We received a very short question a while back about worms that smell bad and shed their skin. More precisely, we were asked “what kind of earthworm sheds its tail and sprays a strong smell in defense?” So, the reader is wondering about earthworms specifically (or technically the tail of earthworms) that produce some sort of odor in self-defense. Regardless of our reader’s interests, though, we suspect he is misusing the word “earthworm.” For reasons we will spell out below, he seems to be describing a millipede, or potentially some type of caterpillar, both of which molt (shed) and can smell bad.
Despite his use of the word “earthworm,” we don’t think our reader is actually interested in earthworms for the simple reason that earthworms don’t molt. In our experience, they also don’t smell like much of anything, nor do they emit a “strong smell in defense,” although they can secrete chemicals that will deter predators from eating them. Millipedes, on the other hand, do molt and emit a strong smell as a defense mechanism. (Indeed, we have dedicated an entire article to the question of why millipedes smell bad.) So, again, our reader seems to be thinking of millipedes, not earthworms.
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There are, however, a couple of things that complicate this suggestion. First, millipedes don’t molt only their tail. Rather, they molt an entire layer of their outer skin, which they often eat to preserve its chitin, the main component of the exoskeletons of arthropods. Partway through the process, we suppose it might look as if a millipede is only shedding its tail, but in fact it is discarding an entire layer of skin. (It isn’t clear if the reader saw some sort of creature that shed its tail, or if he had merely heard of some creature that did this. If the latter, perhaps someone didn’t accurately describe the molting process of millipedes.) It is also worth mentioning that caterpillars molt, going through on average about five or six instars during their lifetime when they shed their outermost layer of skin (this is one reason caterpillars can eat so much food), and several species of caterpillar will excrete foul-smelling odors as a defense mechanism. Thus, what is true of millipedes is true of caterpillars, so it is hard to say which type of creature our reader is interested in.
In any case, though, he does not seem to be after any sort of earthworm, as earthworms do not have the characteristics that our reader ascribes to them. Some millipedes and caterpillars have these characteristics, though, and since these creatures are somewhat worm-like in appearance, we suspect he is talking about one of these two creatures.
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