Saturday, December 11, 2010

Agabus grisseipennis

Three weeks ago, in the middle of November, we had a warm spell in Fresno and the thermometer rose to the high 70's. I took a walk outside to enjoy the day and was surprised by a singular raindrop as I passed a large puddle by the road. The sky was clear and yet I was certain that something had fallen from above into the water. As I stopped to see what was going on, another drop landed on the puddle but this time I could tell that it wasn't water. It was a beetle.

To be more precise, it was a diving beetle (Agabus grisseipennis) a fairly common diving beetle of the Western US. It was obviously taking advantage of the warm day but I was curious about the temporary pool. Why would the predatory beetles be landing in a temporary puddle in November. Obviously this was no place to complete a life cycle.

Then I noticed the dragonflies - lots of them. They were variegated meadowhawks and at one point I noticed ten mating pairs in copula bobbing up and down over the water - the females dropping eggs when the pair got close enough to the puddle. Some dragonflies species are known to drop eggs in temporary ponds or by the side of streams in moist soil. The eggs can survive out of water for quite some time until water returns and covers them up and they hatch.

Then I noticed a backswimmer (a true bug of the genus Notonecta) drop into the water, and I started to figure out what was going on. Both the diving beetle and the backswimmer are predators and I suspected that they were attracted by the dragonflies - and the chance of eating their eggs.

The picture of the backswimmer is a bit out of focus but it is unusual enough that I'm posting it anyway. Backswimmers live almost all of their lives underwater with their bottom (ventral) side up. When they swim, they kick their legs somewhat like a human swimmer doing the backstroke - hence their name. I have been around ponds and puddles for about 30 years watching insects and this is the first time I ever remember seeing a backswimmer resting on top of the water with its backside up. I even found one individual resting beside the puddle on moist soil in the same position. It seemed that these insects were behaving in a peculiar way because of an opportunity - a warm autumn dragonfly fest.

These sorts of entomological spectacles have to be appreciated when you come across them. You may never see the likes of them again. Sadly, they often happen and nobody notices. I saw a couple of people pass the puddle and not notice a single insect. I left for a couple of hours and when I returned the buzz of activity was over. All I saw was a lone backswimmer resting on the muddy bottom - backside down.


Anonymous said...

Interesting - I've never seen a backswimmer on top of the water, much less dorsal side up.

Anonymous said...

Posts like this will help others learn to pay more attention to their surroundings - you have to understand before you can appreciate. I am learning more about insects every day - just learned more from your post. Thanks!

Sam Wells said...

My pleasure. I have to wonder how many natural spectacles I miss myself.