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They serve as a sheath for the other organs, which are more slender. 

 The inner pair of pieces are considered to be the mandibles and the 

 median piece a labrum or hypopharynx. Others call this piece the 

 unpaired piercing organ, the lingua, or the syringostome. There are 

 other interpretations of the homologies of the mouth parts, but the 

 above is the most generally adopted one. The labrum and the 

 mandibles are roughened and constitute the piercing organs which 

 the flea inserts into the host to tap a blood vessel. On the lower 

 part of the head there is frequently a series or comb of stout spines. 

 Similar spines sometimes occur on the posterior border of the prono- 

 tum. These series of spines are called "ctenidia/' and they are of 

 great value in classification. Behind the head are three segments, 

 or zoonites, each bearing a pair of legs. These together form the 

 thorax. The upper surface is called the notum (pronotum, mesono- 

 tum, etc.). The sides are the pleura sometimes "epimera" is used; 

 and the ventral part is the sternum. Each of the thoracic segments 

 has a spiracle, or a breathing pore, on each side. The first segment 

 of the thorax, called the "prothorax," is shorter than the others, 

 and, as above stated, frequently has a row of ctenidia, or spines, on 

 its posterior border. The next segment is the mesothorax, and the 

 third the metathorax. The metathorax usually has some stout 

 bristles in rows on its pleura, which are enlarged and called "epi- 

 physes," formerly called "squama aliforme." The basal one or two 

 segments are sometimes partly covered by the epiphyses or the 

 metathorax. These segments consist of a dorsal plate, or tergite, 

 and a ventral plate, or stermite. Behind the thorax is the abdomen 

 of 9 apparent segments. Seven of these segments have a spiracle or 

 breathing pore on the sides. The last segment, or pygidium, bears 

 the genital organs; in the male certain processes called "claspers" at 

 each side of the genital opening. The anal aperture is at the end of 

 the ninth segment between the dorsal and ventral plates. The 

 claspers have a main curved part, and a slender backward projec- 

 tion called the "manubrium," and at the apex an articulated claw- 

 like process called "the movable finger." At the tip of the abdo- 

 men of the female there is a short median piece called the "style." 

 The legs consist of five parts: The coxa, a large basal piece; the 

 trochanter, a minute piece at the end of the coxa; the femur, which 

 is usually slightly swollen in the middle; the tibia, which usually 

 has stout bristles or spines on its posterior side; and the tarsus, which 

 consists of five parts or joints. The basal joint is often the longest, 

 and the comparative lengths of these joints is expressed by a for- 

 mula, as 60-45-32-18-30. The last, or fifth, joint has been called the 

 "metatarsus," but this name is better applied to the basal joint. At 

 the tip of the last tarsal joint is a pair of stout claws. The coxae of 

 legs II and III show a longitudinal suture. 



