OF INSECTS. 129 



mo it which are indispensable to complete the act of 

 eisjestion ; these are the salivary, hepatic or biliary 

 vessels : along with which the urinary vessels will be 

 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 canal has no opening at its anterior 

 extremity. These are the bot-flies, constituting the genus 

 CEstrus, which are, of course, incapable of taking nourishment. 

 In a few instances the canal has its hinder extremity closed, 

 as is exemplified by the larvae of wasps and bees ; these take 

 nourishment, but reauire to void no unassimilating matter. 



