Thursday, October 22, 2015

Purple Ground Beetle

I found this purple ground beetle, Dicaelus purpuratus, running on the gravel of our driveway at Bull Mills.  They are a Great Plains species found between Illinois and Arizona inhabiting fields and deciduous forests.  Considered infrequent but not rare, they are valued by beetle collectors.

Powerful snail-crushing jaws
This is a predacious ground beetle which specializes in snails.  Many beetles eat snails by reaching in the opening to pull out the innards but this beetle comes equipped with mandibles powerful enough to crush the shells.  Their larvae live under logs or in the duff and also consume snails.

Members of the Dicaelus genus are felt by many to be the most beautiful beetles.  It is interesting to see the color variations between copper, purple, black and green as illustrated in Bugguide.  The color changes partly with the direction of the light beam and I had trouble capturing the color I was seeing with my camera.

Beetle belly with a mite attached to its right hind femur
Many beetle species carry mites on their body, either to their benefit in a symbiotic relationship as in carrion beetles, or as parasites.  They are common on ground beetles although to my knowledge we don't know exact association in D. purpuratus, but there is one seen clinging to the right rear femur above.

Their elytra are hardened forewings, covering hind wings that are no longer functional for flight.  They escape danger by expelling a defensive chemical out their anus, a trait shared with their larvae.  It is described as smoke-like or dark colored liquid with formic acid which would burn a predator.  I can attest to the odor as this specimen left a large amount in my specimen box which I had to throw away.
  
This beetle was shown to be an early example of the converse Bergmann principle.  Back in 1949 this paper in Physiological Zoology showed that Dicaelus purpuratus specimens are larger as you head south. This gets a little deep for us laymen but either bear with me or bail out now.
"According to Bergmann's rule, body size increases with latitude, a temperature effect. According to the converse Bergmann rule, body size decreases with latitude, a season length effect."  oxfordjournals.org
The common application of the better known Bergmann's rule is that mammals tend to be larger in Northern climes (think grizzly and polar bears) where increased body size and fat reduce their relative surface area to weight ratio, helping them maintain body temperature to survive harsh winters.

The newer (to me) converse Bergmann's rule is seen in some arthropods where the species are larger in their southern range.  This is frequently attributed to the longer growing season which means more resources and time to consume them.  And that is where our purple ground beetle comes in.  They are larger in southern states than they are in Canada.

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