36 PROTECTION OF PLANTS. [CHAP. 



love. We have all seen the little brown garden ant, 

 for instance, assiduously running up the stems of 

 plants, to milk their curious little cattle. In this 

 manner, not only do the aphides and cocci secure 

 immunity from the attacks of the ants, but even turn 

 them from foes into friends. They are subject to the 

 attacks of a species of ichneumon, which lays its eggs 

 in them, and Delpino has seen ants watching over the 

 cocci with truly maternal vigilance, and driving off the 

 ichneumons whenever they attempted to approach. 



But though ants are in some respects very useful to 

 plants, they are not wanted in the flowers. The great 

 object is to secure cross-fertilisation ; but for this 

 purpose winged insects are almost necessary, because 

 they fly readily from one plant to another, and 

 generally, as already mentioned, confine themselves 

 for a certain time to the same species. Creeping 

 insects, on the other hand, naturally would pass from 

 each floret to the next ; and, as Mr. Darwin has shown 

 in his last work, it is of little use to bring pollen from 

 a different flower of the same stock ; it must be from 

 a different plant altogether. Moreover, creeping in- 

 sects, in quitting a plant, would generally go up anothe r 

 close by, without any regard to species. Hence, even 

 to small flowers (such as many cruciferae, compositae, 

 saxifrages, &c.), which, as far as size is concerned, 

 might well be fertilised by ants, the visits of flying 

 insects are much more advantageous. Moreover, if 

 larger flowers were visited by ants, not only would 

 these deprive the flowers of their honey, without ful- 

 filling any useful function in return, but they would 

 probably prevent the really useful visits of bees. If 



