Robber flies

Robber fly eating a small white moth

I've had the fortune of seeing quite a few robber flies this year (I first discovered these guys two summers ago). Though seemingly nondescript, they're among the most gruesome of all predators. Relying on surprise, robber - or assassin - flies, lay in wait for an unsuspecting prey to come along and then ambush them, taking the insect out of midair.

Butterflies, moths, and many other nectar-loving insects have long, spindly and bendy proboscises. Not so the robber fly, whose short and hard proboscis is thrust into the body of their prey. It's hard to make out in the photo, but they have a bristly mustache on their face that is potentially a face mask that protects them as their prey flails about.

Then like a shrew, they inject the unfortunate insect with a neurotoxin that paralyzes it and, like a sea star or spider, follows up with an enzyme that liquefies the prey's innards. From there, they make an easy job of slurping up the slurry of guts.

7 bug challenge

What: Several years ago I was up in Morro Bay, CA and I was looking at a salt bush (Atriplex spp.) when I started noticing all these different insects. The longer I stayed the more I saw. I can't remember the number, but it was well over a couple dozen in about 30 minutes. Since then I've often given myself a 7 bug challenge. I choose a shrub or tree and then I can't leave until I spot seven different types of insects (when I get antsy I'll include slugs and snails). This was the first time I'd tried it with an herbaceous plant.

As I was walking in to Centennial Woods this afternoon the sun broke through the clouds and felt absolutely delightful. I stopped at the trail head to sun myself and noticed a big fly sitting on the goutweed doing the same! I decided not to leave until I'd spotted 7 types of flies. I got photos of 6 and stretched it to include a Pimpla sp. (a common type of Ichneumon wasp) because I missed photographing a crane fly.

I'm probably wrong on my identifications here, so please correct me if I'm wrong (I'm using Kaufman's Field Guide to Insects). Scroll over for the name of each.

Ecological notes: True flies are in the order Diptera, which includes the bloodsuckers like black flies, green flies, horse flies, mosquitoes, gnats, deer flies, as well as more benign flies, like crane flies, bottle flies, and s. Other "flies" (e.g. dragonflies, damselflies, mayflies, etc.) are not true flies. All true flies have the word "fly" written as a separate word. True flies also have a single pair of wings and a second pair of balancing rods (really small club like knobs) called halteres. They have cleft claws like the bumblebee I posted on Friday, but their feet are flat


Where: Centennial Woods. Goutweed is a common plant along flood plains and other places with frequent disturbance.