434 Mr. Newport on the Anatomy and 



furca {w), the rudiments of an internal or endo-skeleton, to which the princi- 

 pal muscles of the segments and organs of locomotion, the legs, are attached, 

 and which partially enclose and protect the nervous cord and ganglia, like 

 the rings of vertebrae in the Vertehrata. Some traces of the entrances into 

 these f ureal bones exist in the sternal plates of Perla, but they are flattened 

 and quite unlike those of Pteronarcys. 



Spiracles. — But although the sternal orifices do not communicate with the 

 respiratory organs, the imago Pteronarcys most certainly is endowed with an 

 aerial as well as a branchial form of respiration. It has three pairs of large 

 thoracic spiracles of most complete structure, which are situated in the places 

 usually occupied by these organs in other insects, and which are covered in 

 the pupa of this insect by branchia;. The first pair (figs. 6, 7, 8) is in the 

 tegument which connects the pro- and mesothorax, the second in the junction 

 of the meso- and metathorax behind the first pair of wings, and the third is 

 in the anterior of the segment immediately behind the metathorax, at the 

 base of the second pair of wings. The segment which bears the latter pair 

 bears also on its under surface a pair of branchiae, like the true thoracic seg- 

 ments, and ought perhaps to be regarded only as part of the metathorax in- 

 stead of a distinct segment, the first abdominal. Besides the thoracic spi- 

 racles there are also a series of false abdominal ones, one pair at the sides of 

 each segment. These are situated at the precise spot occupied in the second 

 abdominal segment by the last pair of branchiae (fig. 5 h). They are enclosed 

 by a circular elevation in the tegument (fig. 9) and have an imperfect vertical 

 valvular opening, which leads into a small cavity that is closed internally by 

 a cribriform membrane by which the spiracle is separated from the cavity of 

 a large trachea that is connected with it. These spiracles therefore are me- 

 diate in structure between the branchial and aerial form of organ, and re- 

 semble those which I formerly described in the Transactions of this Society * 

 as common to a genus of Myriapoda, the Heterostoma. The thoracic spi- 

 racles of Pteronarcys, on the contrary, are most complete structures. The 

 three pairs are all similar in formation, the second, or mesothoracic, being 

 somewhat the largest. They are placed vertically in the flexible tegument 

 between the segments, and open and shut by a double valve. The pro- 



* Vol. six., Monograph of the Class Afynaporfa, p. 413. 



