190 ENTOMOLOGY FOR MEDICAL OFFICERS 



The flea about which most is known in this connexion is 

 Pulex {Xenopsylla) cheopis, the species that most commonly 

 infests house-rats in tropical latitudes ; but it is reasonable 

 to infer — and has, indeed, to some extent been demonstrated — 

 that other species of fleas that attack the rat and other 

 rodents may carry the plague bacillus. Thus it was shown 

 in India that both Pulex irritans (the human flea) and 

 Ceratophyllus fasciatus (the common rat-flea of temperate 

 latitudes), and in California that Hoplopsyllus anomalus (a 

 flea that infests the Californian Ground-squirrel and that 

 also attacks the rat) are capable of carrying the bacillus. 



As regards Xenopsylla cheopis it was conclusively proved 

 by the Indian Plague Committee that this species is easily 

 infected from infective rats, that the bacillus multiplies in its 

 stomach so that it in turn becomes infective, and that it 

 may remain infective for a term of fifteen days. The only 

 point that remains obscure is the precise manner in which the 

 contaminated flea passes on the infection. The experiments 

 recorded by the Committee show at any rate that the 

 bacillus does not pass into the body- cavity or into the salivary 

 glands of the flea, but that the flea's faeces are infective, and 

 the hypothesis most favoured is that the infected faeces of 

 the flea somehow or other come in contact with the " bites " 

 that the flea may subsequently inflict ; and this hypothesis 

 receives support from the well-known fact that in many 

 insects that suck juices the act of sucking is accompanied 

 with such violent peristalsis of the whole alimentary tract 

 that the contents of the rectum are simultaneously ejected. 



External Structure of the Flea. — The head, if small com- 

 pared with the abdomen, is large in comparison with the 

 thorax, and is irregularly conical in shape ; the gena, or 

 cheek, is wide, and its lower angle (genal angle) is often 

 pronounced, or even produced backwards. An elegant comb 

 of teeth is found in some fleas on the edge of the cheek, or 

 on the lower edge of the head. Eyes may be present or 

 not ; when present they are simple (non - faceted). No 

 supplementary ocelli are present. Behind and above the 

 eyes are the antennae, which in repose are lodged in definite 

 grooves. Each antenna consists of 2 smaller basal seg- 

 ments, and a larger oval segment, or club, which is more or 



