This black ant, with its lethal-looking jaws outstretched, isn't about to eat the aphid - it's farming it.
This is a mutually beneficial arrangement. The ants 'milk' their herd of aphids, collecting the sweet honeydew that the aphids excrete as they feed on plants sap, and in return the aphids are protected by their ferocious minders. The ants stroke the aphids with their antennae to encourage them to produce a continuous supply of honeydew.
Apparently the relationship even extends to ants picking up young aphids in their jaws and carrying them to uninfested parts of the plant, where the aphids rapidly breed and establish a new 'herd'.