Regents Prep: Living Environment: Laboratory:
Dichotomous Keys

Dichotomous Keys
A dichotomous key is a sequence of steps that allows the identification of a living thing.  The key will consist of a series of choices that lead the user to the correct name of a given item. The term dichotomous means that there will always be two choices in each step of the key until the organism is correctly identified.

Some Key Ideas in Dichotomous Key Construction

1.   Use constant characteristics rather than ones that disappear 
       or vary with the season or other environmental factor.

2.   Use characteristics which can be directly observed. 

3.   Use quantitative measurements with an amount or 
      dimension  rather than vague terms like "big" and "small."

4.   Precede the descriptive terms with the name of the anatomical 
      part to which it applies.


Rules to Follow When Using a Dichotomous Key

1.   Always read both choices, even if the first seems to be the 

2.   Understand the meaning of the terms involved in the key.

3.   When measurements are given, use a scale to measure 
      the specimen. Do not guess at a measurement.

4.   Living things are always variable, so do not base your 
      organism identification in the field on a single observation. 


Using a Dichotomous Key to Identify an Organism

The example below will illustrate the use of a dichotomous key to identify the unknown creature above.

Steps in the
Dichotomous Key

 Identification Process

Taxonomic Key to Stream Water Animals

1.     A.   With a shell            go to  2
        B.   Without a shell       go to  3


**  The creature clearly does not have a shell, so go to # 3.
2.    A.   Shell made of two parts held
              together by a hinge    Clam
       B.   Shell made of only 
              one part                    Snail
3.    A. Body flat, oval 
            and brown                  Water Penny
       B. Body not exactly like a 
            water penny                 go to  4
**  This creature does not have an oval body  it is long, so go to # 4.
4.   A. With six 
            jointed legs                  go to 5                                                     
      B. With more than six 
            jointed legs                  go to 12
      C. With less than six jointed legs; body 
           often worm-like            go to 14
**  The creature has 6 jointed legs, so go to # 5.
5.   A.  With two or three thin, 
             hair-like tails               go to 6
      B.  Without thin, 
            tails                             go to 7
**  The creature has three thin tails, so go to # 6.
6.  A.   With one hook at the end of each 
            leg; usually with three tails, 
            sometimes only two     Mayfly
      B.   With two hooks at the end of each 
             leg; two tails               Stonefly
**  This organism clearly has three tails and only a single hook at the end of each leg which
makes it a Mayfly larva.
7  A.   Body with many long, 
           pointed parts                go to 8
    B.   Body not exactly 
           like this                         go to 9

8.  A.  Body brown or black, often 
          very large               Hellgrammite
     B.  Body white, yellow or tan; not 
          so large                  Beetle larva

9.  A.   Body with hook-like claws at 
            tail end; animal sometimes
            protected with bits of 
            sand, pebbles 
            or twigs                    Caddisfly
     B.   Body without hook-like   
            claws                        go to 10

10 A. Body small, dark, hard and 
          beetle-like               Riffle beetle
     B. Body not exactly like 
          this                              go to 11
11 A. With 3 wide tails        Damselfly
     B. Without tails, but with three 
          short points                 Dragonfly
12 A. With two large claws and eight 
           legs; large                   Crayfish
     B. Without large claws; 
          smaller                         go to 13
13. A. Body flattened side to side; 
           usually white               Scud
      B. Body flattened top to bottom; 
           usually gray                 Sowbug
14.  A. Body with very small legs; usually 
            with a head                go to 15
       B. Body without any legs or 
            head                          go to 16
15  A. Tail-end of body wider than the 
           other                     Black fly larva
      B. Tail-end of body not 
           wider                     Midge

16. A  Body brown, plump, and 
           caterpillar-like      Crane fly larva
      B. Body not exactly like 
           this                       go to 17

17. A. Body with suckers at 
           each end                Leech
      B. Body without suckers; small, thin 
           and worm-like     Aquatic worm

We didn't need to go 
beyond step 6 with the organism we classified 
above, but some 
organisms might 
require the use of 
many more steps 
before its proper identification.

Resources for Dichotomous Keys
Dichotomous Keys of Common Kitchen Beans and Conifers 
(Provides information on key construction and sample keys.)

Internet Front Door Page on Taxonomy and Classification
(Provides information on classification, sample dichotomous keys for several different animal groups, and an introduction to cladistics.)


Created by James M. Buckley, Jr.
Copyright 1999-2003 Oswego City School District Regents Exam Prep Center
RegentsPrep and StudyZone are FREE educational resources.