Kenneth M. Smith 51 



A control experiment of pure potato sap alone had no ill effect upon the 

 apple leaf although left in a damp chamber with the drop of sap in situ 

 for several days, thus showing that some toxin had been added to the 

 potato sap and that some chemical change had taken place, this latter 

 as indicated by the change in colour after exposure to the air. 



The salivary glands were then taken out of the following species, 

 P. rugicollis, L. pabulinus and Psallus ambiguus, and placed each on the 

 surface of a slice of potato; in the first two cases a large quantity of 

 fluid was produced which gradually oozed out as before, and a large 

 patch of tissue was killed; in the third case there was no effect at all. 

 These drops of fluid were picked up with a capillary pipette and found 

 to be toxic to apple leaves though not apparently to fresh slices of 

 potato. 



This seems fairly conclusive evidence that a harmful substance is 

 injected by the bug and that it must be looked for in the salivary glands. 



Observations were made of P. ambiguus in the act of feeding. Before 

 the stylets are inserted into the tissue the proboscis is held upwards and 

 a large drop of saliva gathers at the end, the proboscis is then lowered 

 and the stylets forced into the tissue through the drop of saliva de- 

 posited on the leaf. After the insect has been feeding for some time the 

 surface of the leaf is dotted over with these drops of saliva, these even- 

 tually dry up and leave no mark or visible after effect. If these drops 

 are picked up with a capillary pipette and injected into the leaf no 

 damage results, which is in marked contrast with the effect of the drop 

 exuding from the puncture made by P. rugicollis which kills the cells 

 underlying it. There appears to be a slight difference in the method of 

 feeding of these two bugs, whereas P. ambiguus secretes the saliva and 

 pumps it out before piercing the leaf, P. rugicollis inserts the stylets 

 first and then pumps in the saliva, the latter method certainly seems 

 the most effective and it is difficult to account for the deposition of saliva 

 outside the leaf unless this particular species is in the habit of producing 

 greater quantities of the secretion than P. rugicollis, and pumps it into 

 the leaf as well. 



Sections were cut of tissue, fixed immediately after puncturing by 

 the bug, also of tissue punctured by P. ambiguus; these sections did not 

 prove very successful, but punctures were discovered which penetrated 

 some distance and showed laceration of one or two cells. 



It is difficult to say how harmless bugs such as Orthotylus marginatus 

 and P. ambiguus procure their food if their salivary injection does not 

 kill the cells as it seems impossible that the contents of the one or two 

 cells actually lacerated by the stylets should suffice for their needs; 



4—2 



