General:

Gall Midges (Cecidomyiinae) are a curious group of small flies that lay eggs on plants. When the larvae hatch, they produce substances causing the plant to produce galls, one gall per larvae. Tucked safely inside a gall, the larvae feeds on the inside of the gall until it is ready to emerge.

There are many species of gall-inducing midges, and each produces a unique gall. As such, the gall can be used to identify the midge. The midge that produces this juniper gall is undescribed, but the genus is Walshomyia. I guess we need more midge-ologists.

More photos online at BugGuide.net.