We received a question a few days ago about small worms that spin a “thread like line from vegetation.” (The worms’ threads are probably found mainly on trees, and hence our title, but they could be on other plants as well.) The reader lives in Arizona, and only sees the thread-spinning worms at night, or at least this is when the worms are believed to be creating their threads. (The reader is ambiguous on this point.) The reader was only preoccupied with identifying the small worms, so we’ll limit ourselves to this matter, only concerned with the question “what are the small worms that spin threads and hang on trees?”
Although the reader really only gave us one piece of information to work with – the worms she found spin threads – this is quite telling, as there is one very common creature that spins threads and hangs from vegetation: inchworms. Inchworms are not actually worms, but rather caterpillars. More precisely, they are the larval form of geometer moths, which make up the Geometridae family. There are tens of thousands of species of geometer moths in the world, and well over a thousand are indigenous to North America. They come in a variety of colors and sizes that help them blend into their natural environment, and some have distinguishing body features (like humps or stripes on their backs). Without more information or a picture, it is hard to help our reader identify exactly what kind of inchworm she found, although we are reasonably confident she did in fact find some type of inchworm.
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That said, there are a couple of different inchworms that are especially common and known for the threads they produce: fall cankerworms and spring cankerworms, or Alsophila pometaria and Paleacrita vernata, respectively. (To try to keep names straight, it is worth noting that some people use “cankerworm” as a synonym for “inchworm,” but it is probably more common to use the word more restrictively to refer only to the two species of inchworm just mentioned.) Both fall and spring cankerworms are found all over the United States, and thus could be found in Arizona. Cankerworms will feed on the foliage of trees and shrubs, and when they are ready to pupate, they will drop to the ground by rappelling with a thin silk thread of their own creation. (The threads are sometimes deployed to escape predators as well.) Fall cankerworm moths emerge in fall, and spring cankerworm moths emerge in spring, and hence their names. However, both types of caterpillars live simultaneously, emerging in the spring and eating their way through the summer until they are ready to pupate, and thus our reader could have found either species, assuming she found cankerworms at all.
This last point is important because while there are good reasons for thinking our reader found a cankerworm, we are far from certain this is the case. We have too little information to make a definitive identification. As we said above, though, we think it fairly likely our reader found some sort of inchworm, and perhaps this is all she wants to know. If she wants more specific information, she can look into what kinds of inchworms live in her part of Arizona.
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