EXTERNAL FEATURES 



317 



conceal tlie under surface of the caput. The shape of the 

 sternum and of the labium, and the contour and degree of 

 inclination towards one another of the maxillae, 

 are important considerations in the taxonomy 

 of Spiders. 



The appendages of the cephalothorax, which 

 are the chelicerae or jaws, the pedipalpi or feelers, 

 and the four pairs of ambulatory legs, will be 

 treated separately. 



Pedicle.- — The chitinous investment of the 

 narrow stalk which unites the thorax with the 

 abdomen is for the most part thin and flexible, 

 with only slight indurations of various patterns 

 on the dorsal surface, where it is in most cases 

 more or less protected by the forwardly-projecting 

 abdomen. Beneath, it is usually quite mem- 

 branous, guarded only by a sort of collar formed 

 by the raised border of the anterior portion of 

 the abdomen at the point of insertion. In 

 some Spiders, however (Dysderidae), there is a 

 posterior sternal plate, the " plagula," closely 

 corresponding with the labium in front, which 

 partly embraces the pedicle. In Hermipptis 

 (Zodariidae) the plagula is detached from the 

 sternum, and is succeeded posteriorly by two 

 smaller paired plates. 



Abdomen. — The abdomen differs remarkably 

 in shape in the different groups of Spiders. In some families 

 the prevailing shape is more or less globular, and in others 

 cylindrical, while it may be diversified to almost any extent by 

 prominences or spines. Ordinarily no sign of segmentation is 

 observable, but in Ziphistius it is covered dorsally by seven 

 well-marked chitinous plates. 



In most Spiders the integument of the abdomen is uniformly 

 soft and flexible all over, but it is not rare to find portions of it 

 thickened and hardened to form " scuta." In the Gasteracan- 

 thinae and the Phoroncidinae there is a great dorsal scutum 

 armed with spines, while in several families there are species 

 characterised by the possession of a smooth dorsal scutum ; and 

 in some a ventral scutum is present. 



Fig. 174. — Diagram- 

 matic ventral view 

 of a Spider. Ce- 

 phalothorax — /, 

 Labium ; »!, max- 

 illa ; p, paturou 

 of cheliceia ; 5^, 

 sternum ; i<, un- 

 guis of chelioera. 

 Abdomen — a.f, 

 Anal tubercle ; c, 

 colnlus ; ep, epi- 

 gyne ; s, stigma ; 

 sp, spinnerets ; 

 tr, tracheal open- 



