404 Annals Entomological Society of America [Vol. IX, 



There' seems no reason to doubt that the close association 

 of the Malpighian vessels with the walls of the colon is for 

 the purpose of excreting toxic products just as these authors 

 have already pointed out. In Haltica bimarginata it is probable 

 that most if not all of the digestion and absorption takes place 

 in the mid-intestine. It also seems very likely that on account 

 of the strong continuous layer of circular muscles the liquids 

 absorbed do not at once pass into the body cavity, but flow 

 backward between the basement membrane and the circular 

 muscles. It may well be true that the greater part of this 

 material does not pass outside the circular muscle layer until 

 well along in the colon. Here its passage into the body cavity 

 would be retarded by the peritoneal sheath which completely 

 surrounds this region, and the absorbed liquids would thus be 

 brought into close contact with the distal ends of the Malpighian 

 vessels, which lie closely pressed against the muscle wall of the 

 colon. Here doubtless such poisons and other harmful sub- 

 stances as the intestinal epithelium has allowed to pass through 

 it are taken up by the cells of the Malpighian vessels and 

 excreted before they have had any chance to pass into the body 

 cavity of the insect. 



The writer wishes to acknowledge his indebtedness to Dr. 

 W. A. Riley of this Department, under whose general direction 

 the work was carried on, for numerous suggestions and helpful 

 criticisms. 



