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

A	B	C
D	E	F
G	H	I
J	K	L

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.

	Characteristic	Instruction
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