of the Salivary Glands 269 



burrow dragging with it a salivary thread from the middle of the 

 sheet which is thus pulled into a conical net. In its new position 

 the larva makes violent movements which draw a current of water 

 through the net, which traps the solid particles. The larva then 

 eats the net and the catch. This process may be repeated every few 

 minutes. 



The salivary glands of Drosophila melanogaster have been exten- 

 sively studied by geneticists because of the large size of the nuclei 

 in the cells and the clarity with which the giant chromosomes can 

 be seen. The glands are simple sacs lined with a single layer of 

 polygonal cells (Bodenstein, 1943). The salivary glands undergo 

 remarkable changes during the insect's development. In the first 

 instar the gland is only about 65 /i long and the nuclei 2-4 // in 

 diameter, but in the mature third instar the gland is over 1 mm 

 long and the nuclei 20-25 //. In the larvae and adult insects the 

 nuclei are smaller (Ross, 1939). 



The blood-sucking leeches (Annelidae) produce an anticoagulant 

 substance from simple salivary sacs that invaginate into the muscle 

 fibres surrounding the oval cavity (Haycroft, 1884; Franz, 1903). 

 For many years extracts of the medical leech (Hirudo officinalis) 

 called hirudin were used experimentally as an anticoagulant. With 

 the discovery of heparin, interest in hirudin has waned and much 

 remains to be elucidated on its chemical constitution and mode of 

 action. 



The salivary glands of the octopus are highly developed and 

 seem to serve a dual purpose as a digestive organ, providing anxi- 

 olytic and proteolytic enzymes and a copious amount of mucus, 

 as well as being a source of offensive toxic substances (Ghiretti, 

 1950; Buddenbrock, 1956). 



Extracts of the octopian posterior salivary gland are very toxic 

 and there seems little doubt that the secretion is used to immobilize 

 and kill crabs which are the main food of the octopodes. The 

 pharmacology of octopus saliva is very complicated; originally it 

 was thought that the main toxic constituents were histamine and 

 tyramine which are certainly present in high concentration, but in 

 addition there are large amounts of 5-hydroxytryptamine (first 

 isolated from octopus salivary glands by Erspamer), acetylcholine, 

 and murexin (urocanylcholine) as well as toxic proteins. The reason 

 for the presence of so many toxic substances in this secretion is not 

 clear. 



