These Crematogaster ants (C. cerasi) have a heart-shaped abdomen and so they're known
as Valentine Ants...also called Acrobat Ants. In these photos the ants are tending
a small "flock" of aphids from which they harvest droplets of "honeydew" liquid.
In the last few images it's possible to see one of the ants examining the small droplets
of honeydew exuding from the hindquarters of a couple of tiny aphids.
In the next (last) image an ant is gently carrrying a honeydew droplet in its mandibles.