Subject: What are the ants on my cornflowers doing?

My question is about ordinary black ants (Lasius niger) commonly found in 
gardens. I have noticed that they are very active on the cornflowers (Centaurea 
sp) which grow in my garden. They are particularly noticeable on the buds: 
usually 4 or 5 on each bud, seemingly 'grooming' the bud or scraping at the 
surface. They do not bite it in any way. They are also active on the open 
flowers; again, mostly on the sepals surrounding the inferior ovary. However, if 
a bee (or any other insect) lands on the flower to forage for nectar, the ants 
on the sepals will all rush up to the flower and harass the ant by biting it's 
legs until it flies away. They then return to the sepals below. They can be 
easily tricked into this behaviour by touching a grass stem into the open 
petals. I have never seen the ants forage inside the flower. They pay no 
attention to the nectaries or the anthers. WHAT ARE THEY DOING?? I wrote to the 
BBC Wildlife radio programme several years ago about this, and some ant 'expert' 
said they were feeding. 

Re: What are the ants on my cornflowers doing?