[Odonata-l] Why report hindwing length and not forewing length also?
Jason Bried
jbried at TNC.ORG
Fri Mar 16 14:26:58 PDT 2007
Some neat discussion here.
Following the thread, I have some hindwing length data from Mississippi
collections in mid-June and then again in late-August (same site) of five
familiar species: Pachydiplax longipennis, Erythemis simplicicolis,
Perithemis tenera, Plathemis lydia, Libellula incesta. All specimens were
mature males. Here's the breakdown of a quick analysis, reporting mean
right hindwing length +/- s.d.:
Erythemis simplicicolis: June = 30.8 +/- 2.9 (116 specimens) and August =
30.8 +/- 1.1 (99 specimens); any statistical evidence of difference? No
(p=0.450...I ran a Monte Carlo-based "t-test", which is robust to unequal
sample sizes and error variance)
Libellula incesta: June = 39.9 +/- 1.2 (60 specimens) and August = 39.7 +/-
1.3 (52 specimens); p=0.195
Pachydiplax longipennis: June = 28.5 +/- 1.5 (137 specimens) and August =
27.9 +/- 1.9 (123 specimens); p<0.001
Perithemis tenera: June = 17.0 +/- 0.5 (33 specimens) and August = 15.8 +/-
0.5 (20 specimens); p<0.001
Plathemis lydia: June = 31.1 +/- 0.9 (34 specimens) and August = 30.8 +/-
0.7 (48 specimens); p=0.040
>From a statistical robot perspective, it looks like populations of
Pachydiplax, Perithemis, and Plathemis flying in August tended to have
shorter wings than the June cohort. Interestingly, these results don't
match the patterns of dry mass in the same specimens. On average all
species except Pachydiplax weighed less in August than in June (based on the
same analytical approach), even though at least 50% of the length-mass
variation was explained in most cases. This is all part of a paper in
review right now.
I agree with Dennis and Mike regarding human error. I measured wings from
base to tip with a digital caliper (0.1 mm), but admit in the paper that the
"tip" is ambiguous because it's rounded. I wonder about using a less
ambiguous measure of wing length as a size index, like length of the costa
vein from base to nodus or base to pterostigma?
Dennis brought up latitudinal shifts. Check out Frank Johansson's
"Latitudinal shifts in body size of Enallagma cyathigerum (Odonata)",
Journal of Biogeography, 30:29-34. He found larger animals at low and high
latitudes and smaller ones at intermediate latitudes ("U-shaped" size
cline), which contradicts the hypothesized "saw-tooth" size cline.
Jason Bried
-----Original Message-----
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
To: Odonata-l
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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