ARTHROPODA 



269 



to the air over a wide surface. There are one or two pairs 

 of these near the front end of the abdomen, and at the 

 hind end lie two or three pairs of spinnerets, from which 

 are secreted the silken threads that spiders have the power 

 of forming. Both lung-books and spinnerets are regarded 

 as representing abdominal appendages. 



Fig. 174. — The Follicle 

 Mite {Demodex follicu- 

 lorum), in ventral view. 

 — From Thomson. 



Fig. 175.— The Itch Mite (Sarcoptes 

 scabei), in dorsal view. The two 

 hinder pairs of legs are hidden 

 under the body.- — From Thomson. 



Aoarlna. 



Mites and ticks (Acarina) have neither lung-books nor 

 spinnerets. Many of them live as parasites 

 upon animals or plants, or on decaying organic 

 matter. The chelicerse are often transformed into piercing 

 organs to enable the animal to suck the juices of its host. 

 A system of air-tubes (trachea) for purposes of respiration 

 is often present. There is a larva (p. 284) with three pairs 

 of le°s, followed by a stage known as the " nymph," which has 



