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, 

 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 

 has been used commercially 

 to a very limited extent. 

 They are predaceous and feed on insects, 

 occasional bites which they inflict upon 

 man, they may be considered bene- 

 ficial. 



Mites and Ticks (Acarind) 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. 

 Many mites are parasites on domes- 

 tic animals. The mites on poultry 

 are the best known forms with this 

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

 *See page203 ; Part II. 



FIG. 9. A Tarantula (Arachnida, 

 Araneida). Greatly reduced. 



Except for 



FIG. 10. Scorpion 

 (Class Arachnida, 

 order Scorpionida) . 



