On Oct 7, 2007, WaterCan2 from Eastern Long Island, NY (Zone 7a) wrote:
I've been observing this behavior for about 3 weeks now on the undersides of the leaves of this plant, (Nasturtium). A few weeks ago we had a brief cold front come in which damaged some Nasturtiums I had, it revealed an amazing example of symbiosis. Under the fallen leaves were pastures of Aphis, tended to by ants. Darwin observed this behavior and published a paper on it in the early 1900's. He found that the ants would stroke the abdomen of the Aphid so it would give something similar to honey which the ants would consume. What is amazing is that he found that the behavior had been so inbred that the Aphis would not give milk unless they had their abdomens stimulated. Somehow wasps also 'got word' of this and exhibit the same behavior except that they stimulate the Aphid with their wings.