232 THE INSECTS. 



Pulicinae. Formerly, with the exception of infection with D. 

 canium, the fleas were only under suspicion as carriers of disease; 

 ideas having been entertained as to their being possible transmitters of 

 relapsing fever, typhus fever and kala-azar. As a result of the con- 

 vincing experiments of the British Plague Commission, their role in the 

 transmission of plague has been absolutely established. It is by the 

 bite of the Pulex cheopis that plague is chiefly transmitted from rat to 

 rat, and in bubonic and septicaemic plague it is apparently the in- 

 termediary in human infection. The puncturing apparatus of the 

 flea consists of a pointed epipharynx and 2 mandibles. By the ap- 

 position of the mandibles to the epipharynx a tube is formed through 

 which the blood is sucked up. The antennae are inconspicuous and 

 are in close apposition to the sides of the head, behind the eyes, and can 

 only be well made out with a lens. Fleas have 3 pairs of legs, and the 

 male can be distinguished from the female by its smaller size and the 

 conspicuous coiled-up penis within the abdomen. The body of the 

 flea is flattened laterally. They may or may not have eyes, and certain 

 conspicuous structures called combs are of importance in classification. 

 In the metamorphosis of the flea the eggs are hatched out in dust of 

 crevices, etc., into bristled larvae in about i week. The larva forms a 

 cocoon and develops into" a nymph which has 3 pairs of legs. The 

 nymphs emerge from the cocoon as adult fleas in about 3 weeks after 

 the larva forms it. 



Key to the Fleas. 



A. No eyes, or eyes indistinct. 



1. Densely spinose. Combs on some abdominal segments. 

 Hystrichopsylla. 



2. Combs on head and prothorax, none on abdomen. Typh- 

 lopsylla. 



B. Eyes present. 



1. Combs along inferior border of head and posterior border 

 of prothorax. Ctenocephalus. 



2. No combs along inferior border of head, but on posterior 

 border of prothorax. Ceratophyllus. 



3. No combs. Pulex. 



