[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.

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.