PHYLUM ARTHROPODA 371 



legs, proboscis, and body; the anopheles mosquito transmits 

 the malaria germ; the stegomyia mosquito transmits the yellow 

 fever germ ; the rat flea carries plague germs ; the body-louse 

 transmits relapsing fever; and the tsetse- fly is responsible for 

 sleeping-sickness. 



Millions of dollars are lost every year because of the attacks 

 of insects upon domestic animals. Among these insects are the 

 blood-sucking gnats, buffalo-gnats, horse-flies, gadflies, bot-flies, 

 horn-flies, flesh-flies, ticks, fleas, sucking lice, and bird-lice. 



Even more enormous are the losses due to insects that eat the 

 leaves of plants, bore into their sterns, suck their juices, or de- 

 stroy their fruits. Table XIII presents a conservative estimate 

 of these losses. (Marlatt.) 



6. CLASS V. ARACHNIDA 



The class ARACHNIDA (Gr. arachne, a spider) includes the 

 spiders, ticks, mites, scorpions, and king-crabs. These animals 

 differ markedly from one another, but agree in several important 

 respects: (i) they have no antennae; (2) there are no true jaws; 

 (3) the first pair of appendages are nippers, termed chelicerae; 

 and (4) the body can usually be divided into an anterior part, 

 the cephalothorax, and a posterior part, the abdomen. Twelve 

 orders of arachnids are recognized in this book. The first four 

 orders ARANEIDA, SCORPIONIDEA, PHALANGIDEA, and ACARINA 

 contain most of the living species ; the last order, EURYPTERIDA, 

 is known only from fossils. 



a. The Spiders 



Order i. Araneida. -- SPIDERS. -- Since the spiders are the 

 most common of all arachnids, they are used here to illustrate 

 the anatomical and physiological characteristics of the class. 



External Features. -- Figure 313 shows the principal external 

 features of a spider. The body consists of a cephalothorax which 

 is undivided, and an abdomen which is usually soft, rounded, and 

 unsegmented. 



