Evolution of Bioluminescence in Millipedes

Walk deep into a rainforest at night. Switch off your headlamps. And wait with open eyes. At first, it is so pitch black that you cannot see your own hand if you wave it in front of nose (as Bilbo might have said). As your eyes get accustomed to the darkness, you will realize one thing. Everything glows. Everything. There is fine fuzzy layer of bioluminescent fungus covering dead leaves and the bark of trees, so you can almost make out the forest like some one has traced it out in ghostly yellow-green outline. Little ghostly yellow-green mushroom cap clusters are found here and there. And in certain places, much brighter spots are moving very slowly: millipedes crawling on vegetation. For years I've wondered about the functional utility of these different forms of bioluminescence. The mushrooms caps? Surely it cannot be to attract insects? Or warn off predators (after all, why visually call attention to yourself in the first place when nobody can see you)? My (admittedly, in those days, naive) literature searches yielded nothing. I speculated that the only explanation that made sense was that the bioluminescence was a side-effect of some other metabolic process, and in a place where visual channels were not economical to exploit by predators, there was no cost to it. It seems that I was totally wrong! But that is the fantastically cool thing about science. It is precisely when you are wrong that you learn the most!

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