EXCEETION OP GREEN BUG 27 



sheath — the so-called salivary .sheath. The course which 

 the proboscis takes in penetrating the leaf tissue can be 

 readily traced by these salivary sheaths which, of course, 

 remain after the withdrawal of the proboscis and appear 

 as highly refractile colourless threads. The proboscis fol- 

 lows the most irregular course in penetration, the salivary 

 sheaths branching in all directions. However, the end of 

 the proboscis can always be found on the outer margin of 

 the phloem and at the ends of the medullary rays, and it 

 is these two parts of the tissue which appear to be parti- 

 cularly attacked by the insect. The result is that these 

 tissues as well as the surrounding cells even of the cam- 

 bium and wood become completely brown and dead. When 

 this has taken place the insect withdraws its snout some- 

 what and seeks out a fresh point of attack on a new 

 medullary ray. 



Keuchenius considers that the death of the attacked 

 cells is a direct result of the sucking action of the bug and 

 is not due to any poisonous secretion w^hich it might in- 

 ject into the leaf tissue during the process of feeding. 



Excretion. 



The sole excretion seems to be the honey dew 

 The manner of its ejection has already been described. 

 Ajits attend the bug for the sake of this sugary excretion. 

 They tap the bug gently w4th their antennae in the 

 neighbourhood of the anal plates, until it yields a drop of 

 the fluid in response to these solicitations. The ant feeds 

 on the drop before it has a chance to burst and then 

 proceeds to "milk" another bug in the same way. 



In spite of the large quantity of honey dew thus used 

 up by ants, there is enough left over to cover the upper 

 surface of the leaves with a fine layer of shining trans- 

 parent syrup. This is an excellent medium for the growth 

 of the fungi called sooty moulds, which grow in it and 

 thus blacken the leaf. The fungi do not directly harm 

 the leaf as they derive their nourishment solel}^ from the 

 honey dew. But an opaque film covering the leaf and 

 interrupting the sun's rays may injuriously affect the 

 plant. Not uncommonly in such cases the leaves are 

 seen to drop off before their time. 



