Crawlies with Cri: The Inchworm

Inchworm (Family Geometridae)

This week we’re going to meet a group of crawlies with a singular name. Meet the inchworm (Family Geometridae).
Inchworms aren’t worms; they are caterpillars of geometridae (AKA geometrid) moths. The family got their name from the caterpillars. “Geometridae” is from the Greek geo, meaning earth, and metron, meaning measure because they appeared to be measuring the Earth with their looping movement as they inch along.
While many inchworms look similar – it’s their niche to look like bits of twig – there are 1,441 species of inchworm (thus geometrid moth) in North America. Nearly all inchworms are small and most are brown or green with minimal body patterning.
Some have quite intricate patterns, but their size makes them look “plain” to the naked eye. A few species stray from the “earth tones” color palate and stand out in shades of bright yellow, red or pink.
The “inch” portion of the inchworms’ name comes not from their petite size, but from the unique way they move – approximately one inch at a time.
They are built like all other caterpillars, with six legs near their heads and prolegs at the back. The prolegs act as suction cups allowing the caterpillar to hold on to a firm surface and use the rest of their muscles to move.
While some caterpillars have as many as five pairs of prolegs, inchworms just have two. This creates their “arch-stretch-arch-stretch” walk. They use the prolegs to hold themselves steady, then arch up and out to gain ground, then release the prolegs and pull them up to their real legs, rinse and repeat.
This unique movement has given inchworms a few other nicknames too: spanworms, loopers, and measuring worms.
Having only two sets of prolegs also allows them to stand up and “look like a twig” when they feel threatened.
Adult geometrid moths largely lean toward earth tones as well. Many species are “plain” looking shades of brown with speckled markings. However there are many bolder colored and patterned ones as well.
There are black and white species, several vibrant green species (collectively called “emeralds”) and species that are bright yellow or orange.
Adults vary from small to medium in size and have broad wings. Some of the species have wingless, thus flightless, females.
Nearly all geometrid moths are nocturnal and are attracted to lights, so you may find them around your porch light in the morning. Several species are on the wing throughout the winter months so you can spot a geometrid any time of year.
Most geometrid moths hold their wings out like a butterfly when resting. Also, many have wavy line markings similar to our pictured moth, which line up when the moth is at rest.
If not resting by an outdoor light during the day, geometrid moths have excellent camouflage skills, so look sharp on flat surfaces in your yard, from foliage to man-made materials and you may spot an adult snoozing the day away.
Final fun fact: Geometrid moths’ tympanal (hearing) organs are in their abdomens.