| 1. | a. Thorax possesses three pairs of segmented legs(legs are sometimes minute or fused to the body | go to 2 |
| b. Thorax lacks three pairs of segmented legs | go to 11 | |
| 2. | a. Wings or wing pads are present | go to 3 |
| b. External wings or wing pads are completely absent | go to 14 | |
| 3. | a. Thoracic legs are fused to the body | go to 4 |
| b. Thoracic legs are not fused to the body | go to 5 | |
| 4. | a. Only one pair of developing wings is present | DIPTERA |
| b. Two pairs of developing wings are present | LEPIDOPTERA | |
| 5. | a. Abdomen ends in three tails | EPHEMEROPTERA |
| b. Abdomen ends in one, two or no tails | go to 6 | |
| 6. | a. Mouth is in the form of an elongate beak or cone-like structure | HEMIPTERA |
| b. Mouth is not in the form of an elongate beak or cone-like structure | go to 7 | |
| 7. | a. Labium (lower lip) is modified into a large mask-like structure | ODONATA |
| b. Labium (lower lip) is not modified into a large mask-like structure | go to 8 | |
| 8. | a. Forewings are modified into platelike coverings | COLEOPTERA |
| b. Forewings are not modified into platelike coverings | go to 9 | |
| 9. | a. Abdomen ends in a pair of segmented tails | go to 10 |
| b. Abdomen does not end in a pair of segmented tails (but sometimes in a pair ofunsegmented anal processes) | go to 13 | |
| 10. | a. Most abdominal segments possess lateral gills | EPHEMEROPTERA |
| b. Abdominal gills are absent or, if present, are filamentous and restricted to the first few abdominal segments | PLECOPTERA | |
| 11. | a. Head is distinct and well developed | go to 12 |
| b. Head is indistinct, entirely or partly lacking | DIPTERA | |
| 12. | a. Prolegs are often present on either thorax or end of abdomen or both; if prolegs are absent, then conspicuous hairs and/or distinct processes are present at the end of the abdomen | DIPTERA |
| b. Prolegs are not present | COLEOPTERA | |
| 13. | a. Wing pads or shortened wings are present only on the second thoracic segment | DIPTERA |
| b.Wing pads or shortened wings are present on the second and third thoracic segments | TRICHOPTERA | |
| 14. | a.Mouth is in the form of a beak projecting below the head; eyes are compound | HEMIPTERA |
| b. Mouth is not in the form of a beak, if mouthparts are long and slender, then they project forward; eyes are absent or consist of single or grouped spots | go to 15 | |
| 15. | a. Abdomen usually possesses a forked jumping organ that originates on its underside | COLLEMBOLA |
| b. Abdomen lacks jumping organ | go to 16 | |
| 16. | a. Mouth includes long slender rods that project forward; when held together, they form a sucking tube | NEUROPTERA |
| b. Mouth not made up of long slender rods | go to 17 | |
| 17. | a. Abdomen possesses pairs of short, fleshy leg-like structures on the underside these structures end in a series of tiny hooks | LEPIDOPTERA |
| b. Abdomen lacks pairs of fleshy legs on the underside that end in a series of tiny hooks | go to 18 | |
| 18. | a. Abdomen ends in a pair of short to long prolegs (sometimes fused together) that end in a single hook | TRICHOPTERA |
| b. Abdomen ends variously, but never in a pair of prolegs each having a single hook (if terminal hooks are present, then there is a total of four hooks) | go to 19 | |
| 19. | a. Abdomen possesses well developed lateral filaments | go to 20 |
| b. Abdomen lacks well developed lateral filaments | COLEOPTERA | |
| 20. | a. Abdomen ends either in a single, unforked, elongate filament or in a pair of prolegs, each possessing a pair of hooks | MEGALOPTERA |
| b. Abdomen ends variously but never with a single, unforked, elongate filament and never with a pair of prolegs each having a pair of hooks (if four terminal hooks are present, then two pairs of terminal filaments are also present | COLEOPTERA |