122 



furthermore constitutes a condition essentially different from that 

 discussed in this paper. 



This interesting symbiosis or biological relationship may be 

 summarized as follows : 



1. The bone of contention seems to be the plant-lice [Aphis 

 Cratacgi) which are antagonistically associated with the hawthorn 

 {^Crataegus Oxyacanthd), feeding upon the cell sap of leaves, 

 growing tips and injured or thin portions of the young bark. 



2. A hyphal fungus infests the plant-lice, destroying many of 

 them and finally spreading over the exterior of leaf and stem of 

 the hawthorn. The fungus is therefore decidedly antagonistic 

 to the Aphis and rather indifferently antagonistic to the hawthorn. 



3. Two species of ant, antagonistic to each other and mutual- 

 istic to the hawthorn, feed upon the honeydew of the Aphis and 

 upon the Aphis itself and are therefore antagonistic to these 

 organisms. 



4. Several species of beetles, indifferently associated with each 

 other but mutualistically associated with the hawthorn, feed upon 

 the Aphis, forming therefore a decided antagonism to the Aphis. 



5. One species of ladybird {Diabvotica Soror) feeds upon the 

 fungus and diseased Aphis, thus forming a mutualistic (though 

 perhaps not pronounced) association with both Aphis and 

 hawthorn. 



6. The yellow-jacket feeds upon the Aphis thus forming an 

 antagonistic association with these as well as with the ants, but 

 mutualistic with hawthorn. 



7. A similar association exists between birds, Aphis, ants and 

 hawthorn. 



8. Flies are antagonistic to the interest of ants as well as Aphis 

 and mutualistic to the hawtiiorn itself. 



From this ma/.c of complicated biological relationship it 

 would appear that the plant-lice must be at a decided disadvan- 

 tage in the struggle for existence, since it is very evident that 

 they have numerous enemies and apparently no true friends. 

 Furthermore, as compared with these enemies they are physically 

 helpless, being mostly wingless, slow of motion and without 

 means of offense or defense. These deficiencies are however 



