Adult mud daubers are 3/4 to 1 inch long, varying in color by species from dull black to black with bright yellow markings to iridescent blue-black. They are best identified by the long, narrow "waist," or petiole - the very thin section between the thorax and abdomen.

Mud daubers are solitary wasps. The female wasp provisions each cell in the mud nest with a live, paralyzed spider, upon which a single egg is laid. She then seals the cell and abandons it. Eggs hatch, and the resulting larvae feed on their food stores, growing up to one inch long (you can see the mud nest pictured below is almost exactly 2.5cm, or 1 inch deep). Larvae then spin cocoons and pupate within the cell. There can be several generations each year.

Although mud daubers are capable of stinging (that's how they paralyze the spiders), they are not aggressive and do not defend their nests as do honeybees, paper wasps, or yellowjackets.

I found this (empty) mud dauber nest inside the cowling of my garden tractor - which had been parked inside a barn all summer.