How aphids lose their marbles
Aphids are a common species of insect that lives on and feeds off of plants. They have many natural predators in part because they have soft bodies and are not easily able to defend themselves. This is one of the reasons that many species of aphid have evolved to form plant galls and live within them for the duration of their lives. A plant gall is a growth that forms on a plant in the presence of certain parasites, such as aphids. Galls formed by aphids are hollow extensions of the plant that provide the aphids with a sheltered place to live while still providing them access to the nutrients of the plant off of which they live.
There are two major problems for insects living in such confined spaces. The first is that any liquid trapped in the space could easily drown them because of the dominance of surface forces in small-scale systems. The second is hygiene. Aphids have to eat a lot of plant sap to sustain themselves, and everything that they ingest must ultimately leave their bodies. This excrement is known as honeydew, and its presence in the gall must be carefully controlled to prevent both drowning and the development of pathogens that could harm the aphids.
The aphid has come up with a very clever way to deal with these problems. Aphids have special cells on the outsides of their bodies, which excrete a waxy powder that coats the inner surfaces of the plant gall. As soon as the honeydew leaves the aphid’s body it touches this powdery surface and is coated in a layer of wax that transforms the droplet into a liquid marble. These liquid marbles retain their sphere-like shape and do not wet any surface that they come in contact with.
Within groups of aphids there are specialized aphids, known as soldiers, which have certain extra defensive adaptations such as a hard exoskeleton or pincers. It has been found that in galling species, the soldiers are also responsible for removing the marbles of honeydew from the gall every day. The soldiers move the droplets by “kicking pushing, or walking on their surfaces