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Thursday, July 18, 2013

Male and female gliders?

I know I just posted about Spot-winged Gliders, but there are several reasons to talk about these dragonflies again.  I mentioned that I had seen quite a few recently, but now I think I can say that this is the largest number I've ever seen on Bodega Head.  They seem to be everywhere right now, especially feeding on the lee side of shrubs.

On the southern end of Bodega Head this afternoon there were quite a few perching on the vegetation close to the ground.  Spot-winged Gliders are very active fliers, so whenever you can see them perched, it's a great study opportunity. 

The following picture shows a view with three perched individuals — can you find all of them? 


They're all in the center of the image, two at the top close together, and one near the bottom.  Here's a close-up of the top two individuals:




The other reason to post about Spot-winged Gliders has to do with some questions I now have about how to identify males and females.  Sometimes it's fairly easy to identify male and female dragonflies either by color (as in birds), or by looking at their reproductive parts (sometimes visible in the field, either with binoculars or after catching them and viewing them with a magnifying lens). 

In this case, it's not so easy.  Male and female Spot-winged Gliders have similar coloration, especially when they're young.  And it turns out that their reproductive parts aren't that helpful.

When you read about sexing Spot-winged Gliders in field guides, you often don't receive much guidance.  You may read that adult males have red faces, but that's only helpful if you have an adult and if you can see the face.  (Both adult females and younger males have yellow faces.)  

In Dragonflies and Damselfies of the West, Dennis Paulson mentions that females have "slightly longer and more slender cerci."  The cerci are the paired appendages pointing backwards from the last abdominal segment.  Unfortunately, I'm not sure what "slightly longer" means, and without comparing a bunch of specimens, I can't evaluate this character yet.

Walker and Corbet, in The Odonata of Canada and Alaska, say this about females: "the brown spot of the hind wing slightly smaller than that of the male."  This is interesting and worth checking out further!

When I was reviewing my pictures from this afternoon, I wondered about a character that no one has mentioned but is often a good one for female dragonflies in general: the overall shape of the abdomen.  Females often have broader, more rounded abdomens (appearing swollen), while males have narrow and often constricted abdomens.

Here are close-ups of the upper two individuals from the photo above:



Now compare the two individuals above with the next two dragonflies that I photographed nearby, and look specifically at the shape of the abdomen.



Did you think the first two were broader (more likely to be females), and the last two were narrower (more likely to be males)?  

I realize the angles are different, so this isn't a completely fair test.  But it's convinced me that the next time I see Spot-winged Gliders perched, I'll be checking the shapes of the abdomens (and the sizes of the hindwing spots) to better evaluate whether this might be a good character for separating males and females.

P.S.  I know this has been a long post.  But while reading about Spot-winged Gliders tonight,  I encountered another interesting line in Dragonflies and Damselflies of California by Tim Manolis.  He says that the veins in the hindwing spot "become lighter with age, and the spot may be obscured."  I'm a bit puzzled by this, as my impression is that these are younger dragonflies in these photos (with strong abdominal markings), but they show pale veins within the hindwing spots.  I'll have to ask Tim about this fact.

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