Blue Damselfly Mating Swarm

2018 September 29

On July 7, 2018, we decided to go up to Boston Pond to go canoeing. When we got there, Sandy happened to notice something unusual. It seems that some of the low bushes near the shore were covered in little blue damselflies. They are a bit hard to pick out, so I circled the ones I could distinguish in this picture.


Looking a bit closer, we could see that many of the blue damselflies were coupled up with green damselflies. This was clearly a mating swarm, using the bush as a landmark and roost so that the males and females could find each other.


In these pairs, the blue ones (which would be the males) are holding the green females by the neck with the claspers at the tips of their abdomens.


The mechanics of how this is supposed to work are a bit puzzling. The male presumably needs to get his sperm to her eggs somehow, but she lays eggs from the tip of her abdomen, and he presumably produces sperm from the tip of his, so how do they do this? I found this video which shows the process in some detail (for a different species, but I expect it works the same way):

So the idea is that he holds on to her and they fly around together until she is ready to mate. Then he produces sperm from the tip of his abdomen, and transfers it to his mating organ at the front of his abdomen. She then curves her abdomen around, and picks it up from where he put it. So, not exactly direct, and it definitely requires her full cooperation in order to make it work.

Anyway, these are apparently American Bluets, like the one I posted back in February. They are mostly-blue damselflies in the genus Enallagma. And they are clearly very, very common locally (and working hard at becoming more common still). Interestingly, whenever I see their nymphs in a body of water, I never see any mosquito larvae, so I think I know at least one thing that they like to eat.

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