Due to their long legs and antennae, an ensign wasp on a wall may resemble a spider from a distance, and like spiders, they ought to be welcome guests in a home. These little wasps are unable to sting and harmless to humans, but they are deadly to roaches. Like many other small wasps, ensign wasps are parasitoids: the female ensign wasp lays her eggs only in the egg cases of cockroaches, where the larvae hatch and quickly devour the cockroach eggs.
Ensign wasps (also called hatchet wasps) are members of the family Evaniidae, and take their common name from the distinctive shape of their gaster (rear end). It is flattened laterally, and attached high like a flag. Much like a banner waver, they will twitch their gaster rapidly up and down when disturbed. The species I find around here is also notable for the attractive blue eyes that can be seen under a hand lens. They main body is perhaps 1cm long, with the legs and antennae nearly doubling the size. I found the wasp pictured above hanging around in the hallways of our building on campus, defending us from roaches.