The beetle's elytra (hard, outer wings) are open, with the fly's proboscis embedded in the soft body beneath.

If I’m not mistaken, this Robber Fly (or Assasin Fly) is sucking the life-blood out of one of those cockchafers in Janet’s previous post. Robber Flies are fast and agile predators and eat a wide variety of invertebrates (e.g. beetles obviously, flies, spiders, moths). Once caught, the prey is quickly killed using a hypodermic-type proboscis that injects neurotoxin-containing saliva into the victim. The saliva also contains enzymes that ‘digest the victims insides, allowing the Robber to feed on a liquid diet (rather like spiders do, too). That’s just what this fellow was doing to the hapless cockchafer when I found it. Whilst feeding, Robber Flies seem to remain motionless, focused on the job, allowing for easy photography. They’re clearly beneficial insects- though I’m glad I’m no beetle.

What amazing feet! Cockchafer for lunch!