1. What Inchworms Usually Eat, and the Surprising Exceptions
"Inchworm" basically means the larva of a moth in the Geometridae family. This family is thought to be the second largest among butterflies and moths, with nearly 24,000 species known worldwide [1]. Because this number is an estimate, treat it as a rough guide. With so many species, their diets are likely varied.
Their food is plants, typically tree leaves [2]. However, what they eat depends on the species. Some are "polyphagous," meaning they eat many different plants. Others are "monophagous," eating only a single plant species [1]. An example of a polyphagous species is the Ascotis selenaria moth (known in Japan as yomogi-eda-shaku). It eats grasses from the Asteraceae and Fabaceae families, and trees from the Rosaceae and Theaceae families. It is also considered a pest on fruit trees and tea plants [1].
Eating leaves to grow is what we expect. But does what they eat change what the larva looks like?
2. How Spring and Summer Generations of Oak Inchworms Differ in Looks
For one species living on oak trees in North America, the generation born in spring eats oak flowers (catkins) and grows to look like them. The generation born in summer, after the flowers have fallen, eats leaves and grows to look like small twigs [3].
Is it the season or the food that decides their shape? A 1989 study by Greene tested this. Larvae were raised in two groups: one fed only flowers, and one fed only leaves. Those fed flowers all developed the flower-like shape. Those fed leaves developed the twig-like shape [3]. Flowers are said to have low tannin levels, while leaves have high levels. When researchers fed artificial flower food with added tannins, most larvae developed the twig-like shape [3].
This result suggests that the amount of tannin might be the trigger that switches their shape. However, this is only a possibility based on a study of one species. It cannot be said that this applies to all inchworms. Are there other species that look like flowers or twigs?
3. Many Inchworms Look Like Branches When They Stay Still
It is known that many inchworm species look very much like plant branches, leaves, or new buds when they are still [1]. The larvae from the previous section that look like flowers or twigs are examples of this.
Some larvae in the subfamily Geometrinae are said to carry plant fragments on their backs. So instead of relying only on their own color and shape, they wear plant pieces for camouflage [1]. However, not all species look like branches. Many do, but some do not [1].
How do these long, branch-like bodies move?
4. How Inchworms Move with Only a Few Pairs of Legs
Most butterfly and moth larvae have five pairs of prolegs (belly legs). Inchworms, however, have only 2 to 3 pairs near the tail [2]. Japanese sources state that, with exceptions, only the rear two pairs (on abdominal segments 6 and 10) remain, while others degenerate [1]. Differences in counting methods between sources are likely due to differences between species, but we will consider "mostly two pairs" here.
To move, they grip with their thoracic legs (chest legs) and stretch their body out. Then, they pull their prolegs up right behind the thoracic legs, making the body form a U-shape (an arch) [2][1]. They repeat this motion to move forward. The name "inchworm" is said to come from this posture resembling the gesture of measuring length with the thumb and index finger [1].
Safety Note: When observing insects in nature, do not touch hairy insects with your bare hands.
So far, we have discussed larvae that eat leaves. Are there inchworms that do not eat leaves?
5. Hawaiian Inchworms That Ambush and Eat Flies
In the genus Eupithecia, there are several species known as "fly-catching geometer moths" that ambush and eat insects. These are known only from the Hawaiian Islands [1]. Ambush predation among butterfly and moth larvae is considered extremely rare [1].
These larvae wait, blending in with branches or fern leaves. When prey touches the sensory hairs near their tail, they are said to bend the front part of their body backward and catch flies with spiny front legs [4]. A 2002 article introduced at least 18 species [4].
Staying still among branches looks similar to the "branch mimicry" in section 3. However, from the sources listed here, we do not know if this comes from the same way of using the body. To get closer to the original source, look for Montgomery’s 1983 paper in the journal GeoJournal on the behavior, biogeography, and conservation of Hawaiian Eupithecia [5].
6. Activity: Copy an Inchworm's Walk with Your Fingers
Today’s activity: Mimic the walk with your fingers. Spread your thumb and index finger, then bring them together, then spread them again. The movement where your fingers form a "U" shape is thought to be the origin of the name "inchworm" [1].
Activity when you go out: Look for branches. Gently look at branches of leafy trees. Search for insects that look like parts of the branch without touching them. Do not touch hairy insects with your hands. Twig-like shapes are hard to find, so it is not strange if you do not find any.
If you want to know more, check field guides or insect books at home to see what larvae eat besides leaves. If you want to read papers, Greene (1989) wrote about larvae switching from flowers to twigs, and Montgomery wrote about Hawaiian ambush predation (both in English).