Spanworm Splendor

It’s not a name that is uttered often among everyday folk, nor does it normally find its way through the parted lips of the everyday naturalist, but the Maple Spanworm is a splendid creature worth mentioning. Now is a good time to bring it up.

On chilly October mornings this medium sized moth is frequently found hanging out by back porch lights. Lured there by the siren call of the bulb from the night before, the jilted moth finds itself exposed on an open brick wall by the dawns early light. The creature stands out in the low morning sun due, in part, to her habit of perching with the wings up at a 30-40 degree angle– a habitual position which casts a long shadow across the surface – but also because flat brick is not the surface for which this pose is designed.

As an adult moth, the Spanworm is protectively colored in order to blend into the autumn leaves. The ragged wing edges and spotty orange camouflage create a near perfect imitation of a leaf (like the one pictured below). This trick only works its magic if the moth in question is perched on a natural surface. On an open flat wall, a windblown leaf caught in a spider’s web is about the best imitation that can be mustered. Fortunately, this ploy works pretty well for confused porch moths. I’ve seen a few Spanworms on the museum porch over the past week and they both made it through the day without being eaten by birds. I guess if you are a good leaf imitator, it doesn’t matter where you end up.

The whole life of the Maple Spanworm is about deception, so it’s a wonder they ever know who they really are. Members of the inchworm clan, the larvae of the spanworm are a perfect twig mimics. Sporting fake bud scars and a mottled gray brown skin color, the caterpillar looks more like a stick than most sticks do. When disturbed, the larva suddenly goes “twig” by straightening out. It stays in position until danger is past or until the real twigs around it get jealous and start to taunt it.

When not acting twig-like, Spanworm caterpillars eat a wide variety of tree leaves off of those real twigs.  Alder, ash, basswood, elm, hickory, oak, poplar and, yes, even maple are consumed with equal delight. As you can see, even the name of the Maple Spanworm itself can be a bit misleading given this eclectic menu. Notchwing Geometer, an alternate name for the species, eliminates this maple bias but introduces the need to explain what a geometer is. Officially, inchworms are called Geometers, you see. Spanworm happens to mean the same thing, so I guess this discussion doesn’t need to go any further does it. Where was I anyway?

Apart from expressing admiration for this moth, there is little left to say about the Spanworm without starting to make things up. There is only one generation per year and the notch-winged adults can be found flying from late summer into October. Those that successfully pull off their dead leaf trick, and avoid be eaten by birds or pressed between sheets of wax paper, will lay their eggs and die just as the real autumn leaves die and flutter to the ground.

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