56 PROTECTION OF PLANTS, 1917-18 



conclude that the ant never chases away the enemies of the aphids; too many 

 authors hold the contrary opinion and I agree with them. Exaggeration, however, 

 must be avoided. 



It is difficult to determine the harm which the ant does the plant by protect- 

 ing the aphis. But it is certain that this harm more than counterbalances the ser- 

 vice rendered in the cases mentioned above. The service rendered is accidental, 

 whereas the injury results naturally from the circumstances. The ant feeling that 

 it is stronger than the enemy of the aphis instinctively gives chase to it and will do 

 so every time it meets it. In spring and in autumn the chase will be more vigorous 

 since the number of aphids is smaller. Now it is precisely during these seasons that 

 this insect might be of the greatest help; in the spring by devouring even in one 

 aphis many generations of aphids, and in the autumn by preventing the produc- 

 tion of eggs. 



2. The propagation of the aphids. — This is to my mind the obscurest point in 

 the history of ants and aphids. Many marvellous facts are recorded by the 

 authors, but I fully agree with Mr. Kellogg's opinion that "most of the stories of 

 ants and aphids printed in popular natural history books need to be tested by care- 

 ful observation." {American Insects, p. 175.) It will be sufficient here to recall one 

 of the best known facts without giving any detailed explanation. The aphis 

 found on the root of maize is hatched before the maize has struck root. The ants 

 take it and place it carefully on the roots of various plants, and there protect it 

 until the maize has germinated, and then they bring it back to its proper abode. 

 This fact, which is perfectly authentic, is a striking example of the harm the ant 

 may do. The same instinct is manifested in another way, which is perhaps more 

 important. Even if the aphids are hatched on their favourite plant they may 

 be transported elsewhere. The ants build their nest near a plant not yet infested, 

 and then go hunting for aphids. They spare no labour and sometimes fetch them 

 to the plant from a considerable distance. In the spring this transportation of the 

 wingless generations is fatal to the plants. Eventually of course the aphids might 

 undertake the journey of their own accord, but it would be longer and much more 

 dangerous, for the ant follows a beaten track and carefully protects its precious 

 burden. Now, the multiplication of the aphids depends to a large extent on the 

 quantity of sap at their disposal. In the spring the sap is very abundant, and if 

 the aphids are scattered over a larger number of plants, they have more and better 

 opportunities for propagation. Hence the harm done* is considerable. 



3. The isolation of the plants. — This is the last part of my thesis: "ants are all 

 the more injurious, the smaller the plants are." In developing my thought I shall 

 use only a few of the numerous facts I have observed. 



(a) Some Amaranthus tricolor and Amaranthiis caudatus were literally 

 covered with aphids, while a large number of ants ran about in all directions. In 



