352 ARACHNID A. SCORPIONIDA 



eyes at each of the two outer front margins. The middle 

 region of the body — the mesosoma or pre-abdomen (7 — 12) 

 — is formed of six free segments, which are short and 

 broad ; the chitinous sheath of each segment consists of 

 a dorsal plate or tergum and a ventral plate or sternum. 

 The posterior portion of the body is the metasoma or post- 

 abdomen (13, 14), and is formed of six segments, each being 

 encased in a complete chitinous cylinder, and all, except 

 the first (13), are narrow ; at the end of the last segment 

 is the tail-spine (15), which bears the poison glands. The 

 anal opening is on the last segment. 



The prosoma bears six pairs of appendages : — (1) the 

 chelicerce (fig. 150, 1) are small three-jointed limbs with 

 chelae, placed just in front of the mouth ; (2) the pedi- 

 palps (2) are the largest appendages and are at the sides 

 of the mouth; they consist of six joints, ending with 

 chelae, and the basal joints function in mastication; next 

 come four pairs of seven -jointed walking legs (3 — 6) which 

 end in claws, instead of chelae ; the basal joints of the 

 third and fourth pairs assist in mastication. Between the 

 bases of the last two pairs of legs, and immediately in 

 front of the genital operculum, is a small plate, the 

 sternum. 



On the seventh segment of the body (the first of the 

 mesosoma) there is a small rounded plate — the genital 

 operculum (fig. 150, 7). The eighth segment bears the 

 pectines (s), which are tactile organs and consist of a stem 

 with a row of short processes like the teeth of a comb. 

 On segments nine to twelve, there are, in the adult, no 

 proper appendages ; but a pair of oblique, slit-like open- 

 ings — the stigmata, occur on each of these segments, and 

 lead into pulmonary sacs which contain the lung-books. 

 The metasoma (segments 13 to 18) has no appendages. 



