14 



Arachnids, Centipedes, and Millipedes 

 — Arthropods (Cont.) 



Class — Arachnoidea 



This class includes such animals as spiders, scorpions, mites, and 

 ticks. There is probably no class in the entire animal kingdom that 

 includes such an array of animals that are more repulsive to so many 

 people than this one. A large part of this aversion is unjustified, for 

 many of these animals are our friends, but you would probably not 

 change a person's opinion by telling him that he, or more probably she, 

 should love spiders because they destroy so many harmful insects. 



The list of arachnids seems to include a motley group of animals with 

 little in common, but a survey shows that they have a number of like 

 characteristics which causes them to be placed in the same class. There 

 are two main body parts, the cephalothorax and abdomen, and four pairs 

 of walking legs which are on the cephalothorax. There are no com- 

 pound eyes and no chewing jaws. 



Spiders are the best known of the arachnids because many of them 

 spin conspicuous webs which give evidence of their presence. The 

 cephalothorax bears a number of simple eyes on the anterior dorsal sur- 

 face. These are not well-developed eyes and it is doubtful if the spiders 

 can see more than five inches. They have a pair of fangs at the anterior 

 end of the ventral surface of the cephalothorax which are connected in- 

 ternally with poison sacs. The fangs are like hypodermic needles ; and, 

 when a spider bites, a little poison is forced out of the poison sacs and 

 through the fangs into the wound. Just back of the fangs is a pair of 

 appendages, the pedipalps, which look like a miniature pair of legs. 

 These are mostly sensory in function, but also serve as a copulatory 

 organ in the male and help to squeeze the "juice" out of an insect in 

 feeding. The abdomen is unsegmented and bears three pairs of spin- 

 nerets on the ventral surface. These are sometimes mistaken for sting- 

 ers, but no spiders can sting and these organs are used only for spinning 

 the web. Spiders breathe by a pair of book lungs in the anterior por- 

 tion of the abdomen and also have trachea like the insects. Each lung 

 consists of a single sac with an external opening, through which air is 



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