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Dichotomous Key

Dichotomous Key

A dichotomous key is a tool that taxonomists often use to classify organisms correctly. It is a form of hierarchical grouping that involves making decisions in a series of steps, from general differences to very specific differences. It is called a dichotomous key because there are always two choices. There is a very specific way to set up a dichotomous key.

For instance, one must always move from the general to the specific, and one must always ensure that the two choices in the decision tree are mutually exclusive and jointly exhaustive. Mutually exclusive means that there cannot be overlap between the two options, as this would result in wanting to place an organism in two groups. Jointly exhaustive means that your two options must cover all possibilities, otherwise you won’t be able to place an organism in either of the groups.

Optional Activity: Identifying arthropods using a dichotomous naming key

Aim

To use a dichotomous key to identify arthropods.

Table of specimens

ABC
DEF
GHI
JKL

Instructions

  1. Study the organisms in the table of specimens provided to you.
  2. Use the dichotomous key to find out to which taxonomic group each of these arthropods belong.
  3. Write the letter corresponding to the arthropod, and then your answer.
 CharacteristicInstruction

1a

1b

Arthropod has eight legs

Arthropod does not have 8 legs

go 2 (Arachnids)

go 4

2a

2b

Arachnid has pedipalp with pincers

Arachnid does not have pedipalp with pincers

SCORPION

Go 3

3a

3b

Arachnid drinks blood

Arachnid does not drink blood

TICK

SPIDER

4a

4b

Arthropod has more than 16 legs

Arthropod does not have more than 16 legs

Go 9 (Myriapoda)

Go 5

5a

5b

Arthropod has 3 pairs of legs

Arthropod does not 3 pairs of legs

Go 6 (Insects)

CRUSTACEAN

6a

6b

Insect has hardened fore-wings

Insect does not have hardened fore-wings

COLEOPTERA

Go 7

7a

7b

Insects are social and/ or live in a hive

Insects are not social, do not live in a hive

HYMENOPTERA

Go 8

8a

8b

Insects does not have a sponge-like proboscis

Insects have a sponge-like proboscis

LEPIDOPTERA

DIPTERA

9a

9b

Myriapod with one pair of legs per segment

Myriapod with two pairs of legs per segment

CENTIPEDE

MILLIPEDE

Answers

  • A: Hymenoptera

  • B: Millipede

  • C: Crustacean

  • D: Tick

  • E: Spider

  • F: Centipede

  • G: Lepidoptera

  • H: Coleoptera

  • I: Hymenoptera

  • J: Scorpion

  • K: Diptera

  • L: Hymenoptera

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