26 AMERICAN SPIDERS 



equipment to maintain their place in the extremely diversified hab- 

 itats they occupy. 



The spinning organs of spiders are the spinnerets, fingerlike 

 appendages usually located near the end of the abdomen on the 

 lower surface. They are believed to have been derived from two- 

 branched abdominal appendages of ancient spiders, or their pre- 

 cursors, which were originally put to some other use than that of 

 spinning, perhaps being used as swimming or ambulatory organs. 

 Associated with each of these appendages was a coxal gland in the 

 abdomen that voided its excretory products through a pore on some 

 part of the appendage. From the two pairs of two-branched ap- 

 pendages of the third and fourth abdominal segments have come 

 the four pairs of spinnerets of contemporary spiders. Their devel- 

 opment, modification, and elaboration have gone hand-in-hand with 

 a metamorphosis of the lowly coxal glands into a series of abdominal 

 receptacles for production and storage of distinct types of liquid 

 silk. Originally an excretory product, silk has been put to varied 

 and distinct uses, and it has largely charted the course spiders have 

 followed through their racial history. 



The spinnerets were originally located much nearer the base of 

 the abdomen than their position in most modern spiders now indi- 

 cates, and there was a considerable open space between them and 

 the anal tubercle. The trend has been toward reduction of the num- 

 ber of abdominal segments, and the simplification of the systems 

 inside the abdomen, as well as the segmentation of the outer integu- 

 ment. As the posterior segments became superfluous and were lost 

 or incorporated into the anal tubercle, the relative position of the 

 spinnerets changed also. Ancestral spiders had a long interval of 

 segmented abdomen between the spinnerets and the anal tubercle. 

 In Liphistius this space has been greatly reduced by partial reduc- 

 tion of the size of the segments. In Atypus and Antrodiaetus the 

 space interval has been still further reduced, and in almost all other 

 spiders the spinnerets are immediately adjacent to the anal tubercle. 



Only in the most primitive spiders are eight spinnerets still 

 present as fingerlike projections. The liphistiid spiders have re- 

 tained all of the projections, but both the anterior and posterior 

 median spinnerets are greatly reduced in size, and perhaps figure 

 little or not at all in spinning. In Heptathela only six spinnerets are 

 present, and the so-called seventh one is the fused remnant of the 

 posterior median pair, a "colulus" in an advanced stage of obso- 

 lescence. In the other mygalomorph spiders, the anterior median 



