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        logical. 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,              hair-like              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.)

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