4 MISC. PUBLICATION 5 00, U. S. DEPT. OF AGRICULTURE 



such domesticated animals as cats, dogs, and pigs. During the early 

 part of the summer the fleas lay large numbers of eggs on their hosts, 

 which soon drop off into cracks of the flooring or into debris of vari- 

 ous kinds. Here the larvae hatch and feed, grow to larval maturity, 

 and transform into quiescent pupae. Then about the middle of thG 

 summer, frequently near the time when the occupants of the infested 

 dwelling may be returning from a summer vacation, large numbers 

 of the second generation of adults appear. It is this generation 

 which, because of its great numbers, usually causes so much distress. 



In the United States fleas are serious pests of dogs, cats, and 

 poultry. Dogs may be greatly annoyed by fleas and when attacked 

 spend much time gnawing at the affected parts, particularly the back 

 near the root of the tail. As a result these parts become raw and 

 sore, and infection often follows. 



Poultry are frequently infested by fleas, and the sticktight flea of 

 the South attaches permanently to the heads of its hosts. Here it 

 sucks the blood and causes much irritation. Young chickens may 

 become so heavily infested that they die from the attacks of these 

 pests. 



Some fleas are of much economic importance because they are the 

 transmitters of certain diseases. This is particularly true of those 

 that transmit plague, one of the worst scourges of man. In the 

 United States, as well as in many foreign countries, the oriental rat 

 flea {Xenopsylla cheopis (Rothschild) ) is the most efficient carrier 

 of the bacterial organism of plague. Also in this country four other 

 species that readily feed on man are thought to be occasional car- 

 riers. They are the human flea (Pidex irntans Linnaeus), the cat 

 flea (Ctenocephalides felis (Bouche)), the dog flea (Ctenocephalides 

 canis (Curtis)), and the rat flea (Nosopsyllas fasciatus (Bosc)). 



Plague organisms, developing in the "gizzard" of the oriental rat 

 flea, frequently cause an obstruction so that when the flea attempts 

 to feed again regurgitation results. It is this regurgitation that is 

 believed to be generally responsible for the transmission of the 

 organisms to man. 



Plague first appeared in the United States in California, in 1900, 

 and within 10 years was found to exist in native ground squirrels in 

 several counties. Among these hosts the plague spread slowly until 

 today diseased rodents have been found in the States of Oregon, 

 Washington, Nevada, Idaho, Utah, New Mexico, and Montana. The 

 squirrels act as reservoirs for the disease, and the fleas, becoming 

 infected with the bacteria obtained from these rodents, may later feed 

 on rats or man and thus increase the spread of the plague. 



Fleas also are vectors of endemic typhus. This disease is a mild 

 form of typhus found in several of the warm countries of the world. 



THE STRUCTURE OF THE SIPHONAPTERAN 



HEAD 



In many of the more generalized fleas, such as Ctenocephalides 

 canis or Hoplopsyllus affims, the division of the forehead into a front 

 and a pair of genae is evident. The front extends, with its outwardly 

 curved anterior margin, down to the anteroventral angle of the head. 

 The genae form the lower margin of the head and extend upward at 



