The western edge of Iowa has a unique geological formation called the Loess (rhymes with "us") hills. Loess is a type of wind blown soil, much finer in consistency than sand. In this area are some fairly well preserved tallgrass prairies.
Every year, there is a "Loess Hills Seminar", which is really fun. It sounds pretty boring and academic, but believe me, it is not. People of all ages and social classes attend to learn more about the hills and to socialize. I highly recommend it.
Anyway, one of the flowers that is found in that area is the large flowered beard tongue. This flower was growing in a ditch along the road to the Sy Runkle Preserve (where part of the seminar is held). One day when I stopped, I noticed that almost all of the blooms had this fly present. I don't know what they are, but they were very attracted to the flowers, and I took this picture. I have been back and seen the flies again, but never in the abundance they were on the first day I saw them.
The brown stuff in the background is the loess soil from a cut in the bank. Loess soil is very uniform and can support very steep cuts in the hills--that is part of what makes the soil so desirable for fill material, which endangers the hills themselves.