THE PLACE OF SPIDERS IN NATURE 25 



colors, the abdomen in many groups of spiders is accorded more 

 than its share of elegance and elaboration. In some spiders the 

 dorsum is covered in whole or part by a hard plate, and in others 

 it is armed with curious spines and processes, some of them of great 

 length. The reasons for the possession of such curious structures 

 are no more apparent than are the reasons for those on the cephalo- 

 thorax. Perhaps, because of its many sharp projections, this armor 

 discourages birds from attack. In some of our sedentary spiders the 

 abdomen is drawn out into a long tail, which gives the creature a 

 worm-like appearance. 



The under side of the abdomen is much like the upper in many 

 spiders, and rarely bears conspicuous prominences. Near the base 

 are usually to be seen the two openings to the book lungs, and 

 between them the genital opening. The copulatory organ of the 

 mature female, the epigynum, is located just in front of the genital 

 opening and takes one of many forms. Farther back may be present 

 a second pair of book lungs, a pair of tracheal spiracles, or, near the 

 spinnerets, a single median spiracle. In most spiders of the northern 

 hemisphere is found the single spiracle. At the tip of the abdomen 

 is the anal tubercle or postabdomen, which has the anal opening at 

 its tip. 



Both book lungs and tracheae are found in spiders. The open- 

 ing to the former is a rather conspicuous transverse spiracle, and 

 the area of the lung itself is usually evident externally as a paler 

 patch. In all the tarantulas and their allies, and in one small family 

 of true spiders, two pairs of book lungs are present, the front pair 

 near the base of the abdomen at each side of the genital pore, and 

 the hind pair much farther back near the center of the abdomen. 

 The possession of four lungs is usually considered to be a primitive 

 condition, since higher spiders have the posterior pair changed into 

 tracheal tubes. The tracheae always replace the book lungs when 

 the latter are lost, and probably are not new creations at all but only 

 modified and expanded book lungs without the leaves that ramify 

 beyond the original space limits. In most of the true spiders there 

 is a tracheal spiracle just in front of the spinnerets. In a few tiny 

 spiders all the book lungs have been replaced by tracheal tubes. 



Because in the higher spiders the book lungs have been replaced, 

 at least in part, by tracheae, it can perhaps be concluded that these 

 latter are more efficient respiratory organs. The true spiders are 

 more vigorous creatures of much smaller average size than the four- 

 lunged spiders, and require superior respiratory as well as other 



