THE EVOLUTION OF SPIDERS 103 



way it is related to their greater use of silk, to their more expert 

 spinning, and to the retention of the anterior median spinnerets as 

 functional organs until the principal lines of true spiders were well 

 established. Associated with this divergence from the tarantulas 

 was the gradual change in position of the chelicerae. In modern 

 representatives these are now twisted at right angles from the long 

 axis, with the fangs pointing toward each other. Just what advan- 

 tage this development brought is not completely clear, but it may 

 be that the improving eyes and the new chelicerae could be used 

 together to subdue insects more effectively. Cutting edges were 

 being developed on the coxae of the palpi to aid in crushing the 

 body of the prey. The venom was becoming more potent and the 

 voluminous glands were pressing beyond the limits of the cheliceral 

 segments into the head itself. 



Loss of the heavy abdominal plates was another consequence of 

 the change in life. This armor disappeared gradually and still is 

 vaguely indicated in a very few modern true spiders. P ale o diet yna, 

 a spider from Baltic amber, retains conspicuous plates; and this 

 slow divestiture suggests the possibility of finding many more fossil 

 true spiders retaining dorsal plates. 



The course of true-spider evolution has been charted largely by 

 silk spinning. The Araneoworphae began their history with the same 

 equipment as the parent tarantula group eight functional spinnerets 

 of nearly equal size. But whereas the tarantulas were content to 

 spin in a modest way, the true spiders began to use silk more often 

 and with greater efficiency. Since the lateral spinnerets undoubt- 

 edly were bisegmented at an early date, and had the advantage of 

 greater length and strategic position, it was natural that these should 

 be developed and improved at the expense of the unisegmented 

 median pairs. The great reduction in size and early loss of both 

 anterior median and lateral spinnerets in all but a few relict myg- 

 alomorph spiders reflect their failure as spinners. The true spiders, 

 on the other hand, retained all these spinnerets for a long period, 

 and some still keep the anterior median pair. In this connection it 

 is worth noting that the metatarsal comb the calamistrum used to 

 brush across the spinning field of the median spinnerets, was in all 

 likelihood an early invention, and that all true spiders once spun 

 cribellate threads. The retention of the anterior lateral spinnerets 

 as prime spinning organs, probably made possible by persistent use 

 of the incipient cribellum, was the key to true-spider superiority, 



