A Hanging Thief Enjoys Dinner

Just walking around my yard and photographing the things that I notice is teaching me so much these days.  I often don't realize what I'm seeing until I get inside, download the photos, and take a closer look.  This post describes a case in point....

As I was looking over the asters in my front garden one evening in late July (and wishing this species bloomed before late September!), I noticed an odd insect "knot" down in a dark recess of leaves and stems.  As I tried to get a little closer to figure out what I was actually seeing, the "insect knot" flew a foot or two sideways, into an area with much better light, resolving itself into a robber fly with something in its grasp.  I dutifully took a couple photos, then moved on to find other interesting subjects around the yard.

On my way back into the house, about 30 minutes later, I snapped another photo or two, then went inside to upload my treasures and examine them more closely.

I was quite surprised to see the robber fly had actually captured a paper wasp and was holding onto it while hanging from an aster leaf with one foot.  The wasp seemed easily as large, and probably heavier, than the robber fly. This seemed a little odd, so I went back out to take another look.  Almost an entire hour had elapsed since I first saw the robber fly, but it was still in the same place, with the wasp still in its grasp and still hanging from one leg.  I took another series of photos, but I was losing my light, so I went back inside and started to research.

It turns out that this wasn't just an individual, acrobatic robber fly with a taste for precarious eating experiences.  This was a type of robber fly known as a hanging thief, so named precisely because the entire genus, Diogmites, eats their prey this way, dangling from one leg.  Furthermore, hanging thieves (I LOVE this name) regularly eat wasps and bees, often catching and eating insects as much as twice as large as themselves.  The only downside that I could find to their bizarre eating preferences was a tendency to snack on honeybees, especially if honeybees were the predominant Hymenopterans around.  Needless to say, in this sort of situation, beekeepers wouldn't be particularly fond of hanging thieves that were hanging around.

Like all robber flies, hanging thieves have piercing mouthparts.  Once they capture their prey, they inject saliva, filled with digestive enzymes, into the unfortunate insect, let the saliva liquify the insides of said insect, and suck their juicy meal back out.

This final photo is focused on the prey, the wasp, rather than on the predator, the robber fly.  

This unfortunate wasp was a paper nest wasp, Polistes metricus.  I have no idea where the nest is, but there should be plenty of siblings to help keep the nest well provisioned and thus to produce next year's queens.  In general, paper nest wasps apparently prey primarily on caterpillars, which they chew up and feed to their larvae back at the nest.  The adults eat nectar and are commonly found on flowers.  As far as I can tell, the front of my victim's face is rusty brown, signifying that it is a female.  The face of males is yellow and their antennae also curl at the tips;  males cannot sting, as the stinger is a modified ovipositor (egg-layer).

The balance of nature is a complex thing.  The caterpillars eat the plants.  The paper nest wasps eat both the plants (nectar) and the caterpillars.  The hanging thieves eat the paper nest wasps.  Hanging thieves lay their eggs in the ground.  It's not known for sure what their larvae eat, but they are presumed to be predaceous on other soil invertebrates.

Have you seen any hanging thieves around your garden?