INSECTA 



433 



insects, however, they are specialized in ways which are mentioned 

 later. Such glands are usually associated with the labium; in some 

 insects, however, mandibular and maxillary glands are found. 



The mid gut (Fig. 305) is lined by a layer of cells frequently all 

 similar, which perform almost the whole task of digestion and ab- 



Fig- 305. A, Longitudinal section of wall of oesophagus of a termite. 

 B, Longitudinal section of mid gut of termite in secretory phase. C, Trans- 

 verse section of mid gut of Blatta in resting phase. After Imms. bm. base- 

 ment membrane; c, chitinous intima; cm. circular muscle; cr. crypt; ep. 

 cellular layer ; e. enteric epithelium ; h. striated hem ; l.m. longitudinal muscles ; 

 nc. group of regenerative cells ; pm. peritoneal membrane. 



sorption of all classes of foodstuffs. While secreting, the cells break 

 down and their contents are discharged into the gut cavity. In the 

 absorptive phase the border of the cells has a striated appearance. The 

 same cell may be capable of both absorption and secretion, but the 

 epithelium as a whole often passes through rapid cycles which necessi- 

 tate the constant supply of fresh cells. These are found (Fig. 305) in 



