262 MANUAL OF ZOOLOGY. 



poison-gland which is situated at its base. The abdomen is 

 composed of twelve somites, but there is no evident line of de- 

 marcation between this region and the cephalothorax. The 

 thoracic segments carry four pairs of ambulatory feet. There 

 are six, eight, or twelve simple eyes. The maxillary palpi are 

 greatly developed, and constitute strong nipping-claws, or 

 "chelae" (figs. 98. 100). The mandibles (antennae) also form 

 claws, or " chelicerae." The respiratory organs are in the 

 form of pulmonary sacs, four on each side, opening upon the 

 under surface of the abdomen by as many stigmata, each of 

 which is surrounded by a raised margin, or " peritrema " (fig. 



98, 3). 



The Scorpions are mostly inhabitants of warm regions, and 

 their sting, though much exaggerated, is of a very severe 

 nature. They live under stones or in dark crevices, and run 

 swiftly, carrying the tail curved over the back. They feed on 

 Insects, which they hold in the chelate palpi and sting to 

 death. 



Family 2. Thelyphonidce. The members of this family in 

 external appearance closely resemble the true Spiders, from 

 which they are separated by the possession of a segmented 

 abdomen, and long spinose palpi, and by the absence of spin- 

 nerets. They are distinguished from the Scorpionida by the 

 amalgamation of the head and thorax into a single mass, which 

 is clearly separated from the abdomen by a slight constriction, 

 as well as by the fact that the maxillary palpi terminate in 

 movable claws instead of chelae. Further, the extremity of 

 the abdomen is not furnished with a terminal hook, or " sting." 



ORDER II. ARANEIDA or SPH^EROGASTRA. This order in- 

 cludes the true Spiders, which are characterised by the amal- 

 gamation of the cephalic and thoracic segments into a single 

 mass, and by the generally soft, unsegmented abdomen, attached 

 to the cephalothorax by a constricted portion, or peduncle. 

 Respiration is effected by pulmonary sacs usually in combina- 

 ation with tracheae. (Hence the name Pulmotrachearia, some- 

 times applied to the order.) The number of the pulmonary 

 sacs is smaller in the true Spiders than in the Scorpions, being 

 either two or four, opening by as many stigmata upon the under 

 surface of the abdomen. 



The head bears from six to eight simple eyes ; the man- 

 dibles are simply hooked, and are perforated by the duct of 

 a gland which secretes a poisonous fluid ; and the maxillary 

 palpi are never chelate. 



Spiders (fig. 101) are all predaceous animals, and many of 

 them possess the power of constructing webs for the capture 



