Affinities q/" Pteronarcys regalis. 431 



of the mesothorax, two filaments sometimes arise from a common origin, in 

 which case they are supplied with their trachese from the same root (c), but 

 these are exceptions to the general structure. The manner in which the sacs 

 are supplied with tracheae directly from the great trunks of the body, and the 

 distribution of branches of these to tlie filaments, are important considerations, 

 as the demonstration of these facts fully proves that the sacs in the imago Pte- 

 ronarcys are true respiratory organs. With this object I removed the anterior 

 mesosternal sac from the left side oi Pteronarcys in connexion with a portion of 

 tegument and of trachea (fig. 3 a), and found on examination beneath the mi- 

 croscope that this sac is supplied by a large short branch from the great trunk 

 that passes across the mesosternal surface from the bundle of tracheae poste- 

 rior to the prothoracic spiracle, from which bundle other tracheae pass to the 

 anterior pair of wings, to the oesophagus, and to the dorsal muscles. A pre- 

 cisely similar mode of distribution exists also in the metathorax (fig. 10 i, k). 

 The division of the trachea within the sac differs a little from what has been 

 described and delineated by M. Pictet * in the larva of Perla. In that spe- 

 cies the tracheae are shown to divide abruptly into a multitude of minute 

 ramifications. In Pteronarcys the trachea (fig. 4 b) divides immediately it 

 enters the root of the branchia into two branches. Each division soon again 

 separates into two, and these again each into two others, and this binary mode 

 of distribution is repeated until the whole terminate in ramifications of simi- 

 lar diameter, one of which enters each branchial filament (fig. 4 c), and, gra- 

 dually becoming smaller from its base to its termination, gives off other small 

 ramifications as it passes onwards, and terminates by dividing into a pair of 

 exceedingly delicate closed tubes. M. Pictet remarks f that he has not been 

 able to satisfy himself in what way the ramifications terminate, and thinks 

 that the interior of these is continuous at their apex with the " muqueuse in- 

 terne du tube," the filament. I am not able to confirm this opinion. On the 

 contrary, I have reason to believe that the terminations, becoming gradually 

 more and more delicate, end as caeca. 



Circulation of the Blood. — The blood-corpuscles of the whole body circu- 

 late through the branchiae for the purpose of aeration. The current of blood 

 is always in the vicinity of tracheal vessels, " whether simply along inter- 



* Loc. cit. p. 87. pi. 3. figs. 3 & 4. t Hid. p. 88. 



VOL. XX. 3 L 



