DIGESTION 45 



is pressed to form a solid tube become more complicated, until in 

 many Diptera these annular moulds or presses present the most eleg- 

 ant forms. Sometimes the oesophageal valve bears a rigid cuticular 

 ring against which the cells of the mid-gut are forced (Fig. 8, A, C); 

 sometimes the valve is itself a solid structure made up of large vesicu- 

 lar cells (Fig. 8, B); sometimes, a rather cruder arrangement, the 

 valve is thin-walled but contains blood sinuses which can be dis- 

 tended with fluid and so blown out against the cardiac cells. 



The peritrophic membrane is present in the majority of insects. It 

 has disappeared in most Hemiptera, perhaps because these take only 

 liquid food, but it is present in some Corixids which feed on detritus 

 and other solid matter; it is said to be absent in the blood-sucking 

 adults of Tabanidae, fleas and sucking lice, and it is exceedingly 

 tenuous and delicate in the adults of Culicidae, Simuliidae, Phleboto- 

 tnus, &c. It is wanting also in the Carabidae and Dytiscidae, possibly 

 because their digestion is largely extra-intestinal (see p. 46), possibly 

 because their secretion is of the holocrine type (see p. 46) in which 

 the secreting cells break down completely during the process. It is 

 said to be absent also in adult ants - but here again, if it is very 

 delicate, it may have been overlooked. 



Salivary glands 



Most insects possess salivary glands. Sometimes, as in the Aptery- 

 gota and in the bee, several kinds of glands, probably with very 

 different functions, open in the region of the mouth; and even the 

 single pair of labial glands present in most insects have very varied 

 functions, (i) In the first place, their secretion may serve to moisten 

 and dissolve the food: when a cockroach eats, its mouthparts are 

 regularly moistened with saliva and in the feeding butterfly a drop of 

 saliva is extruded from the tip of the tongue, (ii) Their secretion often 

 contains digestive enzymes which may act outside the body and 

 which continue to act upon the food after it has been swallowed, as 

 in the case of amylase in the cockroach or invertase in butterflies and 

 bees. The saliva of plant-sucking bugs commonly contains invertase 

 and amylase, as in most Aphids; but in some Capsids and Jassids it 

 may contain also lipase and protease which cause much necrotic 

 injury to the plants on which they feed, (iii) In the Hemiptera, 



