100 PROCEEDINGS OF THE ACADEMY OF [March, 



junction of the Malpighian tubes with the gut where it breaks up 

 into small tracheoles ramifying among these tubes and about the 

 circular muscular coat of the gut. 



The distribution of the right visceral trachea (I'vs) and its branches 

 is similar to that of the left; except that, while it gives off tracheoles 

 to the right ventro-lateral fold (No. 4) of the intestinal epithelium, 

 it gives off none to the dorsal fold. 



The left lateral rectal trachea (llrfi) and the right lateral rectal 

 trachea (rlrti) give off tracheoles respectively, to the left ventro- 

 lateral rectal longitudinal fold (No. 8) and the right ventro-lateral 

 rectal longitudinal fold (No. 4), (Figs. 6 and 4). 



It will be seen that, in the main, the abdominal tracheal system 

 of this Mecistogaster larva is similar to that described by Calvert 

 for Thaumatonetira (1915, Plate XVI, and pp. 388, 389) and by Cullen 

 and by Jamieson for Argia (1918, pp. 77, 79, figs. 1, 2; pp. 82, 84, figs. 

 1,2). For convenience in comparison I have used the same nomen- 

 clature and abbreviations as employed by those authors. 



Rectal Respiration. 



A glance at Figure 6, shows that the tracheole supply of the 

 hind-gut is not rich, and is limited to those portions of the latter 

 which have been described as having longitudinal folds of thick 

 columnar epithelium. All of the tracheoles have been traced, as 

 far as possible, to their terminations. Owing to the fact that cell 

 boundaries in the intestinal epithelium are indistinct, it could not 

 be absolutely determined if any tracheoles terminate inside cells. 

 Usually the twigs appear to end just within the basement membrane 

 of the columnar epithelium of the rectal folds; so seem to end, for 

 instance, the twigs seen between sections 300 and 320. Sometimes, 

 like the terminations of llrti (sections 320 to 340) and the caudal 

 branch of rdrti (sections 260-280). the twigs seem to end outside 

 the basement membrane. Sometimes, as in the twigs running 

 cephalad from llrti and rlrti in sections 178 to 200, the terminations 

 may possibly be within the cells. Others may pass between the 

 cells. But one fact is clearly evident and seems to be significant: 

 all the tracheal twigs end on the basement membrane side of the 

 thick columnar epithelium. The tracheoles are at all points separated 

 from the lumen of the gut by the thickness (41 to 83 micra) of the 

 columnar epithelium. 



Furthermore, the columnar epithelium is the only part of the 

 internal rectal wall which has a tracheal supply. No twigs to the 



