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Student Activity: How To Make A Dichotomous Key

Making your own dichotomous key can be easily done for a group of living or nonliving things that you want to organize by physical traits. This can be a good method for helping people make observations and understand some physical traits that are important for learning how to classify things. Follow these basic rules when making a key and you are on your way:

  • Start by observing the group of things to be used in the key.

  • List the most general traits that can be used to divide the organisms into categories.

A Simple Example

[ladybug]


[grasshopper]


[housefly]


[dragonfly]
  
Suppose you have four insects - a ladybug, a housefly, a dragonfly and a grasshopper. After studying the insects, you might use wings covering, body shape and where the wings point towards.

To begin the key, you could start separating the four insects based on wing covering - "wings covered by exoskeleton" vs. "wings not covered by exoskeleton."

The first step in the key will be organized the following way:

1.  
a. wings covered by an exoskeleton
b. wings not covered by an exoskeleton
 
 
 
 
 
 
 
Next, the statements need to lead the observer to the next step to narrow the identification further:
 
1.  

a. wings covered by an exoskeleton
b. wings not covered by an exoskeleton

 . . . . . . . . . .  
 . . . . . . . . . .  
 
go to step 2
go to step 3
 
Step 2 needs to consist of a pair of statements that will allow for the identification of the ladybug and the grasshopper:
 
2.  

a. body has a round shape
b. body has an elongated shape
 
 . . . . . . . . . .  
 . . . . . . . . . .  
 
ladybug
grasshopper
 
Step 3 needs to consist of a pair of statements that will allow for the identification of the housefly and dragonfly:
 
3.  

a. wings point out from the side of the body
b. wings point to the posterior of the body
 
 . . . . . . . . . .  
 . . . . . . . . . .  
 
dragonfly
housefly
 


Notice that there were four organisms to be identified and it only took three steps. There should be one less step than the total number of organims to be identified in your dichotomous key.

If you follow this simple model you will be able to develop keys for many different things. The more similarities the group of things have, the more difficult it is to develop a key. For example, keys used to identify similar flowers will use traits like how the pollen grains are packaged!

Try To Identify These Insects . . .