CLASSIFICATION OF INSECTS. 55 



are shown greatly enlarged in Fig. 24 ; they fre- 

 quently occur in aquaria in which aquatic plants are 

 growing. The sow-bugs (Fig. 25) are also Crustacea. 

 They live about water-soaked wood : with these only 

 one pair of antennas arc well developed. 



Class Arachnida (A-rach'ni-da). 

 The Arachnids {A-rach!nids). 



To this class belong the spiders, scorpions, har- 

 vestmen, mites, and certain other less common forms. 

 These animals differ from the other classes of the 

 branch Arthropoda in having no antennas. They 

 have four pairs of legs fitted for walking, and many 

 of them have very large maxiilary palpi, which re- 

 semble legs. The head and thorax are closely 

 united, forming a region which is called the ceph- 

 alothorax{ceph-a-lo-tho'rax). These characteristics are 

 easily seen in 

 spiders (Fig. 

 26). 



The most 

 common rep- 

 resentatives of 

 the class Arach- 

 nida are the 

 spiders. These 

 are extremely 

 interesting ani- fig. 26.— a spider, 



mals on ac- 

 count of the high development of their instinc- 

 tive powers. An account of the habits of some of 

 the common species is given in the chapter on Road- 

 side Life. 



