Naturespeak A naturalist's view of the world

November 18, 2009

The Case of the Shivering Moth

Filed under: Uncategorized — wykes @ 5:22 pm

I will admit that the title of this blog sounds like one straight from the casebook of Sherlock Holmes – as in the “Case of the Creeping Man,” etc.  But this title instead refers to a case that has already has been solved and doesn’t require the services of that legendary sleuth from Baker Street. We know why the moth shivers, my dear man (as Sherlock would have put it).  He shivers to generate heat so that he can fly on a very cold day.  However, the very fact that there are moths that can fly on very cold days is in itself one of those mysteries of life that never fails to amaze, wouldn’t you say Dr. Watson? I therefore ask you to be the good doctor in this scenario and respond by saying “yes, Holmes, I guess it is a curious case of thermodynamic magic.”

It is a fact that there are a select group of moths living in the northern temperate zone that are actually active in the cold season and inactive during the warm season (see example in photo above). This frigid gang of Nacht-Vlinders , generally referred to as the Winter Noctuiids, completely reverse the normal life cycle of their fellow moths. They emerge as adults in the late fall and  live through the winter. Their eggs are laid and produce spring caterpillars but these ‘pillers “estivate” (or hibernate) through the summer and don’t finish up their business until the following fall. All of this is leading to the reality that these moths need to fly when the temperatures drop well below 50 degrees. In order to do so, they engage in thermogenesis.

Take a look at this video and you’ll see one of these winter moths preparing to fly on a recent 32 degree morning. I believe it is a type of Sallow Moth (see below). Don’t ask me exactly which kind of Sallow because I don’t know. It’s not that I don’t care, but I just don’t know and frankly haven’t had the time to track it down. In my defense, these things tend to be a bit non-descript in appearance and, …anyway, never mind we are getting off point here. This moth needed to get his body temperature up to around 86 degrees F before it could fly. That is the required minimum for flight muscles to work – it is also a 54 degree difference from the ambient air.

The necessary heat, in such a situation, can be generated by intense bouts of shivering – aka thermogenesis. On fairly warm days, it only takes a few minutes to get up to snuff, but there’s a whole lot of shaking needed as the temperatures approach the freezing mark. It takes nearly 30 minutes of shivering to reach the proper core temperature when the ambient air is 32 degrees F. Remarkably, these creatures are able to restrict the heat build-up to the wing muscles muscles only. Through a special set of internal vessels, the thing is able to keep the thermostat down in the abdomen, head, and feet. Because the body is covered with “hair” (actually modified scales called pile) the precious heat is retained.

Unfortunately, I never got a chance to see if this individual flew off. The morning sun beckoned me to other tasks. I’m confident he warmed up sufficiently enable himself to at least move into some protective shelter. Oddly enough these winter moths fly mostly in the evening when the sun is unavailable to provide any additional heat. As winter advances and the temperature falls well below the 30’s, our Sallow Moth will eventually retreat to the cover of the forest leaves and hibernate through the coldest months. Even winter moths can’t produce internal heat when the air is below 30 degrees F or so.

In the Case of the Shivering Moth, then, it’s all done with shivering  muscles, my dear Watson – the most elementary of means.


  1. I guess you could say “there’s a whole lotta shakin’ goin’ on”, eh? 😀

    Comment by Ellen — November 19, 2009 @ 2:22 pm

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