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 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.[1] Indeed, it cannot be said to exist at all except among the mandibulated tribes, for, in a suctorial mouth, the esophagus is in strict continuity 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 Œstrus, which are, of course, incapable of taking nourishment. In a few instances the canal has its hinder extremity closed, as is exemplified by the larvæ of wasps and bees; these take nourishment, but require to void no unassimilating matter.
