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

[George L. Harp]

Dennis mentions the well-documented change in size of Pachydiplax with season.  Although I haven't measured the first one, this phenomenon is also quite apparent in Ischnura posita, at least in Arkansas.  Although I know it is going to happen, the first I. posita in the spring always surprise me because of their relatively great size.  By September, they are really getting small.  I hypothesize this is because during the winter (low temperatures), much more energy is expended in growth, rather than development.  At higher temperatures, not only is development more rapid (Law of Q10?), but relatively more energy is channeled toward development.  I once raised larvae of a species of Chironomus at five different temperatures.  Those reared at 25C began emerging at 90 days from hatching and all had emerged by day 200.  Larvae beginning to pupate were ~ 10 mm in length.  At the other end, no larvae reared at 5C had emerged by day 210, but they were HUGE compared to the largest larvae in the 25C aquarium.  I forget the difference in weight, but it was at least 50-fold, and length was approaching 20 mm.  There was some literature at the time reporting similar trends in a few other species of dipteran larvae.
 
George Harp
 
 
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