NO. 2 ENCYCLOPEDIA OF INSECT ANATOMY — SNODGRASS 21 



ones of the epithelium. In some cases the regenerative cells are 

 contained in crypts projecting on the outer surface of the stomach. 

 In some insects the epithelium is completely regenerated at each 

 moult, and the larval epithelium is always replaced by an adult 

 epithelium at the last moult of holometabolous insects, usually form- 

 ing for the imago an entirely new type of stomach adapted to the 

 special food of the adult. 



Usually there is one or more very thin sheets of secreted material 

 separating the food from the surface of the midgut cells. This is 

 the peritrophic membrane. In some cases (e.g., Diptera) it is clearly 

 produced by a ring of cells at the anterior end of the mesenteron; 

 in other cases (e.g., honey bee) it is delaminated from the surface 

 of the mesenteron. The peritrophic membrane is composed of chitin 

 and protein, and obviously must be sufficiently permeable to permit 

 the ready passage through it of digestive enzymes and of digested 

 products from the food. 



Stomodaeum and proctodaeum : The primary mouth (Gr. 

 stoma) of the arthropods represents the enclosed anterior end of the 

 blastopore, but it is carried inward by the tubular ingrowth of the 

 ectoderm known as the stomodaeum. Thereby the primary mouth 

 becomes the opening of the stomodaeum into the stomach, and the 

 functional mouth of the insect is the external opening of the 

 stomodaeum. 



Likewise, the primary anus (G. proktos) represents the open pos- 

 terior end of the blastopore, but it is carried inward by an ingrowth 

 of the ectoderm that forms the proctodaeum. Thereby the primary 

 anus becomes the opening from the mesenteron into the proctodaeum, 

 and the functional anus of the insect is the external opening of the 

 proctodaeum. 



The words stomodaeum and proctodaeum mean literally "on the 

 way to the mouth" and "on the way to the anus," respectively. And 

 this is just what they are! The -daeum part of these words is 

 taken from the Greek hodaios meaning "belonging to a way" (from 

 hodos, a way or path). By eliding the ho and latinizing the rest 

 of the word, daeum is obtained. 



Head: The insect head is a continuously sclerotized cranium- 

 like capsule. Its simpler or more generalized form and structure are 

 best seen in the head of an insect such as a grasshopper or its rela- 

 tives. In these the face is directed forward and the mouthparts hang 

 downward. The compound eyes then have a lateral position, and the 



