170 NATURAL HISTORY OF WASPS. 
absorbed into the system. If the fluid enters faster 
than it can be strained off, the colon may become 
greatly distended. And to equalize the pressure, 
to allow of very considerable expansion without risk 
of laceration of the bowel, six horny bands are in- 
serted into its walls longitudinally. 
These horny slips mark the limits of the colon; 
below them the canal contracts at once into a strong, 
straight tube—the rectum. Its functions are merely 
to expel from the body what can be of no further 
use to it, namely the substances from which all 
nutritive matter has been extracted, and the excre- 
tory results of the daily wear and tear of insect life. 
Purely mechanical in its functions, its structure 
presents the simplest mechanical arrangements. In 
the higher animals a circular muscle, called a sphinc- 
ter, surrounds the opening of the bowel and secures 
a voluntary control over the evacuations. In the 
wasp the same object is attained by a different con- 
trivance. The open end of the bowel—referrmg now 
to the female wasps—is attached to the base of a 
triangular horny scale, called the vent-cover. This 
generally stands at right angles to the canal, stoppmg 
the passage as effectually as a clip on a bent India- 
rubber tube does. But, at the will of the insect, the 
vent cover can be drawn into a straight line with 
the canal. There is no resistance from an opposing 
sphincter to be overcome, nor is any co-ordination 
of movements required; but, by the most simple 
arrangement, the same movement which opens the 
passage expels the contents of the bowel. 
The whole length of the canal coiled away in the 
abdomen of a healthy female wasp may be reckoned 
