ARTHUR DEWITT WHEDON 409 



be dwelt upon. Tramea is to be understood in the absence of 

 other mention. 



The AUmentary Canal 

 (Plate XXIV, figure 14) 



As Libellulid larvae are short and stout, this is also a marked 

 characteristic of the alimentary canal : its chambers tend towards 

 being pear-shaped and are spacious. 



The Oesophagus always has a sharp, dou})le or S-shaped bend 

 in the hind part of the head and the prothorax after which it 

 dilates gradually into the crop. This in turn constricts near the 

 center of the synthorax and is followed by a characteristically 

 oval gizzard possessing an armature of two pairs of plates, or 

 ridges, canying very large, sharp, recurved teeth, two large or 

 five or six smaller ones. The gizzard of, Sympetrum is not set 

 off from the crop by a constriction as in Tramea, Lihellula and 

 Platherms. 



The position of the gizzard has been found to vary greatly in 

 different specimens. Whether this was due to normal position 

 or to displacement in dissection could not be fully determined. 

 The usual position in Tramea seemed to be in the first and second 

 segments of the abdomen; however, younger larvae often had it 

 in the central portion of the synthorax. Specimens cut into 

 right and left halves with the scissors seemed also to vary; those 

 cut towards the head having the gizzard crowded into the thorax 

 and those cut toward the abdomen with it as far caudad as the 

 third or even the fourth segment. The presence of the trans- 

 verse muscles near the middle of the abdomen seemed to allow 

 of no shifting of the hind gut. In specimens with the dorsum 

 removed there was wide variation. These statements apply to 

 all the species studied. 



It is very probable that shifting occurs normally when the 

 chambers of the oesophagus, crop, and ventriculus are succes- 

 sively filled and emptied. The muscular diaphragm (see page 

 408) is also an exceedingly thin and elastic band which can 

 easily be pressed backward by the pressure of incoming food, 

 but which can just as readily push the gizzard forward by its 

 contraction and consequent flattening. In Tramea this dia- 

 phragm, it will be noted, is attached to the anterior end of the 



TIIANS. AM. ENT. SOC, XLIV. 



