l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



The two ends of the mesenteron must be interpreted as represent- 

 ing the two ends of the blastopore, though no distinct blastopore is 

 formed in insect development. The anterior end of the mesenteron 

 is, therefore, the primitive mouth, and the posterior end the primitive 

 anus. With the ingrowth of the stomodaeum, however, the primary 

 mouth is carried inward, and the anterior opening of the stomodaeum 

 becomes secondarily the functional mouth of the adult. This mouth 

 of the adult becomes surrounded during development by the out- 

 growing mouthparts and finally becomes enclosed by them in a space 

 termed the preoral food cavity (often called the "mouth cavity" al- 

 though, being outside the head, it is not truly a body cavity any more 

 than is the space between the thoracic legs). 



The stomodaeum has a strong muscular sheath consisting of an 

 outer layer of circular fibers and an inner layer of longitudinal fi- 

 bers. In the head there are also numerous dilator muscles from the 

 head wall and from the tentorium. The stomodaeum is commonly 

 differentiated into several parts. That just within the mouth may be 

 called the buccal cavity, next is the pharynx, of different form in 

 different groups, and then the tubular oesophagus. The oesophagus 

 is usually enlarged posteriorly as a crop, though in some insects the 

 crop is a diverticulum of the oesophagus. Following the crop is a 

 short division, the proventricidns, which opens into the mesenteron 

 through a funnel-like infolding of the stomach wall, known as the 

 stomodacal valve. The proventriculus is commonly armed internally 

 with cuticular teeth or other structures that presumably give it the 

 function of a gizzard to grind the food, but in other cases it may 

 possibly serve as a strainer, or merely to regulate the passage of food 

 into the stomach. 



The mesenteron is usually a simple cylindrical sac, and it is the 

 functional stomach or ventriculus of the insect. In some insects, 

 however, it is divided into several parts (see Mesenteron). From 

 the anterior end of the ventriculus there usually projects a circle of 

 blind pouches, the gastric caccac, but tubular caecae may also be borne 

 on other parts of the stomach in some species. 



The proctodaeum is divided into two principal regions, an anterior 

 tubular part, which may be termed the anterior intestine, and an en- 

 larged posterior part commonly called the rectum. The end of the 

 anterior intestine adjoining the stomach is termed the pylorus (gate 

 keeper) ; it gives off the excretory Malpighian tubules. The part 

 following the pylorus is often differentiated into an anterior ileum 

 and a posterior colon. The rectum consists of a large anterior rectal 



