7.3: Part 1- Dichotomous Keys
A dichotomous key is an identification key that consists of a series of choices that ultimately lead to the identification of the object in question. In each step of a dichotomous key, the user has two choices and must pick the one that most correctly describes the object. Each choice leads to a new set of choices. To help you understand how a key like this works, consider the following four insects:
After studying the insects, you might classify the insects by wing covering, body shape, and the direction in which the wings point.
To begin the key, you could start separating the four insects based on wing covering— ”flight wings covered by exoskeleton” vs. “flight wings not covered by an exoskeleton.”
The first step in the key could be organized the following way:
- wings covered by an exoskeleton. . . . . . . . . . . . . . go to step 2
- wings not covered by an exoskeleton . . . . . . . . . . go to step 3
Step 2 consists of a pair of statements that distinguishes between the ladybug and the grasshopper.
- body has a round shape . . . . . . . . . . . . . . . . . . . . . ladybug
- body has an elongated shape. . . . . . . . . . . . . . . . . grasshopper
Step 3 consists of a pair of statements that distinguishes between the dragonfly and the housefly.
- wings point out from the side of the body . . . . . . dragonfly
- wings point to the posterior of the body. . . . . . . . housefly
Notice that there were four organisms to be identified and it only took three steps to identify them. After making a key, you should end up with one less step than the total number of organisms you are trying to identify.
When constructing a key, keep the following in mind:
- Use constant characteristics rather than variable ones.
- Use measurements rather than terms like “large” and “small.”
- Use characteristics that are generally available to the user of the key rather than seasonal characteristics or those seen only in the field.
- Make the choice a positive one—something “is” instead of “is not.”
- If possible, start both choices of a pair with the same word.
- If possible, start different pairs of choices with different words.
When using a key, keep the following in mind:
- Always read both choices, even if the first seems to be the logical one.
- Be sure you understand the meaning of the terms involved. Do Not Guess.
- When measurements are given, use a calibrated scale. Do Not Guess.
- Since living things are always somewhat variable, do not base your conclusion on a single observation. Study several specimens to be sure your specimen is typical.
- If the choice is not clear, for whatever reason, try both divisions. If you end up with two possible answers, read descriptions of the two choices to help you decide.
- Having arrived at an answer in a key, do not accept this as absolutely reliable. Check a description of the organism to see if it agrees with the unknown specimen. If not, there is an error somewhere, either in the key or in its use. For a scientist, the ultimate check of identification is a comparison of the unknown with an authentically named “Type Specimen.”
Procedure
- Examine the 6 specimens from school collections or organisms you collected and use them to construct your own dichotomous key here (use the example above as a template):
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Use someone else’s key to identify 2 specimens. List your path to identification here (letters are fine):
- Specimen 1
- Specimen 2