Name:
______________
Lab: Using and Constructing a Dichotomous Key
Introduction
All cultures have developed names for the living things found in their environments. When various
everyday names are used for the same organism, confusion is possible. So, scientists have developed
an international system for naming and classifying all organisms. Identification guides, called keys,
have been developed to help recognize and identify organisms according to their scientific names.
The word dichotomous comes from the word dichotomy, meaning “two opposite parts or categories.”
A dichotomous key gives the reader a series of opposing descriptions of basic features of an
organism. The reader studies the organism and selects the descriptions that apply to it until reaching a
statement with one species. In this investigation, you will use a typical dichotomous key to identify
the genus and species of several different salamanders. Then, you will create your own dichotomous
key to categorize a diverse group of wildflowers.
Pre-Lab Discussion
1. How many choices does a dichotomous key provide at each step?
2. What are some of the apparent differences among the salamanders illustrated?
3. Based on the information in Figure 2, what is a distinguishing characteristic of the members of the
genus Ambystoma?
4. If you were to use live flowers instead of diagrams, what other characteristics could you use to
identify the flowers?
Use the dichotomous key to determine the genus and species of that salamander. Begin by reading
statements 1a and 1b. One of the statements describes the salamander; the other statement does not.
Follow the directions for the statement that applies to that salamander and continue following the
correct statements until you have identified it. Record the scientific and common name.
1 a Doesn’t have back legs
Siren intermedia, siren
Go to 2
b Has back legs
2 a External gills present in adults
b External gills absent in adults
Necturus maculosus, mud puppy
Go to 3
3 a Large size (over 7 cm long in Figure 1)
b Small size (under 7 cm long in Figure 1)
4 a Body background black, large white spots
variable in size completely covering body and tail
b Body background black, small round white spots in a
row along each side from eye to tip of tail
Go to 4
Go to 5
Ambystoma tigrinum, tiger salamander
Ambystoma maculatum, spotted salamander
5 a Body background black with white spots
b Body background light color with dark spots and/or lines on body
Go to 6
Go to 7
6 a Small white spots on black background in a row along each
side from head to tip of tail
Ambystoma jeffersonianum, Jefferson salamander
b Small white spots scattered throughout a black background
from head to tip of tail
Plethodon glutinosus, slimy salamander
7 a Large irregular white spots on a black background extending
from head to tip of tail
Ambystoma opacum, marbled salamander
b No large irregular black spots on a light background
Go to 8
8 a Round spots scattered along back and sides
of body, tail flattened like a tadpole
b Without round spots and tail not flattened like a tadpole
Triturus viridescens, newt
Go to 9
9 a Two dark lines bordering a broad light middorsal stripe with a narrow
median dark line extending from the head onto the tail
Eurycea bislineata, two-lined salamander
b Without two dark lines running the length of the body
Go to 10
10 a A light stripe running the length of the body and bordered by dark
pigment extending downward on the sides
Plethodon cinereus, red-backed salamander
b A light stripe extending the length of the body without
dark pigment on the sides
Hemidactylium scutatum, four-toed salamander
Number
Genus and Species
Common Name
Constructing a Dichotomous Key of Mythical Organisms
Classification of organisms often requires grouping organisms based on their characteristics. Use the
following list of mythological organisms and their descriptions to complete the analysis.
Pegasus


Centaur
Griffin
Dragon
Chimera
Hydra
Pegasus-In Greek mythology, the winged horse that sprang from the blood of the Gorgon Medusa when
she was decapitated by the hero Perseus. He carried Bellerophon in his fight with the chimera, and was
later transformed into a constellation.
Centaur-In Greek mythology, a creature half human and half horse, wild and lawless. Chiron, the mentor
of the hero Heracles and tutor of the god of medicine Asclepius, was an exception. Their home was said to
be on Mount Pelion, Thessaly.




Griffin-Mythical monster, the supposed guardian of hidden treasure, with the body, tail, and hind legs of a
lion, and the head, forelegs, and wings of an eagle, though in classical times all four legs were those of a
lion.
Dragon-Mythical monster, can grow hundreds of feet long, has a snake-like body, four legs, 1-3 reptile like
heads, 2 wings, scaled and breathes fire.
Chimera-In Greek mythology, a fire-breathing animal with a lion's head and foreparts, a goat's middle, a
dragon's rear, sometimes heads of all 3. Alternately, any apparent hybrid of two or more creatures.
Hydra-In Greek mythology, a huge monster with nine heads, four legs, spiked tail, and scales. If one were
cut off, two would grow in its place.
You will be constructing a key. While constructing your key keep the following in mind:
1. Use constant characteristics rather than variable ones.
2. Use measurements rather than terms like “large” or “small”
3. Use characteristics that are generally available to the user of the key rather than seasonal
characteristics or those seen only in the field.
4. Make the choice a positive one- use “is” not “is not”
5. If possible, start both choices of a pair with the same word.
6. If possible, start different pairs of choices with different words.
7. Precede the descriptive term with the name of the part to which they apply.
Analysis
1. Identify the characteristics you think are most useful in grouping the organisms into separate
groups.
2. Classify the organism into at least 3 groups based on these characteristics
3. On a separate piece of paper, create a dichotomous key to match your classifications.