Growing in the tree they even resemble fruit.
Up close, they look more like tumors.
UC Davis for this description:
The tiny gallwasp begins the gallmaking process in early spring by piercing a selected tree part with its egg laying device and depositing an egg inside the plant tissue. Fluids deposited with the egg cause the plant cell multiplication process to begin. The larva that hatches from the egg produces additional substances that maintain and control cell devision. The larva develops within a cavity inside the gall, feeding on material produced in the cavity lining. At maturity, the larva transforms into a pupa, and later becomes an adult that chews its way out of the gall. By causing the tree to form a gall, the gallwasp has provided food and shelter for its offspring.
It's amazing that a little bug I've never seen can provide so well for its offspring, all in one package, right in my own front yard and probably in your yard too. I feel a connection to these small creatures as I too try to provide for my young in the self-contained package we call home. The results aren't always pretty; somedays it's chocolate chips, but other days it's gnarly tumors.
There is so much more going on in the trees than I can fathom, and to think I once dismissed the galls as yard debris; but isn't there always so much more than meets the eye?