148 HOUSEHOLD INSECTS 



The human flea also occurs in large numbers, at times, 

 on rats, cats, dogs, and in fewer numbers on mice. In 

 fact, it seems quite certain that whenever a house be- 

 comes infested with human fleas, the rats in that house 

 will also be found abundantly infested with them. 



When dwellings are left vacant for a length of time 

 they are often found, on the return of the occupants, 

 abundantly infested with fleas. This has led to the popu- 

 lar opinion that these insects are sometimes spontaneously 

 generated. Evidence seems to show that, under normal 

 conditions, mammalian blood is necessary for the pairing 

 and oviposition of adult fleas. On the other hand, it 

 seems to have been fairly well established by different 

 observers, that adult fleas can increase in deserted dwellings 

 for some time without the necessity of the normal supply 

 of food. Larvae of fleas have been found in sweepings 

 from the cracks of floors in deserted houses and adult 

 fleas bred from them. In the light of these facts, it is 

 not surprising that a vacant house often becomes badly 

 infested with these insects. 



OTHER FLEAS FOUND OCCASIONALLY ATTACKING MAN 



In India and other countries where the plague occurs, 

 the flea (Lcemopsylla cheopis) is the common flea on rats 

 and it has come to be known as the plague flea. This 

 flea now occurs in the San Francisco Bay region and 

 occasionally attacks man. This is probably the principal 

 flea concerned in carrying the plague. 



The common rat flea in the United States is a much 

 larger species, Ceratophyllus fasciatns. It is also fre- 

 quently found on man. 



