ANATOMY AND PHYSIOLOGY. 169 
head of the colon, and appears as a nipple-like pro- 
jection in its interior, forming a valvular constriction 
at this point. As the valve at the bottom of the 
crop was set to prevent substances passing involun- 
tarily onwards into the stomach, so this is set, like 
the valve at the top of our own colon, to prevent 
substances regurgitatmg under pressure, from the 
large, back again into the small, intestine. It may be 
shown most plainly under the microscope by black 
back-ground illumination. 
The insect economy is so different from that of 
the more familiar subjects of physiological obser- 
vation, that it would be unsafe to draw any more 
than general inferences from the higher animals as 
to the nature of the changes in the food which are 
effected in insects by each separate section of the 
alimentary canal. The distinct functions, however, 
of the large and small intestine of the wasp are 
easy to be recognized in their structure. The glan- 
dular element predominates in one, and the muscular 
element in the other; pointing to an important 
difference in their operations. The colon has not 
nearly so abundant a supply of air tubes and glands, 
or glandular appendages, as the small intestine has. 
It is larger, and evidently mechanically stronger ; 
the annular folds of the mucous membrane of the 
small intestines being here replaced by longitudinal 
folds, in which the greater development of the mus- 
cular coat is apparent. But the functions of this 
part of the bowel are not exclusively mechanical. 
The fluid, which was necessary to float the refuse of 
the food and the excretory matter through the 
narrow intestinal or glandular passages, is here re- 
