36 ON PLANTS AND INSECTS. [LECT. 



species, for instance, of aphis and coccus have turned 

 the tables on the plants, and converted ants from 

 enemies into friends, by themselves developing nectaries, 

 and secreting honey, which the ants 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 vigil- 

 ance, 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-fertilization ; but for this pur- 

 pose 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 

 insects, in quitting a plant, would generally go up 

 another close by, without any regard to species. 

 Hence, even to small flowers (such as many Cruciferae, 

 Composite, Saxifrages, &c.), which, as far as size is 

 concerned, might well be fertilized by ants, the visits 

 of flying insects are much more advantageous. 



