INSECTS AND THEIR NEAR RELATIVES 



21 



21. Arachnida. Arachnida is the most important of the 

 Arthropod classes excepting the insects. The class includes 

 Spiders, Mites and Ticks, r 

 Harvestmen or " daddy-long- 

 legs/' Scorpions and many 

 other less common forms. 



Arachnids have four pairs 

 of legs, no antennae and only 

 two body regions, the head 

 and thorax being combined 

 as in the Crustacea. They 

 are typically land animals. 



The Spiders (order Ara- 

 neida) spin webs of silk which *- J 



has been used commercially FlG 9 _ A Tarantula (Arachnida, 

 to a very limited extent. Araneida). Greatly reduced. 

 They are predaceous and feed on insects. Except for 

 occasional bites which they inflict upon 

 man, they may be considered bene- 

 ficial. 



Mites and Ticks (Acarina) are of 

 considerable importance. Mites are 

 usually very small and feed on both 

 plants and animals. They cause gall- 

 like growths in plant tissues and may 

 do great damage. The pear-leaf blis- 

 ter-mite is possibly the most important 



example in the eastern United States. 



FIG. 10. Scorpion 

 Many mites are parasites on domes- (Class Arachnida, 



tic animals. The mites on poultry order Scorpionidd). 

 are the best known forms with this 

 habit. Others cause sheep-scab* and other diseases of 

 * See page 203 : Part II. 



