THE MUD DAUBER/DIRT DOBBER IS YOUR FRIEND~COLLECTING SPIDERS
The mud dauber/dirt dobber is your friend and collects spiders to feed it's larvae. Among those spiders are black widow and recluse. If you leave their little mud nest alone, they will rid your area of a lot of spiders. The post below this picture tells you about the dirt dobber. They're a little black wasp and I've never noticed them being aggressive.
"A Mud Dauber nest fell onto the porch last night, spilling the contents of several brood chambers. The large maggot looking larva is the baby Mud Dauber (wasp) and the spiders strewn all about are its food. The mother goes around collecting spiders, her sting paralyzingly but not killing them, and seals them up in a mud chamber along with a single egg. Once hatched, the larva begins feeding on the still living spiders! So cool."