| 1a | Body dorsoventrally flattened with 6 deep lateral constrictions; 6 regions with a ventral sucker |  |
| 1b | Body not as above | {continue line 2} |
| 2a | Head fully formed, headlike, and distinct from thorax |  {continue line 3} |
| 2b | Head inconspicuous, incompletely formed, often represented by a mere tip of anteriorly tapered body, and/or retracted into thorax |  |
| 3a | Prothorax with proleg(s) | {continue line 4} |
| 3b | Prothorax without prolegs | {continue line 6} |
| 4a | Distal third of abdomen swollen |
|
| 4b | Distal third of abdomen not swollen | {continue line 5} |
| 5a | Fleshy processes or bristles dorsally along body |
|
| 5b | Body without fleshy processes, some hairs at most | |
| 6a | Basal abdominal segments without prolegs | {continue line 7} |
| 6b | At least abdominal segment 1 with prolegs | {continue line 10} |
| 7a | Body segments with 2 or 3 secondary divisions |
Psychodidae |
| 7b | Body segments not subdivided | {continue line 8} |
| 8a | Terminal prolegs present | |
| 8b | Terminal prolegs absent | {continue line 9} |
| 9a | Body broad, broader than head |  Stratiomyiidae |
| 9b | Body very slender |  |
| 10a | 6 or more pairs of well-developed prolegs present | Nymphomyiidae |
| 10b | Weak abdominal prolegs on no more than first 3 abdominal segments |  Dixidae |
