Nature's wierdos. The oil beetle!

The European Oil Beetle
God may have made little green apples, but why on earth would He have made Meloe proscarabaeus, the European oil beetle, or its cousins elsewhere in the world?   He must have been downing a bit too much ambrosia before going into the workshop.  If you don't believe us, here from Wikipedia is one example, of an adult.

The European oil beetle was the honoree on the BBC radio program, The Living World, the other day.  The county of Devon is home to the 4 species of oil beetle found in Britain.  There were once at least 9 species in Britain, but 5 have become extinct as the acreage still in meadow has shrunk.  (Does that mean we've passed peak oil?)  The remaining four species are now found only in southwest England.  Large -- centimeters long -- and oily black with a violet sheen, they are still easy to find if you know where to look.

"It looks like the sort of creature invented by Roald Dahl, really," said the radio presenter.



These guys are called oil beetles because, bizarrely, they produce an oily substance from their joints.  It's a toxic substance and, at least in American species of oil beetle, it is strong enough to blister the skin.  Why?

Larvae on a celandine flower; photo by John Walters

These beetles have a complex position in their ecological niche.  And their life history is interesting.  They're a 'phoretic' insect, that is, their way of getting around is to hitch a lift on other insects. In the autumn, the female lays her eggs in the ground.  Come spring, the larvae hatch and wander around until they find a flower, often celandines, at lest in southwest England, which they then swarm around on, waiting for insects to come along.

When the unsuspecting stand-in helicopter lands on the flower, the larvae jump on and grab hold with their well-developed claws.  They'll grab on to any insect, but the larvae that survive are the ones that find their way onto solitary female bees, because the larva then gets a ride back to the bee's nest where it feeds on the bee's eggs.  Once the larva has destroyed the eggs, it then has the nest and all the nectar and pollen to itself.

These beetles have two pupal stages, the pre-pupa and actual pupa.  They emerge from the pupal stage in autumn, but stay underground until spring, when they start the cycle all over again.  The radio program suggests that different species live exclusively on particular host insects or flowers, but there is a range of variation in all of these oily-critters' habits, each species with somewhat different habits.

Why?
Of course, the standard explanation for all of these exotic features is natural selection--what else?  The fact that each member has its own distinct niche, its own distinct life-cycle stages, specific host, and so on, will not damper such explanations.  But natural selection means that only those who landed on the right bee species or flower, or whatever, survived to reproduce.  Yet, such specificity seems not to have been that important, because the species are so diverse--why would selection  refine each species in such narrow ways? And why such weird traits -- their non-functional wings, their wing-cases, their oily knees, and the fact that they are entirely dependent on chance meetings with the right insects, and so on -- as discussed in the BBC program?

One answer might be that the specificity of natural selection is far less than is usually supposed.  Some insects may have just been on this particular flower, visited by this particular host, and so on, and that's where they found their mates (as their neighbors on Flower Lane, so to speak).  Others had similar kinds of experiences on Flower St, Beehive Avenue, and so on.  They diverged genetically and so on, but perhaps there was no major element of differential reproduction.

What about non-functional wings and short wing-cases?  Are they adaptive specifically for something?  Are they on their way out?  Since there are insects with fully functional wings and others with lost wings, Darwin himself would probably have said they were on their way to loss.  That's how he viewed various barnacle traits, but it's a kind of teleological view--that Nature was heading them towards some specific endpoint, and that in turn is both pure speculation and contrary to fundamental aspects of the modern theory of evolution (based on random mutation and local screening for fitness).  The latter would seem unlikely because the trait seems conserved among these slippery-kneed beetles, and not particularly variable as one would expect for a dysfunctional trait (mutations don't harm it).  Similarly, evolving functional differences are always assumed to be due to selection, selfish genes and all that; but this is also contrary to the standard adaptationist doctrine. 

There are other explanations, of course, including the possibility of fine-tuning natural selection.  But in that case, it must have been very local and variable and it shows in a sense that all the beetle lineages needed was some host, transport, etc. environments, whatever happened to be available in each local place and time, rather than very specific functionally critical requirements. Each species needed its own street, taxi service, and the like.  But in a sense any street or taxi would do.

This more flexible view of evolution may be more realistic than classical views, which may require rather stringent and consistent kinds of nearly deterministic selection.  Something to think about, at least, because Nature's Weirdos are widespread among animals, plants, and even single-celled species.  And if weirdness is the rule, each instance doesn't require a specific fine-tuning adaptive explanation, or such applications even if factually correct (almost impossible to prove) will be rather trivial.