22O L. M. HICKERNELL. 



occupies the greater part of the abdominal cavity in the adult 

 insect and which I have called the posterior crop, is considered 

 as a part of the respiratory system. The chief reason for this is 

 the apparent continuity existing between this sac and the first 

 pair of abdominal spiracles. Mr. Snodgrass has been kind enough 

 to demonstrate dissections to me which seem to bear out his 

 contentions. There are, however, some fundamental objections 

 to his position. 



In the first place, a "tracheal bladder" or respiratory duct of 

 any kind in an insect should show a lining layer of chitin since 

 the tracheal system of insects arises as an invagination of the 

 primitive ectoderm. In a former paper sections of the "tracheal 

 bladder" of Snodgrass through three different regions were 

 shown and none of these showed any evidence of a chitinous layer. 

 It has been suggested that perhaps this structure attained its 

 respiratory function secondarily and hence might not conform 

 in all structural details to expectations. It is hard to imagine 

 how an entire segment or organ of the digestive tube could under- 

 go such a transformation of function. 



In my sections also, I have shown that there are distinct 

 openings at the anterior and posterior ends of this organ, the one 

 at the posterior end leading into a continuation of the digestive 

 tube and that a muscular valve intervened between these two 

 divisions. In view of this evidence it is difficult to conceive of 

 this part of the abdominal contents as having a respiratory 

 function. 



It is easy to be deceived as to the continuity of the lumen of 

 the posterior crop with the exterior through these first, abdominal 

 spiracles. In gross dissections there is only the most delicate 

 epithelial membrane limiting this abdominal sac in the region 

 of these spiracles. In my earlier paper a figure was shown cover- 

 ing this point. If a specimen is allowed to become dry, the por- 

 tion of the wall which is in front of the spiracular opening may 

 easily rupture and then there is an external opening in fact. 



Sections show that the spiracle opens into a very small chamber 

 the walls of which break up almost immediately into a number 

 of tracheal tubes which distribute themselves over the external 

 surface of the posterior crop. I am therefore, still inclined to 

 question any interpretation which gives this organ a respiratory 



