Soldier Fly larvae are about an inch long and are white-ish to dark brown in color. They are typically very active in the compost pile and thrive on moist nitrogen-rich materials such as vegetable and fruit scraps. If you have a large number of Soldier Fly larvae in your compost, listen closely and you can hear the chorus of their collective "munching".
To read more about the Soldier Fly and Solider Fly larvae, see this article by Carol Savonen, from the Oregon State University Extension web site.
blog entry by Mark Murphy, CMG
photo by Reggie Reeves, CMG