46 INSECT PHYSIOLOGY 



Diptera and Siphonaptera, which have mouthparts adapted for 

 sucking up fluids, the saliva is discharged at the tip of the proboscis, 

 and one of its functions may be to keep the tube, up which the fluids 

 are sucked, moist and clean, (iv) And in those forms which suck the 

 blood of living animals, the saliva usually contains an anticoagulin - 

 probably antithrombin. It has been shown that if the salivary glands 

 are removed from the living tsetse-fly, although it continues to take 

 food for some time, ultimately its crop and proboscis become 

 blocked with coagulated blood, (v) Finally, in many insects the 

 labial glands are not concerned with feeding and digestion but 

 become modified for the production of silk or for other purposes. 



The plant-sucking bugs Oncopeltus and Dysdercus produce two 

 types of saliva coming from different lobes of the salivary glands: a 

 watery saliva containing the digestive enzymes, and a viscid material 

 which lines the path made by the stylets through the tissues of the 

 plant to form a 'stylet sheath'. This appears to be composed of self- 

 tanned material resembling the cuticulin of the epicuticle (p. 4). It 

 serves to filter off particles in the sap which might obstruct the suck- 

 ing canal and to prevent the sap from exuding to the exterior around 

 the stylets. 



It is interesting to note that many predaceous beetles (larvae of 

 Dytiscus, Carabus and Cicindela, and the adults of these forms) 

 which lack salivary glands, as well as the ectoparasitic larvae of 

 certain Hymenoptera, eject their intestinal secretion from the mouth, 

 allow much of the digestion of their prey to take place outside the 

 body, and then reabsorb the fluid products. This process is termed 

 extra-intestinal digestion. The flesh-eating maggots achieve the same 

 result by discharging proteolytic enzymes in their excrement. The 

 assassin bug {Platymeris) discharges a toxic and strongly lipolytic 

 and proteolytic saliva which quickly paralyses the prey and liquefies 

 its tissues. 



Secretion 



The secretion of digestive juices in insects is commonly stated to be 

 of two histological types : holocrine and merocrine. In the former 

 type, which occurs notably in Orthoptera and Coleoptera, the active 

 epithelial cells disintegrate completely in the process of secretion, and 



