A finished pot ready to be filled with caterpillars before being sealed. You can see the small egg which is suspended on a silk thread just inside the lip of the pot. The pots are made from clay and take 2 to 3 hours to construct. Each female wasp constructs as many as 10 pots, one for each egg. The wasp grub eats the caterpillars inside before eventually breaking out of the pot as a fully formed wasp. Early broods emerge in the same summer but late broods over winter and emerge the following year. See a blog entry here
http://parrotletsuk.typepad.com/wldlife_in_a_suburban_gar/2013/08/potter-wasp-eumenes-coarctatus.html