120 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



of extra labor : thus there are wax workers, masons, and 

 nurses. Ants (except the Saiiba) have but two classes 

 of workers. While ants live in hollow trees or subterra- 

 nean chambers (called formicarium\ honey bees and 

 wasps construct hexagonal cells. The comb of the bee 

 is hung vertically, that of the wasp is horizontal. 



CLASS 5. Arachnida 



The arachnids are closely related to the crustaceans, 

 having the body divided into a cephalothorax and abdo- 

 men. 22 To the former are attached eight legs of seven 

 joints each ; the latter has no locomotive appendages. 

 The head carries two, six, or eight eyes, smooth and ses- 

 sile (i.e., not faceted and stalked, as in the lobster), and 

 approaching the eye of the vertebrates in the complete- 

 ness and perfection of their apparatus. There are no 

 antennae, the first pair of appendages on the cephalo- 

 thorax being modified into grasping organs. They are 

 all air breathers, having spiracles which open either into 

 air sacs or tracheae. The young of the higher forms un- 

 dergo no metamorphosis after leaving the egg. 



Arachnids number nearly five thousand species. The 

 typical forms may be divided into three groups : 



i. Scorpionida, or scorpions, characterized by very 

 large maxillary palpi ending in forceps, and a prolonged, 

 jointed post-abdomen. The nervous and circulatory sys- 

 tems are more highly organized than those of spiders ; 

 but the long, tail-like post-abdomen and the abnormal 

 jaws place them in a lower rank. The abdomen consists 

 of twelve segments : the anterior half is as large as the 

 thorax, with no well-marked division between ; the other 

 part is comparatively slender, and ends in a hooked sting, 

 which is perforated by a tube leading to a poison sac. 

 The mandibles are transformed into small, nipping claws, 

 and the eyes generally number six. Respiration is car- 



