[Odonata-l] Why report hindwing length and not forewing length also?
Ola Fincke
fincke at ou.edu
Thu Mar 15 13:16:09 PDT 2007
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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