316 Prof. T. Thorell on Dr. Bertkau's 



Tristicta they often unite into a single short stem before 

 entering the common spiracle. The identity of the posterior 

 spiracles in the Dysderoidaj and the unpaired spiracle in the 

 Tristicta also explains why the unpaired spiracle is always 

 wanting in the Dysderoidoe (as well as in the Territelariaj). 

 The position of this spiracle when present is, as is known, very 

 variable ; in general it is drawn backwards to the vicinity of 

 the spinners, but sometimes it has its place much more 

 forward, nay, even in the vicinity of the rima genitalis, just 

 as is the case with the posterior spiracles in the Dysderoidse. 

 That the unpaired spiracle in the group Anyphseninaj, Si7n. 

 (which Bertkau, on the strength of its arborescent tracheae, 

 separates from the DrassoidjB, making of it a separate family) , 

 is situated sometimes very far from the spinners, sometimes 

 in their vicinity, shows clearly enough the little importance 

 of the position of this spiracle. 



Bertkau himself does not always consider the position 

 of the unpaired spiracle and its trachea3 to be of much 

 systematic importance ; he even refers to the same genus 

 {Argy7'oneta) two species, in one of which, the A. aquatica 

 (Clerck), the two stems of the trachea? have their opening 

 immediately behind the rima genitalis and penetrate through 

 the petiolum into the cephalothorax, there dividing into a 

 bundle of fine tubuli ; whereas in the other (fossil) species, A. 

 antiqua, v. Heyd., the spiracle is, according to Bertkau, 

 situated in the posterior third of the abdomen, while the 

 tracheae do not enter the cephalothorax, but divide into a 

 bundle of tubuli before reaching the petiolum*. 



Even the 'paired spiracles of the Tristicta, by which the air- 

 sacs debouch, and which are in most cases situated near the 

 base of the abdomen, may sometimes be thrust far backwards ; 

 in Tetrahlemma medioculatum, Cambr.f, for instance, they are 

 situated far behind the middle of the abdomen, and are, more- 

 over, placed very near to one another. 



That the different form of the tubular trachece does not 

 always offer a reliable characteristic for distinguishing closely 

 allied families, is seen by the fact that Bertkau has been 

 obliged, on the ground of such differences, to separate the 

 genera Thanatus and Tihellus [Metastenus, Bertk.) from the 

 other Thomisoidge, and to refer them to the Heteropodoidee 



* See Bertkau, " Einige Spiunen imd eine Mvriopode aus der Brfiun- 

 kolile von Rott/' in Verhandl. des naturhist. Vereina der preussischen 

 Rheiulande und Westfalens, Jahrg. xxxv. (4 Folge, v.), pp. 357 and 358 

 (1878). 



t " On some new Genera and Species of Araneidte," in Proceedings of 

 the Zoological Society of London, 1873, p. 114, pi. xii. fig. 1. 



