Small Whirligig Beetle
March 18, 2012 was an unusually warm day (the temperature was around 75F), so I went out with an insect net to see what could be found. And I found this little (about 5-6 mm long) black beetle swimming in a large puddle out back. The net wasn’t really designed as an aquatic net, but it worked well enough to catch the beetle and transfer it into a small jar of water. It preferred to float on the top of the water while careening crazily about like some demented motorboat, making it really hard to get a sharp photograph.
But, once out of the water it became comparatively subdued.
The hind legs are big paddles, which are what it uses to scoot around so briskly on the surface while it skims up algae and organic debris for food.
Whirligig beetles can dive, but they prefer to skim around on top of the water. They are pretty hard to catch without a net, because their eyes point upwards so they can see you coming, and the paddles let them dash off in a practically random direction.
At the same time, they can see underwater because their eyes point downward, both to find potential food sources and to spot predators.
At this point, one would be justified in wondering, “How can the eyes point up, and down, at the same time?” Well, the answer is pretty easy: these beetles have four eyes! The pair on top are optimized for seeing things in air, while those on the bottom are optimized for use in water (which has a different refractive index, and therefore requires different optics for clear vision).
Whirligig beetles are pretty common. There will often be whole fleets of them zooming around on top of the water. And aside from the difficulty in catching them, I understand they taste bad, too, so they don’t get preyed on much. Overall, they are harmless, and very amusing to watch. Particularly the bigger species, which I will need to see about catching some of at some point.
 Evolving from 2 eyes to 4 eyes is a lot simpler when you have compound eyes than when you have camera-style eyes like vertebrates do. With compound eyes, they consist of all the tiny individual facets, which can be put together into any shape that suits the environment. In the case of these beetles, they could have started with normal round eyes, partly above the water and partly below. But the facets that were actually at the water line wouldn’t have been able to see anything, so there would have been selective pressure to shift the emphasis to the parts of the eyes that were well out of the water, and to the parts that were completely under the water. Over time, the middle part of the eye was lost, and voila! Four eyes!