Swinging Fly

Grabbing pollen - Linda Bower

I was intrigued by a video that Linda Bower sent me.  It showed what appeared to be a wasp clinging onto milkweed, swinging like Tarzan with its front legs.  You can see yellow chunks of pollen on its other four legs and I assumed that it was grabbing the pollen grains.  Chris Barnhart informed me that it is actually the pollen grabbing the insect's legs, hitching a free ride to the next flower, an innocent purveyor of plant sex.

This "wasp" is actually a male Thick-headed Fly, Physocephala tibialis, of the family Conopidae.  Most of this family are convincing mimics of thread-waisted wasps (Sphecidae).  They feed on nectar and like bees gather pollen incidentally.

Thick-headed Fly, Physocephala tibialis  - Molly Jacobson

More from Linda:

"This is no ordinary fly! As a parasitoid, this species of Thick-Headed Fly (no common name) deposits its eggs into Bumblebees. It catches the Bumblebee mid-flight, sometimes falling to the ground, the fly inserts a single egg between its abdominal segments. The larva will slowly eat the Bumblebee alive, beginning with the non-essential tissues first. The larva forces the Bumblebee to dig itself into the ground so it can overwinter as a pupa (this is called adaptive manipulation). Adults feed on nectar, as shown in this video of a male filmed on Common Milkweed in the Missouri Ozarks, USA, June 27, 2017. Some clips are in slow motion, none have been sped up."

Stylogaster sp. of thick-headed fly with long ovipositor - MJ Hatfield

Inserting an egg under the exoskeleton of a flying bee is no mean trick.  It is accomplished by the specialized ovipositor which acts like a "church key" on a beer can (before pop-top cans for youthful readers).  The eggs of some species also have a barb, functioning like a harpoon that stays in the bee.

There are a lot of interesting details about how the fly cuts its way out of the pupa and how the bee is tricked into burying itself in the ground.  (Hint - it is adaptive manipulation).  I would explain that, but our friend the Bug Lady does it much more entertainingly in this UWM Bug of the Week blog.