Kenneth M. Smith 



47 



The insects were fixed in Carnoy's fluid (second formula) mostly in 

 the early stages, or after a moult, before the chitin was hard. In spite 

 of these precautions great difficulty was experienced in getting good 

 penetration of the fixing fluid, and it was found necessary to detach the 

 legs or make a small hole in the abdomen for the fixative to gain an 

 entrance. Great care was necessary in embedding in the wax, as too 

 long or too short a time caused the chitin to become too hard and brittle 

 to cut. It was thought possible that the glands of P. rugicollis might 

 show some extra secretory cells or that the reservoir itself might prove 

 secretory, but no histological peculiarity was apparent, the salivary 

 apparatus being almost identical in the various species examined, the 

 reservoirs showing as thin collapsed vesicles. 



Fig. 4. Semi-diagrammatic drawing of the salivary apparatus of Plesiocoris rugicollis. 

 One side only. 



The fact of there being no apparent histological differences in the 

 salivary glands of the harmless and harmful bugs does not tell directly 

 against the theory of a toxic secretion, and all other experiments seem 

 to point to this being the true explanation. That the damage to the 

 cells spreads after the bug has moved to another spot, and that the 

 mesophyll dies before the epidermis (except of course where there is a 

 drop of sap and saliva lying on the surface of the leaf) are facts which 

 tell very strongly in favour of this theory. Fig. 4 is a semi-diagrammatic 

 representation of the salivary apparatus of P. rugicollis. 



Injections were made with various dilute poisons into apple and 

 willow leaf in order to reproduce if possible the damage caused by the 

 bug. 



