There are over 1,200 species of Geometridae moths in North America – a group better known by their so-called “Inchworm” larvae. Because there are so many I feel justified in not identifying the particular inchworm pictured here. The exact name really doesn’t matter in this case anyway. This group of larvae are well known as twig mimics and this individual was such an incredible example that I felt compelled to bring it to your attention right away (in other words – hang the I.D.). Whatever it is isn’t as important as whatever it isn’t!
Upon prepping some twigs for a recent campfire I came upon this tiny inchworm clinging to one of the branches. I wouldn’t have noticed it at all if my disturbance hadn’t prompted an un-planned movement on the part of the caterpillar. Upon regaining composure it resumed its perfect twig imitation and once more vanished before my eyes. The camouflaging skill of this fellow is successful due to two levels of mimicry.
Like all inchworm larvae, this one is typified by a long slender body and a distinct lack of legs at mid-body. There are three pair of prolegs next to the head, two sets at the hind end, and a long legless gap in-between. Most inchworms are cryptically colored to blend into leaves, bark, or twigs. This one is patterned in twig mode with mottled gray speckling and a clever set of fake bud scars located about a third of the way down the body.
This color pattern alone would serve well enough but an additional behavioral step improves the ruse to the point of near perfection. Make-up is only part of an actor’s skill. Upon disturbance, the caterpillar stiffens out and holds its body at a low angle away from the twig. The prolegs are gathered tightly into a bundle under the head to give the overall impression of a terminal bud and the creature remains frozen in place.
One might be tempted to believe that a green leaf might spring from the head of this living twig which isn’t.