ARACHNOIDEA 265 



maxillary palps; and the third pair of appendages are whip like. The 

 abdomen is 11-12 jointed. There are two pairs of book lungs. 



569. Order 3. The Araneida, or true spiders, have the abdomen con- 

 nected with the cephalothorax by a narrow waist. The abdomen is 

 usually unsegmented. The chelicerae are chelate and the maxillary palps 

 are similar to the ambulatory appendages. There are four book lungs 

 or, two book lungs and two tracheae or, four tracheae. On the ventral 

 surface of the abdomen is a group of four or six glands from which the 



FIG. 153. The great bird-killing spider, Mygale, of South America. X 5/8. 



web is spun. The web is a fluid secretion which sets on exposure to the 

 air. It is used to build and line the nest, for building traps by which 

 the prey is caught, for winding about the egg masses, for locomotion and 

 a variety of other purposes. The chelicerae are provided with a poison 

 gland. 



570. Order 6. The Opilionidea are the "harvestmen" or " daddy-long- 

 legs." The abdomen is broadly connected with the cephalo-thorax so 

 that the whole forms apparently one continuous body. The chelicerae 

 are chelate and the maxillary palpi are like the legs. The legs are often 

 very long. The respiratory organs are tracheae. 



