If one looks up close onto the back of a carabid beetle, one will see (in most species) very fine, engraved lines which form a pattern, usually looking like honeycombs, or bricks, or long, thin parallel lines. This is called “microsculpture”, and is, for reasons that are unclear, relatively constant within a species, and frequently different between closely related species. Here’s what it looks like on the elytron of a female of Lionepha chintimini (Lionepha is a genus closely related to Bembidion):
You can see the nearly circular little polygons (sometimes called sculpticells) that make up the mesh of the microsculpture. The black bar at the left provides a scale: it is one tenth of a millimeter long. The beetle as a whole is about 4 mm long. The picture above is a closeup of the same part of the beetle marked by the little yellow box in this image of a male L. chintimini:
Some females of Lionepha chintimini have microsculpture that is not as deeply engraved, but it still forms nearly circular polygons:
Lionepha chintimini is a species that lives on top of coastal mountains in Oregon (I’ll write a separate post about them later). Living with L. chintimini is a closely related species, Lionepha casta, that is virtually impossible to tell apart unless one looks at either the microsculpture or the genitalia. The microsculpture of females of Lionepha casta is slightly stretched, so that the mesh is composed of oblong polygons rather than circular polygons. Here are two specimens of L. casta that show these stretched polygons in the microsculpture:
I find it fascinating that L. chintimini and L. casta, otherwise so similar, would have consistent differences in microsculpture meshes. The microsculpture is consistent enough that this is the only sure-fire way to tell the species apart without doing genitalic dissections.
For many Bembidion and Lionepha specimens, looking at the microsculpture is often very important in identifying them to species. It is unfortunate that this requires a rather good dissecting scope. I am lucky enough to have a Leica M165, and with that I can see the microsculpture clearly. But I remember when I was a grad student, using a Wild M5, that I could not understand how Carl Lindroth could see the microsculpture patterns he describes.
I’ll add another post soon with a description of the photographic setup used to take these pictures of microsculpture.