There are nearly 50 North American species of blue-black spider wasps (genus Anoplius). Like other spider wasps, they have long legs, with stout spines at the outer tip of the tibia segment of the hind legs. Many species in this genus are entirely black, with a bluish sheen, while many others have an orange marking on the abdomen.
Females hunt wolf spiders, grass spiders, and similar spiders on the ground, flicking their wings nervously. Like other spider wasps, they drag their spider prey into an underground burrow, then lay an egg on it. The wasp larva hatches and eats the paralyzed but still-living spider.
These wasps are often seen visiting flowers.
Learn more about this and other spider wasps on their group page.
The blue-black spider wasps that have an orange patch on their abdomen apparently are the species that Mydas flies mimic. In this case of Batesian mimicry, the harmless fly looks intimidating to would-be predators because it looks almost identical to the spider wasps, which are capable of delivering a painful sting.
One species of blue-black spider wasp specializes in capturing fishing spiders. In the early 1900s, entomologists — including Missourian Phil Rau — noted an unusual sight: a wasp flying very low over a stream, dragging a spider across the surface film like a wind skier. It remained a mystery species among insect geeks until entomologist and nature writer Howard Ensign Evans identified it as Anoplius depressipes, one of the blue-black spider wasps. It turns out this species hunts fishing spiders (Dolomedes spp.) and possesses specialized flattened front feet that are fringed with hairs, which allow it to walk on water, just like its prey. When transporting a spider, this species grasps the spider with its middle or hind legs, faces forward, then extends its forelegs and uses them like water skis while it propels itself and its prey across the top of the water, beating its wings. They skim across the water surface like an Everglades airboat. This spider wasp sometimes dives down into water to chase its prey, since water spiders often swim underwater when frightened. Not surprisingly, it nests in burrows in stream banks.