

THE EVOLUTION OF SPIDERS 105 



the simplification of various organs was operative. The mutations 

 began at different times and progressed at different rates, so that in 

 modern types generalized features sometimes exist side by side with 

 profound specializations. The tendency has been to simplify the 

 fundamental systems, to make fewer segments and functional units 

 (such as book lungs, tracheae, ostia, spinnerets) do the work of the 

 greater ancestral number. 



The abdomen was developing into a highly developed center for 

 silk spinning, and in most lines tended to become shorter; in some, 

 globose. The spinnerets gradually attained a position at the tip of 

 the abdomen, near the anal tubercle itself, indicating the virtual re- 

 duction of the abdominal segments to four. Some of the spinnerets 

 later became elongated and modified to perform special types of 

 weaving, and others became so reduced in size that in certain cases 

 only the anterior lateral pair remain as functional spinning organs. 

 A notable achievement was the transformation of the hind pair of 

 book lungs into a pair of tracheal tubes soon after separation from 

 the cribellate line; this development was followed in most spiders by 

 fusion of the openings into a single tracheal spiracle. In a few lines 

 the front pair of book lungs was also converted into tracheae. The 

 number of ostia in the heart was reduced from four to three pairs, 

 in some species even to two pairs, and the remaining ostia assumed 

 the function of the lost members. 



Changes of many kinds were also taking place in the cephalo- 

 thorax and its appendages. Especially notable was the migration of 

 the eyes from the original local center at the front edge of the 

 carapace, to the sides and to other positions of greater advantage. 

 The anterior median pair was lost early by a whole group of species 

 that persists until the present time as six-eyed spiders, and whose 

 other characteristics indicate that they are among the most gen- 

 eralized ecribellate true spiders. Other types enlarged their eyes, 

 and, with appropriate changes in the legs and body, came to place 

 considerable reliance on sight as an aid in gaining a livelihood. 



The early ecribellate spiders were at first terrestrial types that 

 stalked over the soil and low vegetation in an upright position, trail- 

 ing their dragline threads behind. Some of the lighter ones began 

 essaying trips into the herbs and shrubs, and learned to use their 

 claws to climb from twig to twig, hanging back-downward from 

 their silken lines. The third dimension was becoming a spacious 

 reality to these extroverts, and with its spaciousness came complete 

 freedom from attack by ground creatures. The egg sac was in- 



