[Odonata-l] dragonfly measurements

[Dennis Paulson]

Hello, all.

Writing field guides to the dragonflies of North America, I wanted to  
include measurements of each species to indicate its size, especially  
to facilitate comparisons. I thought total length and hindwing length  
would be sufficient to indicate odonate size. I considered measuring  
10 males and 10 females of each species in my collection, then  
calculated how long that would take me to do for 450+ species.  
Conclusion: it would be ever so much easier to find an authoritative  
published source of such measurements.

So I turned to my library. I discovered that pretty much all the  
recently published guides had just copied their measurements from  
earlier works. I traced measurements back from a number of recent  
guide books to earlier guides and then to Walker's Odonata of Canada  
and Alaska. I couldn't find earlier sources, so perhaps Edmund M.  
Walker actually measured lots of specimens. Good for him, if that was  
the case! Most references list the total range of measurements they  
recorded, so a species might be listed with a total length of  
46.5-60.5 mm (male Archilestes californicus in Westfall & May), which  
doesn't give me a very precise idea of how large it is, although I  
assume it's smaller than Archilestes grandis, males with total length  
of 50-62 mm. Walker did not measure Archilestes, so the figures in  
Westfall & May came from someone else, perhaps the authors of that book.

So I thought I would check my specimens of these two species. I  
measured 13 male californicus, range 49-56, and 7 male grandis, range  
51-60, so my figures match those from the literature quite well  
although not encompassing the entire range, and I assume measuring  
more specimens would have expanded these ranges. The mean of my  
californicus measurements is 52.2, of grandis 56.1. So grandis  
averages about 4 mm longer than californicus, and I decided that is  
what I wanted to point out in my books, so I will include a single  
measurement rather than a range. One thing I discovered from this  
exercise is substantial geographic variation. A. grandis from North  
Carolina were distinctly smaller than any from the West, and  
additional specimens I measured from the mountains of southern Mexico  
were larger than any I have from the US.

The point of this message is twofold. (1) We need measurements of  
Odonata species from adequate-sized series that list both ranges AND  
means. This has been done surprisingly rarely, even in some of the  
superb generic monographs that have appeared in recent years. (2)  
Such measurements should be grouped geographically and perhaps  
altitudinally, as indicated by these Archilestes. For another  
example, Ladona julia in the western part of its range is  
substantially larger than in the East. In some cases measurements  
might be grouped in time as well as space, as we know that some  
species vary in size seasonally (Pachydiplax longipennis is the best  
example I know).

Most of you are interested in birds and know that bird field guides  
also include indications of size. Some of the older ornithological  
monographs, for example Birds of North & Middle America, included  
lengthy sections on measurements of wing, tail, bill, etc., usually  
in geographic clusters. For a book I was writing on the wildlife of  
Alaska, I was surprised by the variation in length measurements in a  
variety of bird field guides, so I decided to take my own  
measurements of all the common birds of Alaska by measuring the  
length of a study skin I considered just right for that species. It  
turned out that my measurements deviated by as much as 15% from those  
in some popular guides. I think this is because some of those  
measurements have been copied for years (but obviously not by all  
authors), from a time when total length measurements of birds were  
made by stretching a dead specimen as far as possible and measuring  
from tip of beak to tip of tail. Most birds aren't stretching their  
necks like that when we see them! Fortunately, dragonflies don't have  
much of a neck to stretch.
-----
Dennis Paulson
1724 NE 98 St.
Seattle, WA 98115
206-528-1382
dennispaulson at comcast.net



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