ANNULOSA: ARACHNIDA. 243 



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 Scorpionidce 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^rogastra. — This order in- 

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

 gamation of the cephahc 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- 

 tion 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. 80) are all predaceous animals, and many of 

 them possess the power of constructing webs for the capture 



Fig. 80.— Araneida. Theridion riparium (female). 



of their prey or for lining their abodes. For the production 

 of the web, Spiders are furnished with special glands, situated 

 at the extremity of the abdomen. The secretion of these 

 glands is a viscid fluid, which hardens rapidly on -exposure to 

 air, and which is cast into its proper, thread-like shape, by 

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