CHAPTER 19 





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Phylum Arthropoda. 

 Spiders and 

 Their Allies 



HE class Arachnoidea received its name 

 from the Greek word arachne, which means 

 spider. The name is appropriate since spi- 

 ders are the most abundant members of the 

 class. However, it is a heterogeneous group 

 consisting of spiders, king crabs, scorpions, 

 harvestmen, mites, ticks, and several minor 

 groups. These animals differ markedly from 

 one another, but agree in several important 

 respects: (1) they have no antennae; (2) 

 there are no true mandibles; (3) the first 

 pair of appendages are nippers, termed 

 chelicerae; and (4) usually the body can be 

 divided into an anterior part, the cephalo- 

 thorax, and a posterior part, the abdomen. 

 Some have very interesting adaptations such 

 as the spinning apparatus of the spiders. 



The Arachnoidea, for convenience, are 

 here classified into one class and four sub- 

 classes. 



The spiders show interesting arthropod 

 modifications, especially for breathing, for 

 obtaining and digesting juices, and for spin- 

 ning webs. A general knowledge of the other 

 groups is worth while. Since king crabs rep- 

 resent an ancient group of animals, they are 

 sometimes called "living fossils"; scorpions 

 are poisonous to man and are often referred 

 to in literature; harvestmen, or daddy long- 

 legs, are frequently encountered in fields; 

 and many mites and ticks are either di- 

 rectly injurious to man and domesticated 

 animals, or they carry disease germs. 



Ordinarly, arachnoids are not noticeable; 

 and we would hardly know they existed were 

 it not for the conspicuous webs spun by 

 spiders. 



SPIDERS 



External anatomy 



The body of the spider (Fig. 162) con- 

 sists of a cephalothorax and an unsegmented 

 abdomen. 



There are 6 pairs of appendages attached 

 to the cephalothorax. Antennae are absent; 

 sensory functions are in part performed by 



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