[Odonata-l] Why report hindwing length and not forewing length also?

Sat Mar 17 11:48:14 PDT 2007

This interests me a lot, because I have been measuring Odonata for many
years.

One problem with wing measurement is that it is not certain where the base
of the wing is located.  In my recent Orthemis study I have been taking the
notch at the base of the costa that seems to correspond to the major point
of flexure of the wing.  This is pretty difficult to observe if one is using
a millimeter scale on a specimen held beneath the microscope, but it can be
done.  Tillyard more or less shows where it is, but he does not figure it
worth a damn.  My technique is to scan the wing and scale together, and then
to transpose the scale image on to the wing in Photoshop, rotating and
translating the scale to make the measurement.  In this way I can get about
0.1 mm repeatability.  This is great for my own exercises, in which I wish
to show comparisons with other specimens measured also by me.  But how do I
compare my results with those of others?  The simple answer is that I cannot
do this, because I have no idea how others measure wings.  So, how do you do
it, and from where?

And abdominal measurment - don't even talk about that.  Even when the
abdomen is still in one piece, and it is straight, not curved, there is the
problem of shrinkage during preservation.  This measurement is vastly less
certain than wing measurement, and I use the hind wing as the only more or
less reliable surrogate for size.  I still am not quite certain what all
this is accomplishing.  I found in the Orthemis study one specimen for which
I thought I had made a serious mistake - it couldn't really be shaped like
that, with such short wings and a long abdomen.  I pulled the specimen out
and looked  at it again- It really was shaped like that - long abdomen and
short wings.  No mistake at all.  But what was the meaning of its reported
size?

What I would like to see done is weight measurements.  I once found a
species of Nesobasis (damselfly) for which the males tended to be much
larger than the females.  I estimated from dimensions alone that the males
must weight at least twice what the females weighed, which I think is
remarkable.  But a protocol for weight measurement would be very difficult
and would have to involve taking a very portable and accurate scale into the
field.  

A digression.  Years ago I was in my office when the friendly Fishers sales
rep wandered in.  "What do you have that is new and interesting?". I asked.
"Why we have just introduced portable, digital, battery powered scales", he
said with considerable pride.  "I'll bet the drug dealers are scarfing them
up", I replied.  He promptly turned an interesting shade of gray, never
having considered this marketing possibility.

  _____  

From: odonata-l-bounces at listhost.ups.edu
[mailto:odonata-l-bounces at listhost.ups.edu] On Behalf Of Ola Fincke
Sent: Thursday, March 15, 2007 4:16 PM
To: Mike May
Cc: Odonata-l
Subject: Re: [Odonata-l] Why report hindwing length and not forewing length
also?

In many species, it's well-known that size declines over the season, which
is one reason that those of us interested in the effects of size on
reproductive success take that into account (e.g. Fincke & Hadrys 2001; see
also work by Dave Thompson and others, cited in the former). 

There must be a LOT of geographic variation in size; certainly Ischnura
verticalis in MI is much larger than it is in OK. Even within species in
similar areas, size can vary quite a across populations. I am collecting
data on this within Enallagma. 

Ola Fincke
Dept. of Zoology
University of Oklahoma
Norman, OK 73019
Tel: 405-325-5514
Fax: 405-325-6202

On Mar 15, 2007, at 1:52 PM, Mike May wrote:

Dennis, et al.,

I have to agree that a much more useful set of measurements, regardless of
which body parts are selected, would be mean +/- standard deviation (range).
Likewise it would be great to include much more precise distribution data
(e.g., county rather than state level - hooray again for Nick Donnelly's dot
maps) and seasonal data (e.g., flight season at several different latitudes
or within regions of similar climate). The reason for not doing so in NWM
and W&M is simple, and the same in all these cases - time and effort. To a
fairly accurate approximation, one can (read, "Mike can") riffle through the
hundreds of specimens of Enallagma civile at FSCA, pick out several of the
largest and smallest from different regions, spread them out and compare by
eye, select and measure 3-4 at each end of the size range, and write down
the maximum and minimum results. That's exactly what I did, and, although
tedious enough, it's a lot quicker than selecting a random sample large
enough to be statistically relevant (~30), measuring (with a mm ruler,
sometimes viewed through a scope) and recording all of them, and calculating
the appropriate statistics. That doesn't make the former technique great,
especially if one or two atypically large or small individuals are incuded,
it's just easier. There's also no doubt that that human error plays a role
and is likely to lead to a more misleading outcome in assessing range than
aggregated statistics. As an aside, total length and abdominal length are
probably much more subject to error than is wing length because of the
tendency of heads to fall off, abdomens to curl, etc. 

Anyway, I think a genuinely worthwhile project would be to accumulate and
publish careful size measurements of as many odonate species as possible,
based on statistically meaningful and geographically diverse samples. This
just wasn't feasible in doing the manuals, but it would be very useful in
itself and could lead to some interesting questions. E.g., does size vary
geographically or seasonally, and is the variation consistent across
species? This has been shown to be true for several species, as Dennis
notes, but the answer isn't known for most. If such patterns do exist, then
you can ask why. If anyone is interested in undertaking such a project, I'd
be glad to give them the benefit of my experience, for what it's worth.

Mike May

----- Original Message ----- 
From: Dennis Paulson <mailto:dennispaulson at comcast.net>  
To: Odonata-l <mailto:odonata-l at listhost.ups.edu>  
Sent: Wednesday, March 14, 2007 12:17 PM
Subject: Re: [Odonata-l] Why report hindwing length and not forewing length
also?

What is sometimes frustrating is that guidebooks such as these list
measurement extremes without giving any idea of means or explaining the
significance of the extremes, and the handbooks Alex mentioned are among the
best examples of this.

For example, I opened Needham, Westfall & May to a page at random and found
Erpetogomphus bothrops. Length 38-51 mm, abdomen 26-39, hindwing 21-31. This
is a common species in Sonora, which I've visited several times in recent
years, and I always thought the individuals I saw and collected didn't vary
anything like that. So I just measured the hind wings of all my specimens
(from throughout the range of the species) and found this:

males - range 23-26.5 mm, mean 24.9 (n=15)

Eleven were with n 1 mm of the mean, in the range 24-26 mm.

females - range 26.5-29 mm, mean 27.5 (n=6)

Five were within 1 mm of the mean, in the range 26.5-28.5 mm.

Thus in both sexes, the great majority of individuals measured within a 2-mm
range, and that range defines the size of each sex quite well. Combining the
sexes would justify listing the range in "usual" hindwing length of 24-28.5
mm. Total length and abdomen length don't seem to vary any more than
hindwing length.

The entire series of 21 individuals ranged from 23-29 mm. Rosser Garrison,
in his 1994 revision of the genus, examined 91 specimens. He listed the
variation in hindwing length as 27-31 mm, which makes me wonder if we
measure wings differently. In fact we must, because some of my specimens
were among those he measured. Both of u show a considerably smaller range
than that listed in NWM, but between my measurements and Garrison's
measurements, we're approaching the range in NWM, so part of the great
variation listed in that book could be explained by different measurement
techniques and/or acquiring the measurements from all published data.
Unfortunately, ranges like that don't give one a very good idea of how big
the dragonfly is. Perhaps Mike May can tell us how those figures were
obtained. I emphasize that I'm not criticizing NWM, as I'm sure the authors
of that book did the best they could in acquiring these figures.

I do know, from much research on bird size, that guide book writers
sometimes get it wrong, and the incorrect figures have a life of their own,
copied from book to book. I found total length measurements off by as much
as 15% when analyzing measurements in several bird guides, at times a larger
species listed as smaller than a smaller species. How many bird-b ok writers
nowadays measure birds, live or dead?

In many taxonomic papers, the author measures a series of specimens and
gives ranges and means, but for many species there are no such papers, so
writers of guide books have a difficult time getting measurements, with two
options. Either they have access to a collection and spend long hours
measuring specimens, or they take the figures from other publications, the
research literature and/or other guide books. One of my colleagues recently
confessed to me that when he listed measurements for a book on dragonflies,
he just used the midpoint of the measurements in NWM and Westfall & May,
which seems as good a way of doing it as any. I think having a single
measurement in a guide book makes it easier to picture the size of the
animal than when there is a rather substantial range.

Some species vary in size over the season. Pachydiplax longipennis t at
emerge in summer in southern Florida have hind wings about 3 mm shorter than
those that emerge in the winter, and the size varies seasonally at least as
far north as the US Gulf states. I am not aware if seasonal variation has
been established for other species. Just from eyeballing it, it seems to me
that some species vary more in size than others, but perhaps that just
depends on how many of them you see. I was just examining two Sympetrum
costiferum, one of which seemed no more than 2/3 the bulk of the other, and
two Enallagma carunculatum, one of which was surely only half the bulk of
the other and about 2/3 its length. These are unusual extremes, of course.
When I look at my collection or at odonates in the field, individuals of a
species mostly look around the same size.

Another source of variation in odonates is temperature. Damselflies in
Mexico and Central America tend to be larger at higher elevations,
presumably something about temperatu e vis-a-vis development time of the
larvae. I don't know if there is latitudinal variation as well, but there
should be. Nick Donnelly called attention to the very small size of some
damselflies in eastern Texas, and I have seen these same populations of
impressively tiny creatures. I don't know whether these populations are
genetically distinct, or does some environmental factor of the area have
them emerging at a smaller size? I collected tiny Perithemis mooma in
Yucatan, as small as or smaller than Nannothemis bella.

There are probably many more examples of interesting size variation in
odonates, and it would be good to know more about this.

Dennis Paulson

On Mar 14, 2007, at 6:58 AM, Mike May wrote:

Certainly part of the reason is tradition. Once a large body of data is 
available for a particular measurement, the value of that measurement for 
comparative study is enhanced. I suspect, also, that the hindwing may have 
been selected originally because, when a dragonfly is held in the hand with 
the wings pressed together above the back, the hindwing is positioned 
outside the forewing. Although the forces produced by fore- and hindwings in

flight are certainly somewhat different, I feel pretty confident that had 
nothing to do with the decision.

The forewings are nearly always slightly longer than the hindwings. My 
impression is that the proportionate difference does not vary greatly with 
size, taxon or sex, but I don't know whether this has been investigated 
rigorously. Females do typically have longer wings than males in comparison 
to other measures of body size such as total length or thoracic mass 
(probably because abdominal mass becomes greater in mature females because 
of the mass of eggs), but I don't know that the differences between fore- <
SPAN>
and hindwings is affected by sex. Maybe Roy Beckemeyer can illuminate that 
question a bit more.

Mike May

----- Original Message ----- 
From: <aardila at uoguelph.ca>
To: "Odonata-l" <odonata-l at listhost.ups.edu>
Sent: Wednesday, March 14, 2007 4:28 AM
Subject: [Odonata-l] Why report hindwing length and not forewing length 
also?

Hello everyone,

I have the books "Dragonflies of North America"(Needham, Westfall ,and
May, 2000) and "Damselflies of North America" (Westfall and May,
1996). These books are excellent. They report the body size lenght,
abdomen lenght, and hindwing length for every Odonata species in North
America.

However, I don't understand why the size of the forewings is not
reported. Is this by convention? What is the history behind this? Why
were the hindwings and not the forewings chosen? Is there a
statistically and biologically insignificant difference between the
forewings and the hindwings within species (and sex) in Odonates? Is
this selection based on the role of the hindwings in flight? I would
appreciate any guidance that could help me find an answer to these
questions. Thank you.

Alex

-- 
Alex Ardila-Garcia

-----
Dennis Paulson
1724 NE 98 St.
Seattle, WA 98115
206-528-1382
dennispaulson at comcast.net

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