These poor adult ants are struggling to move a pupa that is considerably larger than themselves. They are moving a future "reproductive" ant (either a queen or drone) that will help start a new colony. They are called "acrobat" ants because they often raise their heart-shaped abdomen when disturbed. If these ants had not been so busy trying to move the pupa, they probably would have had their abdomens raised!
If you look VERY closely, you may be able to see the tiny stinger of the ant on the right. I had to be careful not to get stung as I moved in close with my macro lens to get this photo!