[Odonata-l] black in dragonflies

[Dennis Paulson]

Bob,

I think we should at least keep in mind the possibility that black is  
an important color for heat absorption for thermoregulation. If you  
take a latitudinal sample of odonates, you find that the proportion  
of dark species is higher at higher latitudes (I think Philip Corbet  
wrote about this in one or more of his books, and I'll bet Mike May  
has written about it also). Look at the genera that are  
characteristic of high latitudes that are partially or largely black,  
Somatochlora and Leucorrhinia. In fact, corduliids in general fit  
this picture, being mostly a northern group. Neotropical corduliids  
are dark but tend to be up in the mountains. And of course tropical  
odonates could benefit from heat-absorbing colors too.

Sympetrum danae, the only black Sympetrum in North America, has the  
northernmost average range of any of our Sympetrum. Interestingly,  
there is another black Sympetrum high in the Venezuelan Andes. It's  
interesting that Aeshna doesn't show this trend at all, and the more  
northerly species may even have more blue, less black on the abdomen.  
Libellula quadrimaculata could have evolved blackness, but instead  
its cuticle seems to be especially thin and almost transparent,  
perhaps another way to enhance heating during insolation.

I haven't looked at all species, but this trend is probably just as  
significant intraspecifically as interspecifically. in western North  
America, for example, Enallagma boreale and annexum have a higher  
proportion of black on their body in cooler climates, sometimes much  
higher; they are amazingly dark in Alaska. I've noticed that annexum  
on the Pacific coast is also more heavily marked with black than in  
the drier, warmer interior. A thorough analysis of this variation  
would be of great interest correlated with climatic factors.

I did such a thing for Macromia magnifica. Both it and Cordulegaster  
dorsalis, species with a wide climatic range in western North  
America, have their darkest populations in the wettest parts of their  
ranges. Subspecies have been proposed (actually as distinct species)  
in both species to acknowledge this substantial variation.

It would be interesting to do a latitudinal analysis of odonate  
color, not yet done to my knowledge. Such a study done for birds some  
years ago was of great interest. I did such an analysis for red  
coloration and found, contrary to conventional wisdom, that red color  
in libellulid species is no more frequent in the tropics than the  
temperate zone. There are black odonates in the tropics, of course,  
but they are not especially common. I can think of males of a few  
species of Erythemis and Erythrodiplax. Interestingly, there are  
relatively few black-and-yellow ringed and spotted species in the  
tropics, so that's not a strategy everywhere. Perhaps this is merely  
because there are few neotropical cordulegastrids and no macromiids.  
The species that fly up and down tropical streams are more likely to  
have an all dark abdomen with pale spots on segment 7. That probably  
also enhances conspicuousness.

I am impressed by Tom's assessment of the situation, which seems very  
reasonable. I assume he intended to write detectability, not  
detestability, as I know he likes odonates.  ;-)

Finially, it's worth keeping in mind that there are a lot more avian  
predators on odonates in the tropics, and this is another latitudinal  
gradient to keep in mind when thinking about the adaptive  
significance of odonate colors. No one has kept a pet jacamar and  
given it a choice of different-colored dragonflies!

Dennis

On Dec 4, 2007, at 10:37 AM, Bob Glotzhober wrote:

> I am looking for data, opinions, thoughts and ideas on why so many  
> dragonflies have black as a major color. This initially hit me last  
> summer when a couple of us locally were working with 3 plus species  
> of Macromia on the Big Darby Creek near Columbus. Since then, I’ve  
> been invited to talk to a bunch of folks as a part of our state’s  
> wildlife diversity conference – and the entire conference theme is  
> “Back in Black”.  Obviously, an emphasis will be around the  
> resurgence of black bears in Ohio, but also other “black” wildlife  
> including cormorants, black vultures, the (melanistic) Lake Erie  
> Water snake, and others – including black dragonflies.
>
> While my emphasis will be on the diversity of such critters and  
> their ecology, I want to spend a little time discussing the thought  
> of why so many are black. To launch things (my apologies to Tom) I  
> am inserting a communication from Dr. Tom Schultz at Denizen, who  
> has worked on colors in Odonata before. This is a basic good start  
> – and maybe it will stimulate some further thought and discussion  
> within this group. I eagerly await to hear not only hard data but  
> off-the-wall theories as well.
>
> Here is Tom’s correspondence:
>
> “Bob,
>
> I have given this a little bit of thought.  The combination of  
> yellow and black is as conspicuous as any Odonate coloration can  
> be, especially if the visual system of the insect is trichromatic  
> with sensitivities to UV, blue and green (like bees).  The striping  
> provides important contrast that enhances detestability; consider  
> the difference in how C. erronea looks compared to S. linearis when  
> each patrols the same shaded stream.  In dark forest understory  
> black and yellow would be the color pattern of choice. In light  
> flecks, yellow is a conspicuous color  because it reflects the  
> filtered green-yellow light without matching the green background.  
> However, when flying in and out of light flecks the broken pattern  
> of Cordulegaster should make it hard to detect.
>
> The eyes may be part of the color pattern that  these dragonflies  
> use to attract mates or identify themselves, but they should also  
> boost the reception of the wavelengths they reflect (e.g. green).   
> No research has really been done on the roles of color in odonate  
> eyes, but it is a reasonable guess that the emerald green eyes may  
> function like a tapetum in vertebrates.
>
> There are only three studies using electrophysiology to determine  
> the color vision of an odonate species.  Two bluets are know to  
> have receptors for UV, blue and green.  It has been shown that one  
> red libellulid species has an additional  red photoreceptor.
>
> Tom”
>
> Bob Glotzhober
>

-----
Dennis Paulson
1724 NE 98 St.
Seattle, WA 98115
206-528-1382
dennispaulson at comcast.net



-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mailweb.ups.edu/pipermail/odonata-l/attachments/20071204/c2bb5811/attachment-0001.html