Aphids

On the abdomen they have two processes, cornicles, which secrete wax and juice and are used for defence, while the honeydew that particularly attracts ants is produced from the anus.
Michael Chinery et al, Insects: An Illustrated Survey
Sap-drinkers, makers of honeydew;
some kept in herds by dairyman ants—
each stroke of a feeler elicits one drop:
in return, present protection, and symbiotic
survival through millions of years.
Ladybirds, lacewings, are predators,
and hoverflies, fringing whole colonies
with eggs, while wasps use their very bodies
as nests. Soon, marauding larvae
from within and without. Before all this
defenceless, but for small wax pellets
flicked into air … With primal products,
aphids sting their attackers,
sweeten their keepers.