Copper underwing caterpillar - 6 true legs and a bunch of false ones.

You can see the caterpillar's true legs at the front (left) and its prolegs (mid and rear).

While walking one of the trails between bridges along the North Saskatchewan River in Edmonton, I happened to see an interesting caterpillar.  It was unusual in that there was a noticeable hump at its back end (to the right in the image).  This is a defining characteristic of the Copper underwing caterpillar, which will become a moth after it undergoes pupation.

I got an excellent side image of the beast, one that clearly shows all of its legs along the left side.  Caterpillars all have six proper or true legs and a number of false ones, often called prolegs.  The true legs are typical arthropod legs formed with joints (arthropod means joint-footed).  The false legs function through fluid movement (hydrodynamically) and have no joints as such.  Also, the true legs develop into the legs found on the adult of the species while the prolegs disappear completely.

The last pair of prolegs are sometimes called claspers.  Also called anal prolegs, these offer a powerful grip that allows the developing insect to reach across distances, to pivot from their current position, and as an anchor when it is time for the final moult into a pupa.  Like the other prolegs, they are shed in the adult.

Copper underwing caterpillars are not pests and feed upon a wide variety of plants.  They are commonly found throughout much of North America in deciduous woodlands.  If you want to see images of the adult, or read more about the species, click here.

Thanks for reading.

Eric Svendsen     www.ericspix.com