One of the chambers contained a dead spider, legs neatly tucked against its body. What insect had engineered such a intricate ball of life and death?
This wonderous sarcophagus was the work of a mud dauber, a type of thin-waisted wasp. I'm not positive, but judging from the nest's location and shape, I believe it was built by a Yellow and Black mud dauber (Sceliphron caementarium). The female carries mud in her mandibles to a chosen location (they like eaves, wood piles, sheds, etc.) and constructs hollow tubes. She then plasters them over with additional mud to form a nest. Each chamber is stuffed with a spider she has paralyzed. Next, the female lays an egg in each chamber and seals it shut. The spider will serve as a meal for the developing larva.
An interesting fact: mud daubers of different species (there are 5 in the U.S.) prefer different spiders.