In my last post, I talked about the dragonflies that my fiancee and I saw on our recent photographic trip to Sweetwater Wetland in Tucson. We took a lot of photos of dragonflies and as I went through them, it occurred to me that there is an important topic related to identifying dragonflies that I have not covered so far in my blog: odonate polymorphisms.
If you’re like most people (including me until I’d been an entomology grad student for almost a year), you’re now thinking to yourself, “what is a polymorphism?” You probably already know more about polymorphisms than you might think! First, let’s take apart the word to define it. The root “poly” means many. Think back to geometry and polygons – a polygon is a shape with many sides. The root “morph” means shape or form. So, the word polymorphism effectively means “many forms.” Form can refer to shapes, colors, sizes, and other characteristics of biological organisms that might vary between stages, sexes, or individuals within the same species.
You probably know about a few species that exhibit polymorphisms already. Birds are a classic example of sexual dimorphism (di = two, so dimorphic species exhibit two forms): males take one form while females take another. In birds, the males are often one color while the females are another. In most cases, the males attract the females, so the males are the more colorful, showy individuals. In general, the more brightly colored males are healthier and better able to produce strong offspring with a high chance of survival, so females will choose mates that are brightly colored over less colorful males.
As an example, consider peacocks. Peacock males are VERY showy with their long, elegant tails while the female peahens are much less colorful and have much shorter tails. Peacocks with the biggest, baddest tails get all the girls while the less showy males have to settle for leftovers (sometimes younger or less healthy females) or simply cheat their way into getting a mate.
Other animals that show dimorphisms are deer and elk. Again, the males are trying to attract the females and are willing to fight other males for them. Elk or deer with big racks are generally better able to successfully fight other males. As in birds, the buck with the biggest rack is likely healthier than the bucks with smaller racks – they have to be getting enough food and other resources to grow those antlers in the first place. The doe chooses her mate from the available males and usually selects the one that is best able to win fights, the one with the best antlers. Because the females are not fighting amongst themselves and are not trying to attract the males, a doe doesn’t need antlers. So, some elk and deer have horns (the males) and others do not (the females). They are also sexually dimorphic.
Now let’s get back to the dragonflies. Like in birds, elk, and deer, it is the role of the male dragonflies to attract female dragonflies if they wish to produce offspring. Thus, male dragonflies are often much more brightly colored than the females and many species are sexually dimorphic. Take, for example, the blue dashers, or Pachydiplax longipennis. If you follow my blog, you’ve seen this one before, but here he is again in all of his elegant glory:
Blue dashers are very common mid-sized dragonflies across a big section of the United States, including Arizona. The males are easy to identify based on their bright green eyes and the bluish coloration of their bodies. The abdomen is covered in a waxy substance, or prunescence, which can give them a bit of the whitish look you see in the male pictured here. Blue dashers are perchers, so you’ll commonly find the males sitting on emergent vegetation or on bushes and/or other plants alongside lakes and ponds. They sit and guard their territories from their perches, waiting for females to come into their areas so they can mate. In contrast, the females are known to spend a much greater part of their time away from water and only come to the water to mate and lay eggs. I found this female sitting on a tree branch far from the water:
Can you see how different the female looks compared to the male? The males are a whitish bluish color while the females are largely black! The female blue dashers are about the same size and of similar shape compared to the males, but they have very different body colors, making this a dimorphic species. Female blue dashers are also very easy to identify. Just look for a black abdomen with yellowish or brownish stripes on each section of the abdomen. The abdomen tends to be a bit stumpy compared to the male abdomen, so the wings look disproportionately large. In fact, this is the origin of the species name longipennis, which means “long winged.”
Blue dashers are considered dimorphic because there are two main forms, but they are not exactly sexually dimorphic either. Odonates are not sexually mature when they molt from nymph to adult and require a period of a few days to complete their maturation. Immature individuals, including the males, look like the females. So, the mature males are blueish and the immature males, immature females, and mature females all tend to look like the picture of the female.
Many dragonflies and damselflies follow similar patterns. Green darners (Anax junius) are actually polymorphic:
As you can see in the photo, the male (the dragonfly in front) is green and blue while the female is green and green-brown. Immatures of both sexes can have a reddish abdomen and the females can have brown sections on their abdomen. Sometimes the females even have about the same blue on their abdomens as the males! Because the green darners have so many different color patterns, they are considered polymorphic.
Not all dragonflies exhibit dimorphism or polymorphism. Some dragonflies are monomorphic (mono = one) such that all indviduals look about the same, regardless of sex or age.
Next time I’ll post some more photos from the Sweetwater trip and go over how to identify them. I hope you’ll stay tuned!
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