2. AN INTRODUCTION TO THE USE OF TAXONOMIC KEYS©
Laboratory Exercises for ES 300, Biodiversity
09jan00
Copyright 2000
by
Richard Fox
Lander University
BasicPrinciples
Dichotomous Keys
A taxonomic key is a device used to
simplify the identification of unfamiliar organisms. Keys
are prepared and published by specialists sometimes for
use by other specialists but more comonly for
non-specialists. Keys make the task of identification far
simpler than it would otherwise be. In fact, without
keys, routine identification of organisms in
environmental work would not be feasible.
Keys are based on a process of
elimination in which the user is presented with a series
of choices, each with two alternatives. The pair of
alternatives is referred to as a couplet. At each couplet
the user must choose which alternative applies best to
the unknown organism in hand. Rejection of the
inappropriate alternative eliminates from further
consideration ALL the organisms to which that alternative
applies. They need be considered no further and this is
the value of a key. It permits the rapid elimination of
large numbers of species that the unknown cannot be. The
chosen alternative leads to another couplet and another
choice. Once again some species are eliminated from
further consideration and fewer remain to be considered.
This continues until the user has eliminated all species
except that of the specimen in hand, whose identity is
now known.
The ideal key is dichotomous,
meaning the choices are always offered in pairs, or
couplets, rather than in threes or fours. The two
alternatives of a couplet must be mutually exclusive and
exhaustive. You will occasionally encounter keys that
employ triplets but well designed keys avoid them.
Some Advice
When using keys you should:
a. Keep in mind that there is
individual variation in all sexually reproducing species
and it is not realistic to expect that every individual
will agree exactly with a published description or that
the specimens in your collection with be identical.
b. Let the preponderance of
characters make your decision.
c. In a conflict, place more
reliance on illustrations than on words.
d. Do not discount the possibility
that your specimen is not in the key. There is always the
possibility you have a specimen of a species not yet
recorded from the region and thus not included in the
key. It is also possible that you have a species that is
new to science. If you spend a lot of time collecting and
identifying organisms you are eventually going to
encounter such organisms.
e. Many aquatic insects are known
only from the larval stage or from the adult stage and in
many cases the larval and adult stages have not been
associated with each other. Strange as it may seem there
is no way of automatically knowing which larvae
metamorphose into which adults unless you watch it
happen. Making this association requires that the larval
stage be reared in the laboratory until it transformed
into the adult and this simply has not been done for
large numbers of aquatic insects.
f. Young individuals, such as early
instar nymphs, are difficult to identify because they may
not yet have developed their distinguishing
characteristics. Keys are usually written for adults or
late instar nymphs.
g. Keys may not exist for the
geographic area, taxon, or life history stage of
interest. For example, there are no regional keys to
South Carolina mussels or lichens.
h. Regional keys are better than
larger scale keys. Regional keys allow you to omit from
consideration large numbers of species that do not occur
in your region. For example a key to the SC mussels would
be much easier to use than a key to the mussels of North
America. The former might include 30 species whereas the
latter would have 300.
i. Use the most recent keys
available. Our conceptions of the evolution, and hence
classification, of groups changes continually as learn
more and more. Further, new species are discovered and
described continuously. Consequently the composition of
taxa and their names change with time.
j. A key will always give you a
name but not necessarily the correct one. You may feel
that something is wrong as you work your way through the
key and may be suspicious of the answer you arrive at. In
many cases, however, the resulting identification may
seem completely plausible to you but be totally
incorrect. Using the key alone, there is no way to know
if your answer is the correct identification of the
specimen. Whenever possible you should compare your
specimen with a drawing or photograph of the species you
think it may be. This is especially true if you are
suspicious of your determination.
k. Always read the introductory
material that accompanies the key before attempting its
use. You can save yourself a lot of time and frustration.
Be sure the key includes the taxon and geographical
region of interest. In the words of R.O. Brinkhurst
(1968) "No amount of time spent chasing about the
key in growing frustration will ever compensate for
failure to read the preamble to a key".
l. Before attempting to use a key,
first learn the pertinent features of the anatomy of the
group and know the names used for their parts. You must
have some familiarity with the features that are
taxonomically useful in the taxon of interest. All these,
anatomy, names, and important features, vary from taxon
to taxon and you have to know them for all taxa in which
you expect to be proficient.
Procedure
Shop
Fasteners
This exercise is based on a jarful
of nails and screws. You will use these as simulated
organisms to learn the basic principles of using a
dichotomous key. Try to forget that you already know the
names of these taxa and go where the key leads you. This
key exemplifies one of many formats that are used in the
construction of keys. Do not expect all keys you
encounter to look like this one.
Anatomy and
Terminology
Use of a key requires knowledge of
the anatomy, and its associated terminology, of the group
in question. It also requires that you know which
anatomical features are relevant to the identification of
members of the group. The best keys have a glossary,
illustrations of pertinent anatomical features, a
discussion of the taxonomically important features, and
illustrations of each species covered by the key.
Glossary
Head: the widened apical end
of the body.
Shaft: The narrow, elongate,
portion of the body.
Slot: An invagination or
groove on the apical surface of the head of some
fasteners.
Thread: any ridge on the
outside of the shaft.
Taxonomically
Important Features
1. H/S ratio. This is the
ratio of the diameter of the head to the length of the
shaft. It is determined by dividing the head diameter by
the length of the shaft.
2. Slot shape. Determined by
inspection. It may be a simple straight channel, a pair
of straight channels crossing in the center of the head
to form an "X", a quadrate pit, or rarely,
other shapes.
3. Shaft ornamentation. The
surface of the shaft may bear threads which can be
helical, spiral, circular, or rarely, longitudinal.
4. Color. Determined by
inspection.
5. Total length. Determined
by measuring, in mm, the length from the apical side of
the head to the free tip of the shaft.
There is no sexual dimorphism and
neither dissection nor microscopic examination is ever
required for identification of taxa in this group. The
key is applicable to individuals of all life history
stages.
Exercise
Use the following key to identify
some of the items (species) in a collection of common
shop fasteners. Demonstrate some of your results to your
instructor.
A Preliminary Key to Common Shop Fasteners
| 1a. | Shaft threaded. |
Screws and their allies 2 |
| 1b. | Shaft not threaded |
Nails and their allies 9 |
| 2a. | Head without a slot |
threaded nail |
| 2b. | Head with a slot |
screws and bolts 3 |
| 3a. | Shaft cylindrical, not tapered, tip truncate |
bolts 4 |
| 3b. | Shaft tapered, at least at tip, tip pointed |
true screws 5 |
| 4a. | Head flat |
flat-head bolt |
| 4b. | Head rounded |
round-headbolt |
| 5a. | Slot "X"-shaped |
Phillips head screws 6 |
| 5b. | Slot linear, a single straight channel |
standard screws 8 |
| 6a. | Head flat |
7 |
| 6b. | Head rounded |
machine screw |
| 7a. | Shaft black |
drywallscrew |
| 7b. | 7b. Shaft silver |
common Phillips screw |
| 8a. | Head rounded |
round-head
standard screw |
| 8b. | Head flat |
flat-head
standard screw |
| 9a. | Head diameter much greater than shaft diameter, H/S ratio = 0.3 - 2.0. |
tacks10 |
| 9b. | Head diameter not much greater than shaft
diameter, ratio <0.2 |
true nails 11 |
| 10a | Shaft angled, with flat surfaces; H/S
@0.5
|
carpet tack |
| 10b. | Shaft circular in cross section, H/S
@ 1.7 |
thumb tack |
| 11a. | Head diameter slightly greater than shaft,
H/S < 0.1 |
finishing nails 12 |
| 11b. | Head diameter about twice that of shaft,
H/S @ 0.1 |
common nails 13 |
| 12a. | Total length 50-60 mm |
6d finishing nail |
| 12b. | Total length 35-45 mm |
4d finishing nail |
| 13a. | Total length 40-50 mm |
4d common nail |
| 13b. | Total length 60-70 mm |
8d common nail |
Additional Keying
Use the key to identify the
additional specimens provided by your instructor.
Tell the instructor the names of
these specimens, as determined by the key. If there is a
problem, why do you think so and what do you think caused
it?
Bibliography
Brinkhurst, R. O. 1968.
Oligochaeta, I1-I17, in F. Parrish (ed), Keys to watern
quality indicative organisms (Southeastern United
States). Federal Water Pollution Control Administration,
Washington.
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