The key for Diptera can be found in Chapter 13 of the Guide to Aquatic Invertebrates of the Upper Midwest, pages 161-183. Once you have keyed out your insect, you can use the photographs on this website to verify your identification. If you click on the link to each family on the right, you will be directed to a page that has photographs of that family along with close-up photos of their identifying character. The character featured matches the character used in the identification key.
Most Diptera larvae are maggot-like or worm-like. Some posess an obvious head capsule, but this structure is either reduced or obscured in many other Dipteran taxa. In all dipteran larvae, segmented legs and wing pads are absent from the thorax.
Because of the large diversity of aquatic Diptera and the lack of easily observable and consistent characters in their larvae, the identification of larvae can be difficult. Common diagnostic characters for aquatic Dipteran lavae include the number and location of prolegs, shape of the terminal processes, and presence or lack of a head capsule.