the great millipede mystery

Over the past three summers, I’ve happened across the remains of several millipedes which (at least to me), seem to have expired under mysterious circumstances. The species, Sigmoria (Rudiloria) trimaculata trimaculata (Wood), is flat-bodied with segments that resemble some form of armor. Unlike the round-bodied Narceus species, these Sigmoria have far fewer legs and are not so hyperactive and rapid when on the move. I’ve videotaped Sigmoria eating damp moss and their movements are ponderous. While eating, they move their heads side to side, antennae held in a position similar to that of a brahma bull as they slowly chew at soggy moss.

Of the Sigmoria that I’ve found, the dead far outnumber the living. At first, when I found an immobile millipede, my first thought was that it might be in the process of moulting. However, closer inspection soon revealed that most were quite dead, as in the case of the above individual who, when found, was being dragged along by an overly ambitious ant.

Stranger still, most of these millipedes were found hanging over sticks above the ground, or seemed to have died in the midst of walking up a tree or over a rock — in other words, always in a very exposed location (see below). This seems odd to me, as I rarely find living Sigmoria moving in such places. Most that I’ve found have been in damp, mossy, sheltered spots out of direct sunlight.

At this point, I haven’t really got a theory for cause of death. I don’t think the millipedes were killed by larger creatures that regarded them as prey. For one thing, their remains are usually complete and undamaged, with the exception of the odd one which was in the process of being consumed by ants or similar. Also, this species is probably quite unpalatable to most animals as it can release a weak cyanide-type fluid as a form of self-defense. Further, and this adds to the oddness of the situation, on at least a couple of occasions, I’ve found two dead millipedes in very close proximity. Cause of death? For now, that remains unknown.

To view more photos of pedes, visit my millipede gallery on Pbase.

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4 Responses to “the great millipede mystery”

  1. pohanginapete Says:

    Just guessing, Bev, but I wonder if it’s the result of behaviour induced by a parasitic infection?

    Also, Nuthatch, at Bootstrap Analysis, has a recent post about millipedes. She doesn’t mention this, but I’d expect she’d be able to offer some intelligent suggestions.

  2. burning silo Says:

    That seems like a pretty good guess to me, Pete. I’ve wondered about that a bit too – if it might be something like the parasitoids that have an effect on the behaviour of spiders. The only thing that doesn’t seem to make sense is that, a couple of times, I’ve found dead millipedes a few centimeters apart. That seems a little odd to me. Anyhow, for now, it’s one of those good natural history mysteries!

    Thanks for the link to the Bootstrap Analylsis post about millipedes. I went to it, then followed a link that took me to more interesting millipede pages where I found further information that I’ll have to investigate when I have some time. However, today is sunny and the snow is melting, so I’m off to do some hiking!

  3. Liz Says:

    Hi there,

    Any insecticide used in the area or nearby? Millipedes tend to be detritovores i.e. they graze through leaf litter and like you said mossy stuff. If there is any rain, it may wash nearby pesticides into areas that collect water and are therefore usually damp and therefore usually attractive to millipedes.

    The parasite thing is interesting. Most parasites have more than one host in their life-cycle. The millipede could be an “intermediate host” i.e. host to a parasite life-stage before adulthood. It has been shown with other organisms that infection by pre-adult parasites causes the intermediate host to change their behaviour in ways that make them more likey to get eaten by the final host of the parsite.

    For example, mud snails infected with flatworm life-stages don’t bury themselves as well in the sand as uninfected mud snails so they are more likely to get eaten by wading birds- the final host of the flatworm.

    Maybe a dead millipede hanging over a branch is just too tempting, even though it might taste bad?

  4. burning silo Says:

    Hi Liz,

    Insecticide would probably be unlikely as the various millipedes were found in three different conservation areas — although that’s not to say it’s impossible. I’m thinking that the parasite angle might be the most likely answer. Ineresting about the mud snails that don’t bury themselves as well, etc… – bev