Friday, August 18, 2017

Rove Beetles

Rove beetle, 12 mm Philonthus caeruleipennis - REK
I recently encountered two different species of rove beetles, members of the Staphylinidae family.  This is the largest beetle family with over 63,000 species worldwide, an ancient lineage that has been around since the Triassic 200,000,000 years ago.  Question, where do you find someone to count 63,000 species?  I suspect it is graduate students.

Earwig, note pointed cerci - Wikipedia
Rove beetles most obvious characteristic is their short wing covers (elytra) that don't reach the lower half of the abdomen like other beetles.  They resemble earwigs until you notice they lack the pointed cerci pincers at the end of the abdomen.
Ventral view of Philonthus caeruleipennis - REK
They can range up to 1.4" but most are less than 0.4".  Both of my species measured 0.5" (12mm).*  Most of the common species live in decaying plant and leaf litter, under stones and bark or around the margins of streams and bodies of water.  The Philonthus caeruleipennis pictured above was found in a decaying Oyster Mushroom, a common association for the species according to Eric Eaton.  It is commonly found in forest, marsh and prairie litter (often riparian), and sometimes under the bark of logs.

Typically rove beetles have short elytra covering small wings that are seldom used.  Many have evolved to be flexible, able to used their narrow bodies to crawl into narrow places or to shorten their bodies to reduce their surface area to avoid dehydration.  The increased exposed surfaces and junctures in their body means they lose moisture more rapidly.  They overcome this by staying under humid detritus, leaves, and bark.  

Gold-and-brown Rove Beetle (Ontholestes cingulatus) - L Bower 
The advantage of hanging around for millions of years is the time to evolve to live in a wide variety of niches.  There are a wide variety of shapes and colors as demonstrated by Linda Bower's find to the right. 

Specialized lifestyles means that they can lose some now redundant body parts and save the energy needed to grow and use them.  Some species that live in soil and caves have lost their eyes.  Others no longer need long distance transportation to maintain their species and no longer have wings.  

As you might expect with such a large number of families, there is a wide variety of foods that various species specialize in.  While many are saprophytic including fungi feeders, others are predatory (fly larvae and even mosquitoes) and a few eat only plants.  To get even, the Laboulbeniales order of fungi are obligate ectoparasites, living on rove beetles, mites and millipedes.  They get their nutrition from the animal while not usually causing significant harm to the host.

Rove Beetle - Homaeotarsus sp.
My other rove beetle can only be identified down to the Homaeotarsus genus and even to that level I can't find any more details about its lifestyle.  With 63,000 species in the rove beetle family I suspect it will be some time before all of them get fully studied, more work for future graduate students.

Movie monster or rove beetle? - REK
* I hope that the metric conversions don't bother you.  Most scientific articles use metrics so if we are going to continue to read about nature, we need to get used to it.

The University of Florida has a good in depth discussion on rove beetles in this PDF.