What is it about Bembidion heads? And, in particular, what is it with the heads of one subgroup of Bembidion, the Bembidion Series? The Bembidion Series is a large clade of few hundred species, and represents perhaps a quarter of known Bembidion (that’s a bit of a guess – I’d have to add up the numbers to be sure).
Most Bembidion have heads with the dorsal surface relatively flat and simple, with two shallow grooves just inside the eyes, like thus:
Bembidion zephyrum
Within the Bembidion Series, some species have similar heads. Others have more complexly textured heads, including wrinkled ones:
Bembidion (Notholopha) sp., Chile
But within the Bembidion Series there has evolved, at least 5 times, heads with converging, deep grooves on the head. Some of these are shown below.
Bembidion (Nothonepha) sp.
Bembidion (Bembidion) sp.
Bembidion (Trepanedoris) sp.
Why are the heads like this, and only in this one group of Bembidion? Do they feed differently, or on different prey? Or does this have nothing to do with feeding, but instead is about burrowing? Or???
David Maddison
The Subulate Palpomere 
Or perhaps it’s a fashion statement?
Some structural advantage? I’m imagining two little Trepanadoris – each staring down the other. They circle one another…snorting intensely…tossing their heads tauntingly. The tension peaks…the circling stops…they each paw at the soil, flaunting the breadth of their basal tarsal segments as they kick up the dust. At some unseen cue each beast rears back on it’s hind legs lick a cocked hammer waiting to be fired…the universe seems pause. The shot is fired – the beasts gallop ferociously on their collision course…the only structures that can be made out amidst the speeding blur of chitin are the deep, shining frontal furrows, steadily guiding the impending impact. The collision is thunderous…the victor claims the crowd of fertile onlookers – wondering why they too have such structurally advantaged foreheads… 😉
correction “like” a cocked hammer…oops.
Fascinating topic…. certainly a phenomena that must be recognized by anyone attempting to successfully key out Bembidion species from the Pacific Northwest (e.g. Lindroth 1961-1969). .
This topic reminds me of one of my favorite papers about evolution – the classic: Gould, SJ & RC Lewontin, 1979. The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationists programme. Proc. R. Soc. Lond. B 205: 581-593.
There may actually be an adaptive advantage of each of the head morphology types David’s mentioned within Bembidion, perhaps having to do with their prey, hunting tactics, burrowing habits, sensory skills, etc. One must also consider the hypothesis that they could be “spandrels” and may not actually have any special adaptive value at all. For instance, could they be products of the process of morphogenesis in pupal chambers, and of no real significance to the adults (except maybe to help entomologists figure out their phylogeny!).
I have no idea.
James C. Bergdahl
Spokane, WA, USA