300 THE INVERTEBRATA 



ventriculus or gizzard^ most typically developed in insects which eat 

 hard food as in the Orthoptera. The chitinous lining of the fore gut 

 is here greatly thickened and the sphincter muscles in this region 

 control the passage of food between fore gut and mid gut. Into the 

 buccal cavity discharge the salivary glands (Fig. 290), which may as 

 in the cockroach have a very similar function to those of the mammal, 

 in producing enzymes for the digestion of carbohydrates. In other 



Fig, 290. General view of internal organs of Apis mellifica as seen from above ; 

 musculature and tracheal system not shown. From Carpenter, an. antenna; 

 bn. brain ; co. colon ; cr. crop ; e. eye ; ga. ganglion ; mg. mid gut ; mt. Malpighian 

 tubule; oe. oesophagus; rm. rectum; sa.gl. salivary glands (three types are 

 shown) ; pv. pro ventriculus ; il. ileum. 



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. 291) is lined by a layer of cells all similar, 

 which perform almost the whole task of digestion and absorption of 

 all classes of foodstuffs. While secreting, the cells break down and 

 their contents are discharged into the gut cavity. In the absorptive 



