CHAPTER II. 

 CLASS ARACHNIDA. 



In the strict teclmical sense the group Arachnida may be 

 exckided from the Insecta, but in general usage, and to a 

 large extent in entomological practice, these divisions are 

 put together, and it seems desirable that the group should 

 be given a place in any work dealing with the insects in 

 general. 



The group Arachnida includes spiders, scorpions, harvest- 

 men, mites, ticks, etc., and is characterized by the presence 

 of four pairs of legs, the absence of antennse and compound 

 eyes,^ and the lack of distinct metamorphosis, although 

 in certain groups there is a considerable change from the 

 newly hatched or six-legged form to the mature eight-legged 

 stage. 



In general structure the Arachnida agree with other 

 Arthropoda, but the head and thorax are usually merged 

 into a cephalothorax separated from the abdomen by a 

 more or less distinct stalk; in the Acarina, however, this 

 separation is not marked and the body is without distinct 

 separation of head, thorax, and abdomen. 



The economic importance of the group depends upon 

 their attacks upon certain crops, from the fact that many 

 of the species, such as spiders and harvestmen, are uniformly 

 predaceous and serve as important checks upon injurious 

 species; while other forms, such as the mites and ticks, are 

 parasitic upon domestic animals and man, and some of the 

 species occupy a most important relationship as carriers 

 of infectious diseases. 



The subdivisions of the group are, for the most part, very 

 well marked and represent ancient groups which have 

 diverged quite widely from each other. 



The scorpions (Scorpionida), mostly tropical in distribu- 

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