Linnaeus recognized two species of flea, Pulex irritans, the common parasite of man, and Pulex (Sarcopsylla) penetrans, the tropical and sub-tropical jigger flea. More than a hundred species are now listed, and the recent discovery that certain fleas are instrumental in the transmission of the plague has given a new impetus to the study of these tiny pests. A flea that is often commoner in houses than Pulex irritans is the “dog and cat flea,†variously known as Pulex serraticeps, Pulex canis, Pulex felis or Ctenocephalus canis.