n] EXTERNAL STRUCTURE OF A FLEA 29 



The thorax of a flea consists of three segments 

 called respectively the prothorax, mesothorax and 

 metathorax. The chitinous external skeleton which 

 covers each of these three segments is primarily a 

 hoop but each hoop is further subdivided into a 

 number of complicated plates. Attached to the thorax 

 are the three pairs of legs which are characteristic of 

 all adult insects. The hind pair are very much the 

 strongest (Fig. 3). They are the organs of hopping. 

 It has often been pointed out that if men had the 

 leaping powers of some fleas they would bound with 

 ease backwards and forwards over the cross on the 

 top of S. Paul's Cathedral. Each leg consists of 

 four segments beautifully articulated and plentifully 

 supplied with bristles. At the end comes the foot 

 with five very short segments. The last segment 

 is provided with a pair of more or less formidable 

 claws. Fleas use their legs for leaping, for running, 

 and for clinging to their hosts. They also use their 

 mouth-parts for the last purpose and it is worthy of 

 note, as we shall see later on, that in those fleas in 

 which the mouth-parts are shortest and weakest the 

 legs are most liberally supplied with bristles and 

 possess the stoutest claws. The legs of a flea are 

 unique in the insect world owing to the enormous 

 development of the segment nearest the body called 

 the coxa. Most leaping insects rely for their activity 

 on the muscles of the lower joints. In a grasshopper 



