This structure was photographed in mid December in Eastern Iowa, USA along a path within a public use area with tall grasses framed by small to medium trees. It is approximately 5 cm (2 inches) long and 10 cm (4 inches) in diameter. It had the light airy appearance of honeycomb but was one solid ribbed structure attached to a small branch. I'm not sure if this was something used over the summer by an insect or if something will come out of it in the spring - winter temperatures often go below -18 degrees C (0 degrees F) in this area of the country. Any help would be appreciated.
This is a mantis (Mantodea) egg sack, or ootheca. The female mantis makes it in the fall and it will remain over winter until (juvenile) nymph mantises emerge from it in the spring.
The following site introduces how to raise mantises from eggs but also has some general background information including on how seasonal temperature changes can affect the ootheca: