In addition to the difficulty in identifying a specimen in two dimensions, the compression process as well as the process of mineral replacement can lead to a great deal of distortion of various body features. Heads, thoraces, and abdomens can often look like parts of insects that have been squashed on cards or abdomens will be bloated due to a long period of submersion in liquid before any amount of sedimentation took place to preserve the specimen. Wings and their venation generally are the parts of the insect least subjected to this distortion and, if in good condition, are the major identifiable features of Diptera compression fossils. When the original organic or replaced mineral material exfoliates from the fossil leaving only an imprint on the sedimentary matrix, this is called an impression. Trace fossils give evidence that a particular type of organism was occupying a specific medium at one time.
Most trace fossils are examples of the work of an organism.
These structures and tracks can easily be destroyed through wave or wind action, but under the right circumstances can be preserved and undergo fossilization. In the case of Diptera, trace fossils listed in this catalogue include leaf mines, galls, and trails. Those listed in this catalogue include subfossil and tar pit or brea recovery. In these fossils, all or parts of the insect specimen are visible in three dimensions much the same as those imbedded in amber or copal. Subfossil recoveries of Diptera have been made in peat bogs, caves, and ancient middens of small mammals.