Key Measurements

[Margaret Thayer]

>Why are measurements so important to identification?
They're not always.  They are merely a kind of character and may or may not
be useful in any given situation.  In many cases there may be difficulty in
finding non-mensural characters that are usable and useful in a key (e.g., a
key consisting only of couplets offering "male genitalia like fig. X vs. not
like fig. X" is not worth the space it takes, and is utterly useless for
identifying females).  I ran into this problem in a key I published a while
back:  the species were quite distinguishable on the basis of male genitalia
but very difficult to separate on external or non-sexual characters. I wound
up using a number of measurements and proportions in the key, against my
general inclinations, but there seemed to be no choice.  I was especially
loath to do that because of very small sample sizes (n=1-5) for several
species.  To my amazement, most of the measurements I used have held up
pretty well as I've gradually seen more material.

>I can understand why quantities (e.g. 5 stamens, 6 fingers) and relative
>measurements (e.g. petals smaller than sepals) can be definitive ...
These can vary within taxa, too, and be just as misleading if used
uncritically.  The important thing is that a key needs to be constructed
*after* extensive study  determines what features are useful in
discriminating among the taxa involved.  One must figure out what the limits
of the taxa are first, then devise effective means for (others) to
discriminate them.

>1) Don't measurements vary based on the age/growth stage of the specimen?
If >so, should all measurements be averages?
It depends on the manner of growth of the organism.  Individual adult
insects, for instance, do not change in size, although there may be either
tremendous or very little variation among adults of a species.  It's rare
for a key to insects (except perhaps at the ordinal level) to include both
adults and immatures because in most cases there is little or no resemblance
between them.
Whether averages are useful depends on the nature of the variation within
species (or whatever taxa are being dealt with).  In some cases, maxima
and/or minima might be more useful.  What is useful also depends on the
context; in an interactive key, one could be comparing any taxon with any
other.  In a non-interactive key, the context is usually more limited and a
particular measurement (or any other character, for that matter) may be
extremely useful for distinguishing two particular terminal taxa in, say,
couplet 8 (after a number of other taxa have been eliminated), but
completely worthless in another context.

>2) Isn't any measurement necessarily (because of difficulties measuring using
>analog instruments such as rulers, for example) approximate?
Of course.  Measurements are only useful when the ranges/averages/whatever
are significantly different between the taxa or groups of taxa being compared.

>3) If three things are identical in every way but one is twice the size of the
>first; and the other three times the size of the first would there be cause to
>say that each thing being identified might be identified as a unique thing
>(using "thing" broadly).
Again, this is something that has to be determined (ideally, at least) on
the basis of additional characters and their concordance, i.e., a
posteriori, not a priori.  I don't think *I* would want to draw such a
conclusion solely on the basis of size, without some context (concordance of
other characters, perhaps patterns of variation in closely related taxa),
but others might.

To summarize this very briefly, identification is best done using tools
(keys or any other) constructed on the basis of careful and detailed study
of all the taxa in the pool of potential ID's.
Measurements of particular structures are potential characters that, like
any others, must have their usefulness assessed in the course of that
detailed study.


Margaret K. Thayer      thayer at fmnh.org  (use this short form for best results)
Field Museum of Natural History - Zoology, Insects
Roosevelt Road @ Lake Shore Drive
Chicago IL 60605, USA   tel. 312-922-9410, ext. 838