Gall aphids: Nature’s waste-disposal system

Gall of Pemphigus spyrothecae aphids
Aphid gall. Image courtesy of Wikimedia Commons user AfroBrazilian, CC BY-SA 3.0

The aphid Pemphigus spyrothecae lives in galls which helps prevent attacks from predators. Soldier aphids remove waste from the galls by pushing or rolling balls of defecated honeydew out of the entrance to the gall.

Gall aphid with honeydew
Gall aphid projecting honeydew from anus. Image courtesy of Wikimedia Commons user Amada44, CC BY 4.0

The balls of honeydew are liquid marbles created by coating the honeydew in a powdery wax secreted by the aphids. The wax is hydrophobic and creates a microscopically rough surface on the inner wall of the gall. Weakly compacted needles of this wax render the surface superhydrophobic.

Honeydew on leaves
Honeydew on leaves. Image courtesy of Wikimedia Commons user Dmiti Don, CC BY-SA 3.0

See here for details of how we can create liquid marbles in the laboratory using hydrophobic sand.