ARACHNOIDEA RESPIRATORY ORGANS 



529 



and anteriorly in the abdomen. Two sharply distinguished forms of 



tracheae occur : tubular traehese and book-leaf tracheae. The former 



essentially agree with the tracheae already known to us in the Pro- 



tracheata and Antennata. The 



latter, which are also called 



lungs, lung tracheae, lung 



sacs, or leaf tracheae, have till 



now only been met with in the 



Arachnoidea. 



Tubular tracheae appear 

 in three modified forms, between 

 which, however, intermediate 

 stages occur. (1) The principal 

 trunk arising from the stigma 

 is branched like a tree in the 

 body, as in the Inseda and most 

 Myriapoda. Separate tracheal 

 trees are connected together by 

 anastomoses. A spiral thread 

 becomes differentiated in the 

 chitinous cuticle of the trachea?. 

 Such branched tree-like trachea? 

 are found in the Solpugidce, 

 Cypliophthalmidce (Fig. 372, Sj), 

 Phalangidce, a few Pseudoscor- 

 pionidce, and a few Acarina 

 (Gamasidce, Ixodes). 



(2) The principal trunk 

 arising from the stigma gener- 

 ally divides only once into 2 

 chief branches. On each of 

 these principal branches, at 

 irregular intervals, are attached 

 tufts of long finer unbranched 

 tracheal tubules. Only one 

 such tracheal tuft is sometimes 

 found lying at the end of the 



principal trunk. Such trachea? are found in many Araneidce, many 

 Pseudoscorpionidce, and in most of those Acarina which are as a rule 

 provided with tracheae. This second tracheal form, and especially the 

 modification of it last mentioned, leads over to the third form. 



(3) A common tracheal trunk, arising from the stigma, is wanting. 

 The separate tubules of the tracheal tuft branch directly from the 

 stigma. We are the more justified in tracing back this third form to 

 a shortening and later disappearance of the common tracheal trunk, 

 since the posterior tracheal tufts of a few Pseudoscorpionidse (Chernes 

 cimicoides) still rise from the end of a short tube. Such simple tufted 



VOL. I 2 M 



Fig. 372. Diagrammatic representation of the 

 tracheal system of Gibbocellum Sudeticum (after 

 Stacker). 1-6, 1st to 6th pair of limbs, only the first 

 (chelicerse) drawn fully ; au, eyes ; go, genital aper- 

 ture ; Sj, anterior pair of stigmata (for the tree-like 

 tracheae) ; so, posterior pair of stigmata for the tufted 

 tracheae ; an, anus. 



