SIPHONAPTERA 879 



the action of muscles attached to this lever, the blade of the mandible 

 can be forced in and out. The number of segments of the labial palpi 

 varies from two to seventeen. 



The three segments of the thorax are quite distinct from one 

 another although closely joined. The ventral part of the prothorax 

 extends forward under the head, so that the first pair of legs appear 

 to depend from the head. There is usually one, but there may be 

 two or three, transverse rows of bristles on each thoracic segment. 

 There are no vestiges of wings. The legs are long and strong and 

 fitted for leaping; the hinder pair are largest and the middle pair 

 next in size. 



The abdomen is composed of ten segments; the first seven of these 

 are comparatively simple in structure; the last three are specially 

 modified for sexual purposes. The variations in form of the sclerites 

 of these segments and of the genital appendages afford characters 

 much used in the classification of these insects. The ninth tergite 

 bears what is evidently a sensory plate; on the surface of this there 

 is a number of clear areas from each of which there projects a long 

 slender and extremely fine hair. The function of this organ is unknown . 



A conspicuous feature of many fleas is the presence of a series of 

 short, stout spines on various parts of the body; these are known as 

 combs or ctenidia. The presence or absence of ctenidia and their 

 location when present are important distinctive characters. In the 

 dog-flea (Fig. 1 1 2 1) there are ctenidia on the genas and on the posterior 

 border of the prothorax. 



Large bristles placed at the dorsal angle of the seventh abdominal 

 segment are termed the aniipygidial bristles. 



The eggs of fleas are scattered about the floors of dwellings and 

 in the sleeping-places of infested animals. The larvse are slender, 

 worm-like creatures, with a distinct head and without legs. (Fig. 

 1 121). They have biting mouth-parts, and feed upon the decaying 

 particles of animal and vegetable matter always to be found in the 

 dirt in which they live. When full-grown the larva spins a cocoon 

 within which the pupa state is passed. 



Fleas are parasitic only in the adult state. Some species infest 

 birds, but by far the larger nimiber prey upon mammals, and most 

 mammals are subject to the attacks of these parasites. Although the 

 different species of fleas infest different hosts they are not so restricted 

 in their host relations as are many parasites, and may pass from 

 their normal host to another species; for example, both the dog-flea 

 and the cat-flea frequently attack man. 



Formerly fleas were regarded as merely annoying pests of man and 

 his pets ; but it has been found that fleas are the carriers of bubonic 

 plague; this fact has greatly increased the interest in these insects. 

 A result of this increased interest is that extended studies of the 

 Siphonaptera have been made recently and are being made now.* 



*It is now known that the bubonic plague, which has caused the death of many 

 milHons of people, is a specific infectious disease caused by one of the bacteria, 

 Bacillus pestis; and that it is primarily a disease of rodents, especially of rats. 

 It has been shown by experiments that this disease is transmitted from one ro- 



