PHYLUM ARTHROPOD A 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 stems, 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. Tw r elve 

 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, EURYPTEREDA, 

 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. 



