Terminal Segment
Diabolical Dichotomies
John Acorn
E
ntomologists seem to love dichotomous keys—identification guides
based on sequential this-or-that
choices. It might be that botanists love them
more than we do, but I doubt it. I suspect
that every entomologist has used at least
one key (perhaps one of the standard textbook keys to insect orders), and for many
of us, keys are a daily event. Some are written, some are diagrammatic, and others
are based on software, but the basic idea
is always similar, and entomologists seem
to possess a deeply held collective belief
that keys are the best, most objective, and
perhaps the only way to identify specimens.
Keys, however, are not without their
frustrations. A few weeks ago, my friend
Matt Bertone posted the “worst couplet
ever” on Facebook. Here it is:
“2 (1). Front carinate above antennae,
carina complete and usually widely
separated from labrum; tarsomere
3 lobed, 4 never lobed; prosternal
sutures double. 3
- Front carinate above or not, but if
carinate, carina widely separated from
labrum; tarsomeres 3 lobed. 5”
It takes a moment or two to figure out
why this couplet is so terrible, but once
you do, it should be clear that (if the specimen is carinate) there is no possible way
to decide whether to proceed to couplet 3
or couplet 5. This example comes from a
key to click beetles, by Downie and Arnett
(1996), but I suspect that poor editing may
have been to blame, since the equivalent
couplet in Arnett (1960) makes perfect
sense, not to mention the inconsistent
pluralizing of “tarsomere” above.
I first came to realize how difficult key
construction is when I was a teaching
192
A key is like a path to a goal, with branching points, signals, and switches. Some will also recognize the yellow object on the track—it is called a derailer.
assistant for a course called Entomology for Non-Biologists, taught by Bruce
Heming, back in the early 1980s. On a lab
exam, I asked the students to construct a
simple identification key to four very different groups of insects. It seemed easy
enough to me, but the results were just
plain depressing. Only one student came
up with anything even vaguely resembling
a functional key. Another had both alternatives of each couplet leading to the next
couplet (1a and 1b go to 2, 2a and 2b go
to 3, and so on), and ran out of time at
around couplet 30. A third student had
plans for something like 58 couplets, and
also ran out of time.
This experience helped me realize that
writing a key is a matter of logic, and not
everyone is as logical as your average entomological taxonomist. I have written a
few keys in my time, and I won’t say my
keys are perfect, but I have learned a few
important lessons. For example, the number of couplets in a dichotomous key will
always be one fewer than the number of
possible outcomes. I really should have
told my students that a simple, dichotomous key to four groups always has exactly three couplets. It seemed so obvious at
the time. For me, it was also important to
realize that keys do not contain information that is better, or different, from the
information in a good field guide or monograph; they simply organize it in a different way. Back in high school, I wondered
how birders could possibly identify birds
without keys. It seemed so non-linear and
error-prone to use “field marks” instead.
So I wrote my own key to the flycatchers
and, to be honest, never used it, since
(continued on page 190)
American Entomologist • Fall 2013
Terminal Segment, from page 192
by the time I finished the key I had also
learned how to recognize them, as well as
how difficult some identifications can be.
Let’s return, though, to the joy of poking fun at our least favorite keys. A good
key should provide the user with clear,
mutually exclusive choices, right? So why
do we sometimes have the experience of
interpreting a couplet, deciding which
lead to follow, and then encountering the
exact same couplet further on in the key?
It happens. Or perhaps you are asked if the
specimen is red or brown, and you decide
that it is red. Two couplets later, you are
asked if it is light brown or dark brown.
The next couplet uses an adjective you
have never heard of, and can’t find a definition for. Now there are teeth marks on
your microscope. How can highly trained,
intelligent people create such crazy-making science without anyone noticing at the
time? Have you seen couplets that ask if
your one and only specimen is “variable
in length” or whether it is “consistently
small?” Argh! Or couplets that ask you
something about males, when all your
specimens are female? Again, you pound
your forehead on the eyepieces! How could
the author have been so stupid?
Well, for at least the latter sorts of problems, I do think there is an explanation.
Like any other aspect of science communication, keys are intended to direct
our thoughts and perceptions. As anyone
who writes or teaches knows, recreating a
thought or a perception in another human
mind is never an easy task. In my opinion,
part of the problem lies in the fact that
users and authors often think about keys
in significantly different ways. To a user,
a key is a reliable identification shortcut.
Assuming they know the basics of insect
morphology, and assuming the key is well
constructed and well illustrated, any competent entomologist should be able to take
a single specimen, “run it through the key,”
and come up with an expert identification. Metaphorically, a single specimen
“follows” a (logical) “path” to a (identification) “goal” or “endpoint.”
Authors, however, sometimes think differently, and I suspect that many of the
most problematic keys are based on a
different metaphor, in which the author
sequentially subdivides a huge, imaginary
set of all possible specimens of the taxon
treated by the key, and absent-mindedly
describes the characteristics of the groups
(which can, for example, be “variable”)
instead of the characteristics of individual
specimens (which can’t). The two metaphors are compatible, but different, since
the specimen either follows its path alone,
or as a member of a series of sequentially
smaller groups. In reality, of course, the
specimen is always “alone” until it has
been identified, but this is easy for some
authors to forget.
Of course, this doesn’t explain all of the
oddities we encounter in keys, but I think
it’s a start. Sometimes, the features used in
keys are based partly on the structure of
the insects, and partly on the vagaries of
human perception, resulting in characters
that are, shall we say, somewhat metaphysically unstable. One minute they look
like this, the next they look like that, and
you find yourself fiddling with the lighting on your microscope and wondering
why you can’t see things more clearly. It
feels a lot like trying to find those hidden
3D pictures in posters made of colored
dots—you expect the pattern to somehow
suddenly appear, and indeed, sometimes it
does. When it doesn’t, you blame yourself,
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not the picture. Perhaps the author had
a better microscope. Perhaps you really
weren’t cut out for entomology after all.
Not long ago, while working on a key
to Alberta ants with James Glasier, we
encountered such a feature, involving
some shallow punctures on the heads of
Formica workers. James felt that he could
see these things, I said I couldn’t, and
when we asked Heather Proctor for a third
opinion, she couldn’t see them either. So
we took a look under the scanning electron microscope and, what do you know,
the feature simply does not exist, at least
on our specimens. We decided that they
must have something to do with the play
of light and shadow in the translucent
cuticle, and agreed to find other features
to use in the key. In a similar vein, my
friend David Maddison tells me that some
of the classic characters used for keying
out Bembidion ground beetles are equally quasi-fictive. There are vague wrinkle
patches that form, or fail to form, “doubled
furrows” on the clypeus, and margins of
shoulders that are either “rounded” or
“angular” without really being either. For
the most part, what is needed is for your
mind to “see” a pattern in an impossibly
vague sort of stimulus. You get the idea.
To David (and I agree completely), what
matters here is whether someone “buys
into” the character, especially when the
character is thought to separate two “real”
taxonomic groups. Once you believe that
the groups are real, the character seems
real as well, since it is part of the essence
of both groups. But by the time you understand these things, you also know what
the insects look like, and you are no longer a naïve user of the key. So specialists
“see” the feature, when really what they
are seeing is the specimen as a whole, and
what they believe the ambiguous character
should look like. As a consequence, they
generally get the right answer, and find
nothing wrong with the key.
So what is the solution to all of this?
To me, there are three important things
to keep in mind here. First, any key will
work better the more you learn the group
in question, since keys are just a way of
organizing information, not foolproof
shortcuts. Second, some keys just simply
don’t work, so you shouldn’t feel bad if
you resort to comparisons with properly
identified specimens, comparisons with
lengthy descriptions, ‘picture-booking,”
or Bug Guide. And third, if you yourself
ever write a key, please write it for the
ideal first-time user: a sincere, intelligent
person who just finished learning about
insect morphology and now wants to pursue a career as an entomologist. We owe
that person the best keys we can produce,
don’t you think?
References Cited
Arnett, R.H., Jr. 1960. The Beetles of the United States (A Manual for Identification).
The Catholic University of America Press.
Downie, N.M., and R.H. Arnett, Jr. 1996. The
Beetles of Northeastern North America.
Sandhill Crane Press. Volume 1.
Glasier, J.R.N., J.H. Acorn, S.E. Nielsen, and
H. Proctor. 2013. Ants (Hymenoptera: Formicidae) of Alberta: a key to species based
primarily on the worker caste. Canadian
Journal of Arthropod Identification 22: 1-104.
John Acorn lectures
at the University of
Alberta. He is an entomologist, broadcaster, and writer, and is
the author of fifteen
books, as well as the
host of two television
series.
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