OF INSECTS. 
129 
into it which are indispensable to complete the act of 
digestion ; these are the salivary , hepatic or biliary 
vessels : along with which the urinary vessels will he 
noticed. 
Before proceeding to the separate consideration of 
each of these parts, it is of importance to remark 
that they never all co-exist in the same species. 
Sometimes one is absent, sometimes another, and 
they are often found to differ materially in the same 
individual, according as we examine it in the larva 
or perfect state. 
The pharynx cannot be very decidedly distinguished 
as a distinct feature, as it merely forms the distended 
aperture of the canal where it opens into the cavity 
of the mouth.* Indeed, it cannot be said to exist 
at all except among the mandibulated tribes, for, in 
a suctorial mouth, the esophagus is in strict contin- 
uity with the sucking tube ; it can only be defined 
therefore as the distended opening of the esophagus 
in masticating insects. The mouth and pharynx are 
usually upon the same plane, but in such insects as 
chew the food for a length of time, it lies a little 
higher, doubtless for the purpose of preventing the 
aliment finding its way into the esophagus before 
* Singular as the assertion may appear, some insects exist 
in which the alimentary canul has no opening at its anterior 
extremity. These are the bot-flies, constituting the genus 
(EstruB, which are, of course, incapable of taking nourishment. 
In a few instances the canal has its hinder extremity closed, 
as is exemplified by the larvse of wasps and bees ; these take 
nourishment, but require to void no unassimilating matter. 
I 
