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

Wed Mar 14 09:17:17 PDT 2007

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 within 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 us 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-book 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  
that 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 temperature 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-
> 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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