A while back, a reader asked us which types of millipedes emit a horrible odor when threatened or stepped on. (Technically, the reader asked about “minipedes,” but we’ll assume this was a typo, unless they really are interested in knowing the nonexistent smell of an online site that sells baby shoes in the UK.) In other words, which millipedes smell bad when threatened or attacked in some way. This is a simple enough question, but we’ll have to answer it in somewhat general terms because there are around 10,000 species in the Diplopoda class, which millipedes make up, so we of course can’t list every single millipede that smells bad (nor in truth do we know of all the millipedes that smell bad). So, we will focus on the following, more general question: why do millipedes smell bad?
First, we should note that a bad smell is somewhat relative concept. What offends the nostrils of some will not bother others. For instance, there is a species of millipede called the “almond-scented millipede” (Harpaphe haydeniana) that smells a bit like bitter almonds, and while some might find this difficult to tolerate, others may not. Disagreements over the unpleasantness of smells are also tied to the sensitivity of one’s olfactory perception. Certain smells might be endurable if detected faintly, but they would rise to the level of repulsive if the smell became strong. That said, there does seem to be general agreement about the types of things that smell bad, and one of the things that commonly smell bad are millipedes.
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Actually, an undisturbed millipede may not have a discernible odor at all. As the reader alluded to, it is primarily when millipedes are threatened or attacked that they emit chemicals or liquid secretions that smell bad. For this reason, the unpleasant odor of millipedes is a direct result of one of their main defense mechanisms. Millipedes do not bite or sting, so they have to protect themselves through a chemical defense. (Millipedes are also known to coil up tightly, another primary defense mechanism employed by the creatures.) To some insect predators, these secretions can be devastating, as the chemicals can eat through exoskeletons. To humans, though, the effects are not nearly as dramatic (in part because we do not have exoskeletons to be eaten through). These secretions can cause skin irritation for some people, and they can also cause pain if they manage to get into a person’s eye. Other than that, though, the chemicals primarily just smell bad, and that’s why millipedes have a reputation for emitting an unpleasant odor.
Unfortunately, we cannot compile a comprehensive list of all the millipede species that smell bad – that is perhaps a task best left to a patient scientist or grad student – but we can say that it is a common defense mechanism, so there is nothing unusual about a foul-smelling millipede. On the plus side, they generally don’t smell bad if they remain undisturbed, so if you leave them alone, they’ll likely leave your nose alone as well.
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