Cattail Borer

2013 March 9

The next of the May 26, 2012 moths is this fine fellow, with his hunchback, his broad pale stripe going back from the head, and his elongated wing spots.

This is about as good of a match as one could ask for to the Cattail Borer, Bellura obliqua. And his feathery antennae mark him as a male.

I kind of like the morose look he seems to be giving me when we zoom in on his fuzzy face:

The caterpillars are particularly known for boring into the stems of cattails, which are very plentiful in the marshy area on the northern part of our property and across the road from us. So I’d probably be able to find the caterpillars of this moth pretty easily if I looked. They also bore into the stems of a few other wetland plants, like Pickerelweed, Arrowhead, and Skunk Cabbage, although of those, cattails are by far the most abundant locally[1].

Since its food plants all grow in flooded wetland areas, these caterpillars are able to manage in water much better than other kinds of caterpillars. To the point where caterpillars in this genus are sometimes referred to as “divers”. They are sometimes found with their heads underwater, with enough of their bodies exposed that their breathing spiracles can still get air[2].

[1] In fact, I don’t think I’ve ever even seen a skunk cabbage.

[2] These moths are probably on the way to re-evolving aquatic larvae. This sort of thing goes on all the time: first an animal evolves to live on land, and loses its aquatic features (like fins and gills and streamlining). Then some of its descendants re-evolve to go back to the water, and end up re-developing similar (but not identical) aquatic features. But one thing that hardly ever[3] seems to get re-evolved is gills. Even after going back to the water, animals evolved from land-living ancestors mostly keep breathing air, even at the cost of some inconvenience. Dolphins, whales, sea turtles, sea snakes, mosquito larvae, drone fly larvae, water bugs, diving spiders – they all kept on breathing air even after re-committing to the water[4]. I think the big reason for this is that air is a much, much better source of oxygen than water is. The solubility of oxygen in water is generally less than 10 parts per million by weight, which is a tiny amount of free oxygen compared to the 21% oxygen in the air[5]. Plus, to get it, you have to move huge masses of water over your gills, which costs energy. So as long as you evolved lungs that can handle air, you can actually get more oxygen into your system by going to the surface periodically for a deep breath and holding it for an extended period (or by sending up a snorkel), than by staying submerged and trying to extract it from the water. As an added bonus, if you are breathing air then you really don’t care if there is dissolved oxygen in the water or not. So you can live in stagnant, oxygen-free water where all the gill-breathers can’t survive, happily sipping air through your snorkel.

[3] I wouldn’t say “never” because it’s a big world and evolution is nothing if not thorough in trying out all the possibilities. For example, it looks like black fly larvae have re-evolved gills, which they were able to do because they live in shallow, cold, fast-moving, well-oxygenated water. Between the good oxygen levels, and the fact that all they have to do is hold onto something while the water flows past them, they are in one of the environments where gills beat out lungs. Similarly, predaceous diving beetle larvae look like they have gills, but they live in shallow water and hold their tails so the gills are just barely under the surface, which is where the water is richest in oxygen. And even then, they switch back to air-breathing once they become adults. There is a nice summary of the various underwater breathing solutions developed by insects on this page.

[4] There are a lot of insects with aquatic nymphs and larvae that do have gills, but these are almost all insects that never had purely land-living ancestors. The ancestral insect appears to have had aquatic nymphs, and dragonflies, mayflies, stoneflies, caddisflies, and the like evidently developed from that first ancestor without ever giving up the water (or their gills).

[5] Direct comparisons of the amount of oxygen in air versus water can be confusing because of the huge difference in the density of air and water, and the need to distinguish between the quantity by volume versus the quantity by weight. It’s probably clearest if we consider the quantity by volume. On that basis, say that we have a cubic meter of air. This has a mass of about 1.3 kg. Of that cubic meter, 21% of it is free oxygen gas, for about 0.27 kg oxygen. If we have a cubic meter of water, which has a mass of about 1000 kg, then 10 parts per million gives me only about 0.01 kilograms of oxygen. So per cubic meter, air contains about 27 times more oxygen than water does.

3 Responses
  1. jrr permalink
    March 9, 2013

    Ooh, recursive footnotes!

  2. Sara permalink
    March 9, 2013

    Beautiful moth.

    Ironically, we have tons of skunk cabbage and virtually no cattails where I live.

    I had never considered how inefficient gills are, it was a very interesting footnote.

  3. March 9, 2013

    Fabulous photography. Image borrowed, link posted a bit later. As for the moth, he looks like he’s lived a charmed life. There are no marks on him that I can see. Clearly, he’s a member of the 1% in the cattail borer world.


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