1.7 Keys
1 jointed limbs ........................................... 2
If you want to identify an organism whose name you do not know, you may be able to find a picture of it in a book. However, not every organism may be pictured, or your organism may not look exactly like any of the pictures. If this happens, you can often find a key that you can use to work out what your organism is. A key is a way of leading you through to the name of your organism by giving you two descriptions at a time, and asking you to choose between them. Each choice you make then leads you on to another pair of descriptions, until you end up with the name of your organism. This kind of key is called a dichotomous key. ‘Dichotomous’ means ‘branching into two’, and refers to the fact that you have two descriptions to choose from at each step. Here is a key that you could use to identify the organisms shown in Figure 1.22.
no jointed limbs ...................................... earthworm
2 more than 5 pairs of jointed limbs ....... centipede
5 or fewer pairs of jointed limbs ........... 3
3 first pair of limbs form large claws ....... crab
no large claws .......................................... 4
4 3 pairs of limbs .......................................
locust
spider
4 pairs of limbs .......................................
To use the key, pick one of the animals that you are going to identify. Let’s say you choose organism B. Decide which description in step 1 matches your organism. It has jointed limbs, so the key tells us to go to step 2. Decide which description in step 2 matches organism B. It has more than 5 pairs of jointed limbs, so it is a centipede.
D
A
B
C
E
Figure 1.22 Organisms for practising using a key.
Original material © Cambridge University Press 2014
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