The External Anatomy of Spiders 



region of the body. It extends forward on each side of the 

 wedge-shaped hind end of the head, and covers to a greater or 

 less extent the sides of the thorax. It is often marked by a 

 median furrow and several radial furrows (Fig. 118); these are 

 lines along which muscles are attached to the inner surface of the 

 body-wall. The median furrow is sometimes called the dorsal 

 groove. 



The Sternum. — The plate forming the ventral wall of the 

 thorax is the sternum (Fig. 119, 5.); it occupies the entire space 

 between the two rows of legs; and usually each lateral margin 

 bears four notches for the reception of the coxae of the legs. 



The sigilla (sig'il-la). — In many tarantulas the sternum 

 is marked by circular or oval, impressed, bare spots, normally 

 four on each side; these are termed the sigilla. It should be noted 

 that sigilla is a plural noun; the term sigillae, sometimes used, 

 is, therefore, incorrect. 



The Epimera (ep-i-me'ra). — Correlated with the great 

 development of the tergum of the thorax there is a marked re- 

 duction of the lateral portions, each of which usually consists 

 of a narrow band between the legs and the tergum. In some 

 cases this band consists of a series of four sclerites, one above each 

 leg. These sclerites are termed the epimera; each epimeron 

 represents the pleural portion of a segment. 



The Legs. — There are always four pairs of legs. Each leg 

 consists of seven segments which are named, beginning with the 

 one next the body as follows: coxa, trochanter, femur, patella, 

 tibia, metatarsus and tarsus (Fig. 120). The tarsus usually bears 

 at its tip two or three claws. 



In certain spiders, which have very long and slender legs, 

 the tarsus is rendered flexible by a large number of secondary 

 joints or "false articulations." 



The prxtarsus (prae-tar'sus). — The claws are outgrowths 

 of a small terminal portion of the leg which is not ordinarily 

 distinguished from the tarsus. This terminal part with its 

 appendages has received the name prcetarsus (de Meijere 'oi). 

 As a rule the praetarsus in spiders is withdrawn into the pro- 

 jecting ventral part of the tarsus; but sometimes it is a distinct 

 segment (Fig. 121). This segment is termed by Simon ('92, p. 52) 

 the onychium, an unfortunate use of the term, as ordinarily it is 

 used as a synonym of empodium. (See p. 123.) 



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