Mosaic Darner #3: Black-Tipped Darner (Blue-Green Stripes and Blue Spots)

2012 March 10

I originally thought these were pictures of the same dragonfly as was in Wednesday’s post, but looking more carefully, I realized that (1) they were taken the next day, on July 19, 2010; and (2) this dragonfly had pronounced blue spots running down the abdomen. So, I don’t remember taking these pictures either, but it is clearly a different individual, and probably a different (but closely related) species.

It still has pretty much the same body shape, (lack of) wing markings, and general color pattern as the previous two Mosaic Darners, so I think it is safe to say it is in the same genus.

The most striking difference is the double row of bright blue spots running all the way down the abdomen. Incidentally, I think that this one is a male, because in addition to the two leaf-like appendages on the tip of the abdomen there is also a central “finger” that would allow him to grab onto a female dragonfly.

I think that this one is probably a Black-Tipped Darner, Aeshna tuberculifera. That one has the best match of color and exact pattern of markings, and the most similar face coloration.

And, of course, here is the obligatory face shot. For once, I got the eyes in focus, so that you can see the individual eye facets:

Let’s have a closer look at that eye:

The tiny size of the facets gives some idea of the visual resolution of the dragonfly eye. Basically, the smaller those facets are, the higher the resolution, allowing the eye to see more detail. At the same time, extending the area of the eye increases the angle of coverage. So, with those tiny eye facets the dragonfly can pick out tiny details to quickly identify prey, while the fact that the eyes cover practically the entire head gives it pretty much 360 degrees of visual coverage. It occurs to me that the nerves coming from the many individual facets may very well outnumber the neurons in the dragonfly’s actual brain. Which makes me wonder whether the eye facet neurons might not be a lot of the brain, doing a lot of the visual processing right there in the eye itself.

So, anyway, dragonflies don’t miss seeing much. Their one visual weakness is that they are mainly sensitive to sideways motion. So if you want to creep up on a dragonfly for a good close look, you want to slowly move straight in, without even the slightest sideways motion. If you are careful, and don’t cause any sudden brightness changes by letting a shadow fall on them, they generally won’t even register your presence.

You know, given that I have now caught three Mosaic Darners, and all three of them turned out to be different species, I really have to wonder how they sort themselves out for mating. Since they are all flying at about the same time of year, they must actually be able to tell each other apart based on fairly subtle differences in color patterns or behavior (or maybe scent, although given how tiny their antennae are, that doesn’t seem likely). Either that, or some of the distinct “species” are actually hybrids.

4 Responses
  1. March 10, 2012

    Tim, this is fabulous stuff. Links and postings are on their way.

  2. March 11, 2012

    Thanks! I remembered that you’ve mentioned being fond of dragonflies in the past, so I thought you might like these.

  3. March 11, 2012

    Wow. You just made my day, Tim. That was so generous of you!

  4. March 12, 2012

    Glad you like it. I’ll be sure to get some more dragonflies and damselflies this summer for you.

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