Inchworms, which get their name from their peculiar form of movement, are the caterpillar or larvae form of several species of geometer moth. This type of moth gets its formal name Geometridae from two Greek roots that refer to the visual spectacle of inchworms "measuring the earth" as they move forward. They are also variously known as loopers, spanners, or measuring worms. There are 1,400 species of geometer moths in North America, with a total of 35,000 different types worldwide.
The peculiar movement style of inchworms is the result of its lack of legs in the middle portion of the body. Inchworks have two or three paired sets of legs on each end of the body, but none in the middle. As a result, inchworms propel themselves forward by drawing the back end upward to form a loop shape, then reaching forward with the front portion of the body. When disturbed by a potential predator, inchworms often freeze in place so they resemble a still twig.
Inchworms are generally hairless and have smooth bodies. The color can be brownish, green, gray, or black depending on the type of moth that the inchworm larvae will become. Some species have black spots on the head. Inchworms tend to be camouflaged from predators by blending into the surrounding environment.
Most species of inchworms primarily eat leaves of deciduous or coniferous trees, but at least one species is know to be carnivorous. Many species of inchworm, such as the cankerworm, are considered pests by farmers and gardeners. Farmers sometimes use the pathogen Bacillus thuringiensis to treat inchworm infestations. Natural predators include some types of birds and wasps.
The adult geometer moth can vary in size but typically is about 0.39 inches (1 cm) long, with some species ranging up to 2 inches (5 cm). Unlike many other types of moths, which tend to fold their wings on top of the abdomen when at rest, adult geometer moths keep their wings spread erect after landing, similar to a butterfly. The adult moth is generally noctural, but some species do operate during the day.
In each species of geometer moth, one generation is born annually after adult moths lay large egg clusters during the winter season. The eggs, which are gray and shaped like cylinders, can typically be found on tree limbs. After the eggs hatch in the spring, the inchworm larvae eat leaves for up to five weeks before being ready to pupate. They then create silken cocoons in a shallow layer of soil.