480 



Insects. 



pairs of branchial sacs, from the exterior of which proceed numerous elongated, setose 

 filaments, which together form a thick tuft on each sac. These branchiae are situated, 

 as described by Pictet in the larva state of Nemoura cinerea, Pictet, over the proper 

 spiracular orifices or entrances to the great longitudinal trachea of the body, at the in- 

 ferior lateral parts of the thorax and basilar segments of the abdomen. The first pair 

 of sacs is in the tegument of the neck, between the head and prosternum ; the second 

 and third pairs, each of which is composed of two tufts, between the prosternum and 

 mesosternum, behind the coxae of the first pair of legs : the fourth and fifth between 

 the mesosternum and metasternum, behind the coxae of the second pair of legs : and 

 the sixth pair behind those of the third pair of legs, at the junction of the thorax with 

 the abdomen. The seventh and eighth pairs, formed each of single tufts, are attach- 

 ed more laterally, the seventh to the first, and the eighth to the second basilar seg- 

 ments of the abdomen. These latter branchiae correspond in situation in the segments 

 to that of some apparently closed or obsolete spiracles at the sides of the succeeding 

 segments. The situation of the branchiae themselves is thus as anomalous as their ex- 

 istence in the perfect insect. In most instances branchiae are arranged along the sides 

 of the abdominal segments of the larva, and are often employed to assist in locomo- 

 tion : but they cannot be of use for this purpose in the larvae and pupae of these Perli- 

 dae, which move by means of large and powerful limbs. In Pteronarcys the two pos- 

 terior pairs of legs of the pupa have the tibiae densely ciliated, for swimming, like those 



Pteronarcys regalis. 



of the Dyticidae, so that the delicate filamentose branchiae can afford little, if any, as- 

 sistance in this function. The structure of the filaments themselves differs also from 

 that of the filamentose branchiae of the Sialidae, in which these organs are said to be 

 quadri- or quinque-articulated, and are employed as organs of locomotion. In Ptero- 

 narcys they are simple unarticulated filaments. Each filament is soft, delicate, and 

 gradually tapered from its base to its extremity, and ends in a slightly obtuse point. 

 Internally each filament is traversed longitudinally by a tracheal vessel, which becomes, 

 like the filament itself, more and more slender, and at last divides into two branches, 



