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
To use a dichotomous key to identify arthropods.
Table of specimens
- Study the organisms in the table of specimens provided to you.
- Use the dichotomous key to find out to which taxonomic group each of these arthropods belong.
- Write the letter corresponding to the arthropod, and then your answer.
Arthropod has eight legs
Arthropod does not have 8 legs
go 2 (Arachnids)
Arachnid has pedipalp with pincers
Arachnid does not have pedipalp with pincers
Arachnid drinks blood
Arachnid does not drink blood
Arthropod has more than 16 legs
Arthropod does not have more than 16 legs
Go 9 (Myriapoda)
Arthropod has 3 pairs of legs
Arthropod does not 3 pairs of legs
Go 6 (Insects)
Insect has hardened fore-wings
Insect does not have hardened fore-wings
Insects are social and/ or live in a hive
Insects are not social, do not live in a hive
Insects does not have a sponge-like proboscis
Insects have a sponge-like proboscis
Myriapod with one pair of legs per segment
Myriapod with two pairs of legs per segment