Inchworms are actually caterpillars.
Named for their peculiar way of moving, inchworms are the caterpillar or larval form of several species of geometer moths. This type of moth gets its formal name Geometridae from two Greek roots that refer to the visual spectacle of earthworms “measuring the earth” as they go. They are also known as measuring loops, wrenches or worms. There are 1,400 species of geometer moths in North America, with a total of 35,000 different types worldwide.
The peculiar style of movement of the worms is the result of the lack of legs in the middle part of their body. Inchworks have two or three pairs of legs at each end of the body, but none in the middle. As a result, the tracks are propelled forward by pulling the rear part up to form a loop shape and then extending forward with the front part of the body. When disturbed by a potential predator, the caterpillars often freeze in place, looking like a stationary branch.
Inchworms are generally hairless and have smooth bodies. The color can be brown, green, gray or black, depending on the type of moth the caterpillar larva becomes. Some species have black spots on their heads. Inchworms tend to camouflage themselves from predators by blending in with the surrounding environment.
Most caterpillar species primarily eat leaves of deciduous or coniferous trees, but at least one species is known to be carnivorous. Many species of caterpillars, such as the locomotive, are considered pests by farmers and gardeners. Farmers sometimes use the pathogen Bacillus thuringiensis to treat caterpillar infestations. Natural predators include some types of birds and wasps.
The adult geometer moth can vary in size, but is typically about 0.39 inches (1 cm) long, with some species up to 2 inches (5 cm). Unlike many other types of moths, which tend to fold their wings on top of their abdomen when at rest, adult geometer moths keep their wings outstretched after landing, similar to a butterfly. The adult moth is usually nocturnal, but some species operate during the day.
In each species of geometer moth, one generation is born annually after the adult moths lay large groups of eggs over winter. The eggs, which are gray in color and cylindrical in shape, can usually be found on tree branches. After the eggs hatch in spring, the caterpillar larvae eat the leaves up to five weeks before they are ready to pupate. They then create cocoons of silk in a shallow layer of soil.