DIGESTIVE GLANDS. 



627 



membrane " and it prevents aD contact of the food with the 

 walls of the alimentary canal. 



The glands which open into the intestine are (i) the salivary 

 glands which normally open into the stomodaeum and secrete a 

 fluid used in digestion. There are generally a pair on each side 

 of the oesophagus, but there may be as many as eight pairs in 

 the bee and often a reservoir as 

 well. Their united ducts open on 

 the hypopharynx. In the case 

 of certain blood- sucking insects, 

 e.g. mosquitoes, the fluid is con- 

 veyed down the hypopharynx 

 into the tissues of the animal 

 bitten and it is of a somewhat 

 poisonous nature. Unless some 

 of it be sucked up again with 

 the blood it can take no part in 

 digestion. The irritation is said 

 to be caused by the excretions 

 of certain yeast-cells which are 

 found in the reservoirs, (ii) The 

 silk-glands of certain larvae e.g. 

 the silkworm, also open on that 

 part of the Jabium called the 

 lingua or hypopharynx ; it is 

 tempting to regard them as sali- 

 vary glands ; they are said to 

 entirely disappear during pupa- 

 tion, (iii) The caecal glands of 

 the mid-gut are usually two, six 

 or eight in number or very numer- 

 ous and scattered over the an- 

 terior end of this region of the 

 gut. Their acid secretions act somewhat as does the pancreatic 

 fluid of the Vertebrata. (iv) The malpighian tubes are the 

 chief organs for nitrogenous excretion. 



Excretory organs. The malpighian tubules are caecal ex- 

 tensions of the hind-gut and they usually open into its anterior 

 end, but in the Psyllidae they are said to have shifted forward to 

 the mid-gut whilst in certain Hemiptera they open into the 



FIG. 384. Longitudinal section through 

 the body of Sphinx ligustri (after 

 Newport). MX maxillae forming the 

 proboscis ; t palp ; At antenna ; Gs 

 brain ; Gi suboesophageal ganglion ; 

 N thoracic and abdominal ganglia ; 

 V oesophagus ; V suctorial stomach ; 

 M mesenteron ; Vm malpighian tubes ; 

 H heart ; G testes ; E rectum ; A 

 anus. 



