116 THE ALIMENTARY AND EXCRETORY SYSTEMS [CH. 



In the larvae of Zygoptera, the rectum carries, throughout 

 its entire length, three longitudinal pads of high columnar cells, 

 closely resembling those found in the vestibule of Aeschna (fig. 51). 

 One of these lies mid-dorsally, the other two latero-ventrally. 

 Anteriorly, they are separated by short portions of flattened 

 pigmented epithelium. Posteriorly, these pigmented portions 

 become larger, and project as folds into the rectal cavity, being 

 supported beneath by a mass of cells derived from the fat-body, 

 and carrying tracheae. Folds closely similar to these separate 

 the rectal pads in the vestibule of Aeschna. 



In the larva of Calopteryx, Dufour and Hagen both recognized 

 by dissection, long ago, the presence of three longitudinal folds, 

 which they considered to be gills. In sections of the rectum of 

 this larva, these folds are seen to be the three pads described 

 above, but somewhat specialized in form. The epithelium is 

 only about half as high as that shewn in fig. 51, though otherwise 

 closely similar to it. The pads are thrown out into the rectal 

 cavity in the form of three long double folds or loops, which over- 

 lap one another at their free ends. Calvert has recently dis- 

 covered very similar folds in the rectum of Thaumatoneura. 



It would appear that Zygopterid larvae do not rely wholly 

 upon their caudal gills for respiration, but use the rectum as an 

 auxiliary respiratory chamber. The rectal pads or folds are not, 

 however, true tracheal gills, and must be carefully distinguished 

 from the highly specialized structures found in the rectum of 

 Anisopterid larvae, since in no case are they known to possess 

 any specially developed tracheal supply. Their action must be 

 rather that of blood-gills, liberating carbonic acid gas from the 

 blood into the rectal water supply, by diffusion, and receiving air 

 in exchange. 



In the imagines of all Dragonflies which I have so far exam- 

 ined, the rectum contains six large longitudinal pads, one being 

 mid-dorsal, one mid-ventral, two latero-dorsal, and two latero- 

 ventral. These are homologous with the so-called ''rectal glands" 

 of other insects. Their epithelium closely resembles that seen 

 in fig. 51. Curiously enough, the three pads which are not de- 

 veloped in the larva (viz. the mid-ventral and the two latero- 

 dorsals) tend in the imago to be more strongly developed than 



