VI 



ARACHNOIDEA— RESPIRATORY ORGANS 



531 



/ir i^jf"^'^*"" ""' -P«^*>«^A and the tetrapneumonic Araneidce 

 {MijgaM(x) have only book-leaf tracheae. In the dipneumonic Araneidw 

 book-leaf and tubular trachea exist simultaneously. 



Number and Position of the Stigmata. 



The Seorpionidcc- possess 4 pairs of book-leaf ti-acheie and 4 pairs of stigmata, 

 lymg laterally on the ventral side of the 3d to the 8th abdominal segments 

 (Fig. 362, p. 517). ^ 



The Fedipalpi have 2 pairs of 

 book -leaf trachefe with 2 pairs of 

 stigmata on the ventral side of the 

 2d and 3d abdominal segments (Fig. 

 364, p. 519). 



Among the Araneidce the ar- 

 rangements are different in the Tetra- 

 pneumones and the Dijjneumones. 

 The Tetrapneumones {Mygalidm) 

 have 2 pairs of book -leaf trachea 

 and 2 pau-s of stigmata (Fig. 374, 

 ft, s) lying on the ventral side of 

 the base of the abdomen. The Di- 

 pTieumones have only one pair of 

 book -leaf trachese, corresponding 

 with the anterior pair of the Tetra- 

 pneumones. Besides these, however, 

 they have, as the equivalent of the 

 second pair of book-leaf tracheje 

 of the Tetrapnuumones — tubular 

 tracheae, which generally open 

 through an unpaired stigma in the 

 shape of a transverse fissure placed 

 far back on the abdomen. This 

 unpaired stigma no doubt arises 

 from a posteriorly placed pair of 

 stigmata, which have united. This 

 supposition is supported by the fact 

 that in some Armieidoe (Dysdcra, 

 Segestria, Argyroneta) two separate 

 more anteriorly placed stigmata for 

 the tubular tracheae occur (behind 

 the pair for the book-leaf tracheae), 

 and also by the circumstance that 

 in a few cases (Dictyna) the tracheal 

 trunks which open through the un- 

 paired stigma are distinctly recog- 

 nisable as double. 



The Solpugidce have tubular 

 tracheae with tree - like ramifica- 

 tions, opening through 3 pairs of 

 stigmata, the first pair lying in the first thoracic segment, the second and third pairs 

 in the second and third abdominal segments. The position of the first pair of stig- 

 mata in the thorax deserves to be specially noted. 



FiQ. 374.— Mygale, from the ventral side. The 

 ventral wall of the cephalo-thorax removed to show the 

 large cephalo-thoracio ganglion {hg) and the 2d small 

 ganglion at the base of the abdomen. The ventral wall 

 of the abdomen on the left side opened out. m, Ventral 

 muscles of the abdomen ; I, lamellse of the book-leaf 

 tracheae ; ft, book -leaf tracheae ; s, stigmata of the 

 same ; ov, ovary ; sp, spinning mammillae ; 1-6, 1st to 

 6th pairs of extremities. 2-6, not completely drawn 

 (Rkgne animal). 



