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 ship 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?