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 



