What type of small, green worms hang on silk threads? A reader recently sent us this question after observing some worms “floating” outdoors (and, to be fair, small worms that hang on thin threads of silk often do look like they are floating around). They didn’t mention where they found the worms, but since the reader seems to be describing inchworms (sometimes written as “inch worms”), there is a good chance they were hanging from oak trees. So, what exactly is an inchworm, and why do we think this is the creature our reader found?

Inchworms are actually not worms at all. Instead, inchworms are the larvae of geometer moths, which makes them caterpillars. (That’s what caterpillars are: the larval form of moths, or butterflies.) Geometer moths are more precisely known by the name of the family they constitute, Geometridae, which belongs to the order Lepidoptera (and by extension the class Insecta, the phylum Arthropoda, and the kingdom Animalia). The family Geometridae is very large, as it contains over 35,000 species of moths, of which 1,400 are indigenous to North America. For this reason, inchworms vary widely in appearance, including color, although they tend to be around one centimeter to one inch long.

The main type of inchworm that produces silk is the cankerworm. There are two types of cankerworms, fall cankerworms (Alsophila pometaria) and spring cankerworms (Paleacrita vernata). Both species live in North America and feed on deciduous trees and shrubs. Because of their tendency to feed on plants, they are often considered pests (as are other types of inchworms), at least if they show up in large enough numbers. Oak trees in particular seem to be susceptible to this type of inchworm, but they are also happy to damage other types of shade and fruit trees by consuming foliage. After consuming the foliage of a tree, cankerworms drop to the ground on silk threads. Our reader presumably saw cankerworms during this stage of their life cycle, which comes right before the pupate stage (i.e., the stage when they rest and slowly develop into moths).

Check out our article about inchworms producing silk if you are interested in learning more about these creatures.

• 
•