Why carrion beetles shlep a heaving carpet of mites around on their backs

Here's a fascinating and squicky insight into the marvellous symbiosis between carrion beetles and the mites that live on them, courtesy of Greg Laden:

From the carrion beetle's perspective, there is a strong possibility that there are already fly larva (maggots) on the carcass eating it when it arrives to lay its eggs, or soon after. This situation is probably made worse by the fact that house flies are quick and seem to be able to travel long distances, while beetles are slower and travel less far than flies in a given period of time. The only way carrion beetles will ever be able to raise their young in this scenario is to do a better job of accessing or using the mouse carcass than the fly does, so any variation that arises in carrion beetles that facilitates this will be strongly selected for.

This is known as the life/lunch dichotomy. In the competition for the use of the meaty carcass of a dead mouse, if the fly loses out it gives up the equivalent of lunch ... there are still other opportunities, in this case, poop, for it's young to eat. The carrion beetle, however, may be giving up its life (or the life of its offspring, really) because mouse carcasses are very rare, so if the one carcass it manages to locate is eaten up by fly maggots, it's offspring will not survive.

This is where the mites come in.

The carrion beetle pays a huge cost carrying the mites around wherever it goes, because they are heavy and affect its ability to move and fly. But otherwise, the mites do nothing .... they just hang on for the ride, waiting for the beetle to locate a dead mouse. Then, when the beetle does located a dead mouse, the mites do not eat it. Rather, they eat the maggots, the fly eggs, and larva of anything that is not a carrion beetle. They clean the carcass of the potential competitors of the carrion beetle's larva.

Strange insect encounter: Carrion Beetle with Mites


  1. So the beetle is basically a heavily armored troop carrier, a vast star destroyer loaded with tie fighters, a Lovecraftian juggernaut crawling with flesh-eating minions. Wicked.

  2. yo, that is so awesome. the beetle puts itself under such pressure just to ensure the success of its offspring. who knew

  3. This is quite a good metaphor for the current political situation in the UK. With the current coalition government, the Conservatives are the carrion beetle, and the Lib Dems are the mites. When the “beetle” is confronted with an election, the “Lib Dems” get rid of the Labour competition, allowing the Tories to win.


  4. This is truly great.

    I wonder what the exact trait is that the beetle evolved in order to get her cape of fighter-mites? This symbiotic relationship must be two-ways, or the mites would never bother latching on. Maybe the beetle excretes some substance that is particularly delicious to mites?

    1. Isn’t the free, unlimited bus ticket to food the main benefit to the mites? Doesn’t that make it symbiotic? Or does there need to be some other direct benefit for it to count as symbiotic?

      Pretty cool, but imagine waking up with a bunch of those in your bed?

      Not that it’s very likely considering not everyone who reads boinboing has carrion in their bed on a regular basis (Prostitutes can be fragile).

    2. Actually, many mite species hitch rides on beetles (and other invertebrates) just to get around (try walking long distances on those short legs). This is called phoresy. It’s highly probable that these mites started that way and quickly adapted to exploit the added bonus of an excellent food source.

  5. Seriously, to abuse the possessive apostrophe, not once, but TWICE!
    And from a professional writer, too (not that I don’t make my own egregious typos, but still…)

  6. Fascinating!

    Reminds me of the Amaryllis Azure, which lays its eggs on mistletoe which grows on a tree that has been colonised by little black ants of the genus Papyrius.

    When the eggs hatch, the ants provide shelter for the caterpillars in their nest at the base of the tree or under the bark. The caterpillars are shepherded by the ants to a clump of mistletoe at nightfall to feed and then led back to their shelter at dawn.

    See http://showcasejase.blogspot.com/2009/03/mistletoe.html

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