, which compose the arthropod phylum, have certain common characteristic features. We find a body made up of a series of more or less completely similar segments placed one behind the other. In this they resemble certain worms which are far less highly organised. The body is elongated, symmetrical on either side, and the mouth and anus are at opposite ends. There is, however, an important advance on the segmented worms. Each typical segment carries a pair of appendages which are very different from the foot-stumps that are found on certain worms. These appendages of arthropods are divisible into distinct limb-segments, separated from one another by moveable joints, and acted upon by special muscles.