Last Saturday afternoon, while gardening in the backyard I noticed this small fly (~ 7 mm long) sitting motionless near the tip of a stalk of buds about a half a meter above the ground. I quickly got my camera and took some pictures of it. Its eyes, so big relative to the overall dimensions of its body, attracted my attention. It looked like it was keeping an eye on everything around it, including myself. What else would it do with such oversized eyes? Luckily, it ignored me and the camera and stayed where it was. After I finished with picture taking, I walked away to another corner of the garden.
About five minutes later when I returned, it was still there. But I noticed that there was now a small object attached to the front of its head where its mouth would be. So I got the camera back out and took another series of pictures of it. The fly stayed at its perch for a long time with the small object that appeared to be a tiny insect hanging in front of its mouth. The fly didn’t appear to be eating it, for its mouthparts weren't making any noticeable motions and the prey wasn't getting any smaller as far as I could tell.
Later, after I downloaded the pictures to my computer and enlarged them on the screen, I saw that the fly had its proboscis inserted into its victim's body and was obviously sucking up its insides.
This was a robber fly* (family Asilidae). A week ago Nuthatch had a post on these predators of other insects over at bootstrap analysis.
The small yellow organ visible just below its wing, near the junction of the thorax and the abdomen is, if I'm not mistaken, one of the halteres (there is another one symmetrically located on the other side of the body). The halteres are believed to help maintain equilibrium during flight.
*Note added 7 August: Giff Beaton kindly responded to my e-mail inquiry to identify the genus of the fly in the picture as Cerotainia.
More information on robber flies and pictures are available at these sites:
Asilidae Homepage by Fritz Geller-Grimm
3D Robber fly
Giff Beaton's Robber Flies of the Southeast