CLASSIFICATION 407 



a tiny puncture, and then the serrated mandibles enlarge the 

 hole by an up and down sawing motion. As these organs are 

 sunk into the flesh of the host the labial palpi bend back like a 

 bow under the flea's head. The two grooved mandibles, placed 

 in apposition, form a tube for the outflow of saliva, while the 

 epipharynx, which is also grooved, forms a tube with the man- 

 dibles for the inflow of blood. The digestive tract is provided 

 with a pharynx which acts like a suction pump, and a very large 

 and distensible stomach. 



Classification. — Several hundred species of fleas have already 

 been described and it is probable that many more species will be 

 found. Although some authors split the fleas into a consider- 

 able number of families, it is more usual to recognize only two 



Fig. 179. Heads of common fleas, showing distribution of ctenidia or " combs'' ; 

 A, human flea, Pulex irritans, without combs; B, dog flea, Ctcnocephalus canis, 

 with combs on both head and pronotum; C, rat flea, Ceratophyllus fasciafus, with 

 only pronotal combs. 



well-defined f amiUes or groups — the Pulicidse and the Sarcopsyl- 

 lidae. The former family includes all the "ordinary" fleas, whereas 

 the Sarcopsyllidse is a very specialized group of fleas with a much 

 shortened thorax, which appears as if mashed between the head 

 and abdomen, with slender anterior and middle legs, and with 

 feeble labial palpi of only three segments. Whereas all of the 

 Pulicidse lay their eggs singly, or in small groups, and develop- 

 ment of the embryos occurs after the eggs are deposited, in some 

 of the Sarcopsyllidse the eggs, during their early development, 

 are retained in the abdomen of the female, which swells up to 

 such a size that the head and thorax appear as a small append- 

 age at one end of it. 



