| 1a |
Head without tusks; some gills may be feathery but not all |
{continue line 2} |
| 1b | Head with large anteriorly projecting tusks, all gills on abdominal segments 2-7 fringed and feathery; Tusks curved upward at tips and without spines at the distal portion; Gills dorsal, curving up over abdomen, tibia of front leg heavy and thickened, modified for digging |
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| 2a | Middle dorsal thoracic plate is modified into a carapace-like structure that covers the gills on abdominal segments 1-6 |
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| 2b | Thoracic notum not modified into carapace; gills exposed; Gills on abdominal segment 2 similar to other gills or absent | {continue line 3} |
| 3a | Gills absent from segment 2 |
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| 3b |
Gills present on abdominal segments 1-7 or 2-7 |
{continue line 4} |
| 4a | Head flattened dorso-ventrally, eyes and antennae dorsal, bodies flattened with outspread legs, gills are single and plate-like but often with a feathery tuft |
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| 4b | Head and bodies not flattened, eyes and antennae lateral, gills variable | {continue line 5} |
| 5a | Front legs with 2 rows of long hairs on inner margin |
 Oligoneuridae |
| 5b | Front legs without dense tufts of hair | {continue line 6} |
| 6a | Abdominal segments 2-5 have gills which are forked or double and elongate or with fingerlike projections or in clusters or filaments |
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| 6b | Gills usually plate-like, may be double or single | {continue line 7} |
| 7a | Always with 3 well-developed tails; spines usually present on the sides of abdominal segments 8 & 9; antennae shorter than twice the width of the head |
 Siphlonuridae |
| 7b | Middle tail may be reduced; antennae long (>2x width of head) |
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