On a walk along the edge of our field, I noticed these round ball-like growths on the stems of some tall dried weeds. These swollen sections looked unnatural to the plant, which got me curious. I broke off the section containing the oddity and sliced it open with my knife. Inside the ball there was a small hollow cavity containing a soft white grub-like creature.
After a little research, I discovered that the plant is a goldenrod and the little guy inside the sphere is the larvae of the goldenrod gall fly. In the spring the female goldenrod gall fly lays her eggs on the stems of goldenrod plants. When the egg hatches the larvae begins feeding on the stem. The stem then swells in reaction to this injury, thus creating what is called a "gall". The larvae lives inside the gall, well protected and well fed by the gall itself. When it is done growing it will chew itself an escape tunnel out of the gall, stopping just short of breaking through to the outside. The larvae survives the winter protected by the globular scar tissue. The next spring the fully developed adult gall fly emerges through its pre-made tunnel, ready to repeat the cycle.
Besides the goldenrod, there are many other types of galls, gall plants and gall makers (perhaps 2,000 in North America alone). I'll definitely be on the look out for more of these fascinating creations.