THE TRUE OR WEB SPIDERS 



eyes; but in the structure of its appendages Kxnenia is peculiar. The mandibles 

 are large, pincer-like, and composed of three segments, but the palpi and all the 

 legs are alike, being long, slender, and composed of a number of segments. The 

 legs of the first pair, however, are the longest, as in the Pedipalpi. 



THE TRUE OR WEB SPIDERS Order 



In many points of their organization, the true spiders approach the tailless 

 Pedipalpi. They have, for instance, a deep waist, separating the cephalothorax and 

 the abdomen; the limbs are arranged radially round the cephalothorax, which is 

 covered below by a single sternal plate, to which a labial piece is united in front, and 

 above by a carapace bearing, in the majority of cases, eight eyes. Moreover, in 

 some instances, there are four pairs of lung sacs, as in the Pedipalpi, although gen- 

 erally the hinder pair are replaced by tracheal tubes. The differences between 

 the two orders are, however, striking enough. Thus the four pairs of legs are alike, 

 being composed of seven segments, and used for locomotion; while there are no great 

 seizing limbs, the appendages of the second pair being short and leg-like, though 

 composed of but six segments; of these the basal is termed the maxilla, on account 

 of its function as a jaw, and the remaining five the palpus. The mandibles, too, 

 are larger than in the Pedipalpi, and contain a poison gland, opening at the tip of 

 the second segment, which is transformed into a strong fang. In the abdomen, a 

 marked characteristic of most spiders is the absence of segmentation, its covering 

 consisting of a soft, hairy integument, or 

 a hard horny cuticle, while on its lower 

 surface there are two pairs of shortened ap- 

 pendages, called the spinning mammilla, 

 upon which open the silk glands. These 

 mammillae are perhaps the most distinctive 

 feature in spiders. Although varying con- 

 siderably in shape and length, they are 

 usually short and composed of two, or rarely 

 three, segments. Each spinning appendage, 

 however, is primarily composed of two 

 branches, an outer and an inner, the outer 

 forming the two- or three-jointed mammilla, 

 while the inner branch or intermediate 

 mammilla is always one jointed. Conse- 

 quently there may be as many as eight 

 mammillas; usually, however, there are but 

 six, owing to the disappearance of the in- 

 ner branches of the first pair of spinning ANATOMY OK COMMON CROSS SPIDER 

 appendages. (Araneus diadcma}. 



The Oldest known form (ArthrolyCOSO) J - Foot with claws and hairs; 2. Mandible with 



f 4-1, rv u -r i-rr poison gland and duct ; 3. Face and jaws ; 4. Spin- 



6 UarbOmferOUS, differs from nearly all ningmammilUe; 5. OneofthespinningpapilUe. 



