INTRODUCTION 5 



wail, if above, tlie epipliarynx; below, the liypopharynx. Into the mouth 

 cavity open a series of glands of varying function but all called salivary 

 glands. Among these are the silk glands found in a great many groups of 

 insects. All of these glands and their ducts are produced as invaginations of 

 the wall of the pharynx. 



2. CEsophagus, a slender tube thru which the food passes rapidly. In 

 caterpillars it is rather short but in many insects it reaches fully half the 

 length of the body. 



3. Crop or proventriculus, which receives and often holds the food for 

 a considerable time practically unchanged. In bees this organ is the 

 honey sac. In many insects there are large glandular pouches called coeca 

 opening into this cavity. 



4. Stomach valve, which provides for the intermittent passing of the 

 food from the crop to the stomach. Often this region is called the gizzard 

 when it is large and provided with a thick muscle layer. The interior 

 cuticle may at the same time be hardened into a very elaborate system of 

 teeth. 



Mesentron. 



5. Stomach, usually the largest portion of the digestive tract. The cells 

 of the stomach wall are large and gland-like; there are no glands opening 

 into this region, and no interior cuticular lining such as is found in all other 

 parts of the digestive tract. 



Proctodseum. 



6. Ileum or large intestine, often appears as part of the stomfach, the 

 line of separation however can always be made out by the attachment of 

 the malpighian tubes, which are long slender glands serving as excretory 

 organs. 



7. Colon or small intestine, furnished with muscles somewhat like those 

 of the oesophagus, passing the contents along rapidly; and in consequence 

 this region is usually found empty. 



8. Rectum, commonly marked out by the thick overlaying rectal glands 

 and is often highly muscular with the interior wall thickened in ridges 

 resembling • somewhat the gizzard and which serves as a filter pump to ex- 

 tract the liquid portion of the contents before passing the solid portion out. 



The most elaborate system of internal organs are the tracheae or air tubes. 

 They are called tracheae because of a general resemblance in function and 

 structure to the wind pipe in human anatomy. There are no organs in the 

 body of an insect comparable to the lungs, the tracheae taking the air di- 

 rectly to every part of the body. In aquatic insects the original openings 

 may finally close, and the exchange of oxygen and carbonic acid with the sur- 

 rounding water is made thru tracheal gills. 



Tracheae develop as invaginations of the body wall in quite a late stage 



