Whirligigs in a Ditch

It’s May Day and Darwin is back at what he does best – observing the natural world. I find it ironic that after several days of dining with the British bigwigs in Rio, today he is back looking at animals living in a ditch on the side of the road. His short entry today states:

“Worked at a host of fresh water animals with which every ditch abounds.” (May 1)

So what exactly did Darwin find in the ditch? I imagine there were lots of fun critters to capture and preserve, but one entry from early May in his Zoological Journal states:

“They are not however, so numerous as in England.— Gyrinis frequent & might be seen dancing on the surface of a clear ditch; forcibly bringing to the recollection of an Entomologist his walks at home.” (Zoological Diary)

Now Gyrinis seems to be an old-world genus, but what Darwin is describing here is a member of the family Gyrinidae – the whirligig beetles.

Gyrinidae are found all over the world, and the over 700 species look fairly similar (certainly enough so that I could not ID them at the species level). The 2 long front legs are used for grasping prey. The four back legs are adapted for swimming – which whirligigs are really good at doing. In fact, it is this rapid, erratic swimming around (often in circles), that give the little beetles their name.

Gyrinis natator (from Edmund Reitter’s “Fauna Germanica: Die Käfer des deutschen Reiches” (1909) via Wikipedia Commons)

Whirligigs live on the water surface and feed off of floating prey and detritus. Interestingly, they also tend to clump into groups which have a complex “social structure” with rules for determining who gets to be in the middle and who gets to be on the outside of the congregation. One source suggests that whirligig behavior is even an excellent model for programming nanobot behavior!

Whirligig video clip (posted to YouTube by grcapro)

Another link to a video lesson on whirligig beetles.

Now as adaptations go, whirligigs have a pretty cool one. Each eye is divided into two parts – located on the upper and lower part of the body. This means that when they are floating on the water surface, one eye is in the air looking up and the other is in the water looking down. How cool is that!

Whirligig (Gyrinus sp.) eye (from the Tree of Life Project)

Whirligig’s also have the ability to trap a bubble of air under their abdomen when they dive. That bubble serves as a little oxygen tank – allowing the beetle to stay under water for a long time and still breath. And you thought humans invented scuba tanks….

Darwin was a huge fan of beetles, so expect to see more posts on them in the future. It is only fair, as the species in the order Coleoptera (beetles) number in the 100’s of thousands – roughly 1/5 of all the identified species on Earth! (RJV)

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