Tuesday 18th November, 2008

treeblog's cleft-headed looper (Biston betularia) - larva of the peppered moth

By Ash

A couple of posts back, in the most recent grey alders and Scots pines update, I published a couple of photos of a rather spiffing caterpillar pretending to be a twig on treeblog’s grey alder No. 1. That was on Friday the 7th of November. A week later on Friday the 14th, when I was taking photographs for the cider gums update, I saw that Bud-head (for so I called the caterpillar, after its spectacular bud-mimicking head) was still on alder No. 1. What a beaut! But when I went to check on old Bud-head a couple of days later on Sunday, it was gone. Vanished! Nowhere to be found!

Bud-head on Friday the 14th of November.

What species was Bud-head? I wondered. What would it look like as a moth or butterfly? I didn’t hold much hope that I’d ever find out, after failing to identify the other caterpillars that called the grey alders home this year – see these posts from October and August. But after opening my Insects of Britain and Northern Europe Collins Field Guide, I found what I was looking for pronto. Bud-head is a Biston betularia (L.) – a peppered moth (or, when in caterpillar form, a cleft-headed looper). I was pretty impressed. Peppered moths provide one of the best-known examples of survival of the fittest – probably the most taught example in British education establishments! For those who aren’t in the know…

There are two forms or morphs of peppered moth; one light-coloured and the other dark-coloured, or melanic. Before the Industrial Revolution, most of the peppered moth population was made up of the light-coloured form. Yet once the Revolution kicked off, the dark-coloured form rose to dominance. The accepted theory puts this change down to air pollution. All of the soot and smoke kicked out by the dirty industries of Britain coated buildings and trees. The light-coloured moths used to have a good set of camouflage, but on the blackened, sooty surfaces of the Industrial Revolution they stood out like sore thumbs. At the same time the melanic form fitted in nicely; they suddenly had the superior camouflage. So while their lighter brothers were sitting ducks for predators, the dark-coloured moths thrived. Nowadays, after the decline of our heavy industries, there is less soot and smog in the air, so guess what? The lighter-coloured form is making a comeback.

I’ve just whacked all that down from memory, but if you do some digging, I’ll bet there are some good papers out there with the science to back it all up. Incidentally, judging by its colouring Bud-head is probably of the melanic form (f. carbonaria). Another interesting fact (lifted from the Collins Field Guide):

The name of the family [Geometridae] means ‘ground-measurer’ and is derived from the behaviour of the caterpillars. These are generally long and slender and they have only two pairs of prolegs… When walking they grip the substrate with the prolegs and then stretch out, as if measuring length, to find a hold with the thoracic legs. Having found a hold, they draw the prolegs up close to the thoracic ones and in doing so they throw the body up in a loop – leading to their common name of the loopers.

The Collins also adds to my light-dark story. Apparently the melanic form was first reported in 1848 (the Industrial Revolution took place around the late 18th and early 19th centuries). And:

It is believed that the larvae of the melanic form are hardier than those of the normal moths in the presence of slight air pollution – insufficient to blacken trees and walls. Industrial melanism occurs in many other moths, and in some other groups of insects as well, but in recent years there has been a noticeable drop in the numbers of melanic individuals as a result of smokeless zones in many regions.

Bud-head from behind - now probably gone off to pupate in the soil for overwintering.

Read about the peppered moth on Wikipedia.

Update (26th November 2008): I've had it brought to my attention that this post would be improved by including a picture of the peppered moth. I'll go one better and include two.

Probably the intermediate form, Biston betularia f. insularia. © naturalhistoryman (Flickr).

The melanic form, Biston betularia f. carbonaria. © naturalhistoryman (Flickr).

tags: caterpillars + grey alder + info + photos

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