Friday, October 13, 2017

Mite-y Bess Beetles

Click to enlarge
I came across several of these beetles when a rotted piece of firewood broke apart.  Their distinctive appearance makes them easy to identify as a horned passalus (Odontotaenius disjunctus),  a.k.a. Bess beetle or patent leather beetle for its shiny body.  It is a member of the Passalidae family.  It is unique in the family in its ability to stand freezing temperatures.  Unlike many insect species which are hard to separate, the horned Passalus is the only species in the family which occurs in the US north of Texas and Florida.

The horned passalus is the "largest showy beetle in the US." *  "The pronotum (back plate of the thorax) is square with a deep middle grove, separated from a deeply grooved  elytra by a deep waist."**   It has powerful mandibles capable of chewing through oak but do not bite and can be safely handled (if handling beetles is your thing).

Bess beetle ventral view - note the hairs and brown bumps under the "chin"
The horn on the head extends above the eyes and is quite obvious in a side view of the head.  There are fine golden hairs on the middle pair of legs, around the edges of the pronotum, and on the antennae.  Newly emerged beetles are a deep orange-brown but soon turn all black.

Newly emerged from its pupa
These beetles and their families live their entire lives in well rotted wood.  I call them families because they are a rather tightly knitted group.  The white grub-like larvae cannot digest wood by themselves and are fed with food which has been chewed by the adults including their parents or previous generations.  Like termites and anything else that eats wood, they require bacteria in their gut to digest the cellulose.  They acquire these bacteria by eating the feces of the adult beetles.  The larvae won't develop in sterilized rotted wood.

Adults communicate by stridulation, squeaky sounds made by rubbing body parts together.  They have at least seventeen different calls, more sounds than any other known arthropod.  I can picture a patient graduate student with a tiny microphone hovering over a rotten log, wondering what they are saying to each other.  I suspect they are saying, "Doesn't this highly educated, advanced biped have something better to do?"   Check out their stridulation at this link.  Recorded by Will, Kaiden and Hilton at the WOLF School. ****  

A Bess beetle larva (grub) is a prodigious eater, taking in lots of wood and passing most of it out, a virtual frass factory as seen in this video.  These grubs can even make their own sounds, rubbing their tiny stub of where the back leg should be against the rough area on the back of the second leg.  The tip of the reduced leg has a variety of teeth so the grub can create many different sounds.

You may have noticed the brown bumps below the chin in a photograph above.  These are mites that are frequently carried underneath the head and between the legs, occasionally climbing up on top for some fresh air.***  Many beetles carry mites as passengers.  This is particularly true of beetles that live underground or in dead vegetation. We discussed this in the blog about Sexton beetles

You "mite" be even more interested in the beetle/mite association.  If so go to Macromites Blog.  Yes, there is a group that is passionate about mites, an acquired taste to be sure but still a fascinating subject.

Like a few other beetles, when on their back on a flat surface they seem to be unable to roll over without help.  I suspect that if you spent your entire life in rotted wood chambers with something to hang on to, you would consider learning to roll over a waste of time.  When I put it back in its log, I thought I heard it squeak out a "Thanks."
A comprehensive resource is at this  University of Florida site. 
 *      Beetles, Peterson Field Guides
**    National Audubon Field Guide to North American Insects and Spiders, p. 555.

***  "Passalids almost invariably have associated mites - several families of mites are found ONLY in association with passalids, and many genera and species are similarly restricted - suggesting a very long association between the two groups.  I know of no mites that are harmful to the passalids, although there may well be some (e.g. tracheal inhabitants)."  (
**** WOLF is a Springfield Public Schools choice program in partnership with Wonders of Wildlife and Bass Pro for 5th grade students focusing on nature studies and conservation.