Knock, knock, yes someone is home, snug in this goldenrod gall ball. A gall is an abnormal outgrowths of plant tissue caused by parasites, insects or mites.
A fruit fly causes this particular gall. Each gall contains a single plump larva, which overwinters inside the gall.
The quarter-inch-long larva slows its metabolism and utilizes glycerol as an antifreeze. If undisturbed by predators, the larva tunnels to just short of the surface, goes back to the chamber to form a pupa, and emerges in a few weeks as an adult fly able to pop through the thin-walled porthole, clearly visible in the photo.
After all this, the adult goldenrod gall flies do not eat and last only about 10 days, living only to mate and produce a new generation of gall-causing young.