114 THE ALIMENTARY AND EXCRETORY SYSTEMS [CH. 



swollen up into an immense pad. In the pre-rectal ampulla, the same 

 pad is present, but is divided into two parts projecting latero-ventrally 

 into the cavity of the ampulla, one on either side (fig. 44, pa) and 

 separated ventrally by a wide zone of ordinary flattened cells. These 

 pads are easily seen from without in the intestine of the dissected 

 larva. They are formed of high columnar cells, remarkable in having 

 their nuclei all placed close under the cuticle, at the inner ends of 

 the cells (fig. 50). Strong tracheal branches penetrate into these 

 pads on either side. 



The function of the pads is not known for certain. In the 

 pre-rectal ampulla they are extremely mobile, and can be protruded 

 into the cavity, or retracted from it, by strong muscular action. 



In the imago, the small intestine (fig. 44 A, ^7) forms a single 

 complete whole, intermediate in form between the short intestine 



Fig. 50. Lateral sagittal section through epithelium of one of the pads of the 

 pre-rectal ampulla, from larva of Libellula ( x 50). After Sadones. 



and the pre-rectal ampulla of the larva. It is of moderate length, 

 and shews the six longitudinal muscle-bands very clearly developed. 

 The ventral pad appears to be divided into two portions, not so far 

 apart as in the pre-rectal ampulla. 



The small intestine ends posteriorly with a strong sphincter, 

 which is capable of closing it off entirely from the rectum. 



2. The Rectum. 



This portion of the hind-gut is of very small extent in the imago, 

 occupying less than the last three segments of the abdomen, and 

 is practically of the same structure throughout. In the larva it is 

 considerably longer, extending from the seventh segment backwards. 



It is in the larvae of Anisoptera that the rectum becomes highly 

 specialized, and it is in these also that our knowledge of this part of 

 the hind-gut is most complete. In this suborder the larvae breathe 



