192 THE RESPIRATORY SYSTEM [CH. IX 



The Triquetml Gill 



The best-known example of a triquetral gill is the lateral gill 

 of the Calopteryginae (fig. 40). In cross-section this is an isosceles 

 triangle with its longest side (base) placed internally, facing the 

 median gill. The thickened mid-rib of the gill lies at the vertex 

 of this triangle, externally. There are two main longitudinal 

 tracheae. The edges of the gill may be spiny. The shape is 

 evidently suitable for resting either on the ground or against any 

 moderately flat surface, while still exposing a large area to the 

 water. 



The Saccus, or Saccoid Gill (figs. 35, 84 c, 87). 



Saccoid gills appear to be confined to the Epallaginae, Thorinae 

 and Protoneurinae. In the first two cases, the saccus is a complete 

 or simple one; in the last, it is very distinctly constricted, at a 

 point somewhat distally placed from the middle. That the saccus 

 is a development from the triquetral gill appears clear from a study 

 of the genus Diphlebia. Here the young larva has triquetral gills. 

 These gradually swell up, as the larva grows, until they assume a 

 nearly rounded saccoid form (fig. 35). 



In Pseudophaea (fig. 87) the gills are so swollen up as to appear 

 quite rounded, except for a small tapering tip, which occurs also 

 in Diphlebia. In Cora [31] the gills are also swollen and rounded, 

 but the apex is broad and irregularly broken off. 



In the Protoneurinae (fig. 84 c) beautiful constricted sacci occur 

 in Neosticta and Nososticta. The basal portion is a short thick 

 sausage-shaped bladder; the distal portion arises from a very 

 constricted base, swells out considerably, and then tapers to a 

 point. 



All saccoid gills are very dark and opaque, so that it is not 

 easy to make out the course of the tracheae. There are from 

 two to four large longitudinal tracheae in each saccus. 



The Lamellar Gill (figs. 41, 42, 84). 



Lamellar gills are typical of the Lestidae and the Agrionidae. 

 The median gill of the Calopteryginae is also a lamella. 



