Passalid Beetles

© University Museum of Zoology, Cambridge 2013

© University Museum of Zoology, Cambridge 2013

Dr William Foster, Curator of Insects here at the museum, writes:

The Passalids, or Bess Beetles, are one of my favourite insects. They live deep inside rotting logs in tropical rainforests, and the main reason I like them is because they are so smooth, chunky and handsome. They live in small family groups, and only rarely emerge into the daylight. They are quite hard to find, but it is a huge thrill to open up a log and suddenly find a family of about 20 large clean red and black beetles, together with their young larvae, bumbling around in the damp wood and sawdust. They are highly social animals. The larvae and adults sing to each other, and some people claim that they have the widest song repertoire of any animal. The adults sing by rubbing their abdomen against their wing cases. The larvae have only 4 walking legs; the two hind legs are modified as small paws that are scraped against the thorax to produce a range of sounds.  The Bess Beetles are probably the most social of all the beetles. They always live in family groups, and probably help to feed each other. The larvae are not able to make their pupal cases on their own, and they rely on help from other adults in the colony to pack sawdust around themselves to make a solid case: without this protective case the pupae will die.

We studied several colonies in mountain forests near Chiang Mai in Thailand. We were trying to see whether the colonies had a single reproducing female and male, with all the other adults being sterile “workers” as in termites, but this seems not to be the case. All the adults stay with the family until fully grown, but they all appear to be able to reproduce and none of them were sterile.

The photograph shows part of a draw of Passalids from the Insect Room of the University of Zoology. The different species all look very similar, with sold tank-like bodies and complex comb-like antennae. They do differ in size and this is quite a good way of trying to tell the different species apart. I think that one day we will find a species that lives in really large family groups and is fully social, like termites and ants, with a single reproducing queen and king.

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