THE PLACE OF SPIDERS IN NATURE 15 



Arachnida, and it is through their stimulation that these animals 

 are best able to know their environment. 



THE NEAR RELATIVES OF SPIDERS 



The spiders and spiderlike animals belong to the class Arachnida, 

 one of the major divisions of the Arthropoda. They differ at sight 

 from most other arthropods in completely lacking visible antennae, 

 the sensory appendages on the heads often appropriately called 

 "feelers." Although frequently confused with insects because of 

 similar size and general superficial appearance, the arachnids are 

 not close relatives of these creatures, which have only three pairs 

 of legs and have developed wings. All adult arachnids have four 

 pairs of legs, except in rare instances, and they never have wings. 



Important and interesting in their own right are the arachnid 

 relatives of spiders, such as the scorpions, harvestmen, and mites, 

 which in this book can be mentioned only briefly in passing. Some 

 were among the first animals to crawl out upon the land and adjust 

 themselves to a terrestrial existence. And, rinding the land a most 

 suitable zone for their development, almost none have returned to 

 the water to live even part of their lives, as have many insects. A 

 few of the mites have invaded both fresh and salt water, where they 

 largely live parasitically on the bodies of aquatic animals. 



Each of the major groups, or orders, of the Arachnida has de- 

 veloped a distinctive form, and the various types are quite familiar 

 to most people. The members of the following four orders have 

 the abdomen broadly joined to the cephalothorax by a thick waist: 



Order Scorpiones the Scorpions 



Order Pseudoscorpiones the Pseudoscorpions 



Order Opiliones the Harvestmen 



Order Acari the Mites 



The remaining orders of the Arachnida have the abdomen narrowed 

 and attenuated in front to join the cephalothorax by a narrow 

 waist: 



Order Solpugida the Solpugids 

 Order Ricinulei the Ricinuleids 

 Order Pedipalpi the Whip Scorpions 

 Order Palpigradi the Micro- Whip Scorpions 

 Order Araneae the Spiders 



