Crypsis? Mimicry? Crypsimicry?

Continuing with the previous post’s theme on crypsis, here is an interesting insect that I photographed in north-central Oklahoma in late June 2014. I was checking standing and fallen trunks of large, dead eastern cottonwood (Populus deltoides) trees in Woods Co. near the Cimarron River, where just a few days earlier I had found a jewel beetle (family Buprestidae) that had eluded me for more than 30 years—Buprestis confluens. I had found only a single individual and returned to the spot in the hopes of finding more. As I searched the trunk of one particularly large, fallen tree—its trunk still covered with bark, I noticed movement but couldn’t make out right away what I was seeing. A closer look revealed the movement to be from a wasp-like insect, its antennae curiously quivering in a manner that reminded me of an ensign wasp (family Evaniidae). More careful looking, however, revealed the insect to be not a wasp, but a longhorned beetle (family Cerambycidae), which I then recognized to be the species Physocnemum brevilineum.

Physocnemum brevilineum (Say, 1824) on fallen cottonwood (Populus deltoides) | Woods Co., Oklahoma

This beetle is commonly referred to as the elm bark borer, a reference to the larval habit of mining within the bark of living elm trees, but as far as I can tell this beetle is anything but common. Like the Buprestis confluens that I had found a few days earlier, this was a species known to me only by pinned museum specimens (I’m always amazed when a woodboring beetle species is apparently common enough to warrant a common name and is said to reach pest status in some cases, yet eludes my net for decades!). At any rate, my impression based on these pinned specimens and published images was that the species is another of the many longhorned beetles that seem to mimic ants (Cyrtinus, Cyrtophorus, Euderces, Molorchus, and Tilloclytus being among the others). Like many of these other mimics, the species is dark with small amounts of red and bears polished, ivory-colored ridges at mid-elytra to give the illusion of a narrow waist. After seeing a living individual, however, and especially its behavior—in particular the very wasp-like manner in which it moved its antennae, I’m not so sure that ant mimicry alone explains the appearance and behavior of the species.

A lateral view reveals the beetle, but is it trying to mimic an ant, or a wasp, or both?

Of course, there is no reason why it must be ant mimicry or wasp mimicry (or crypsis, for that matter). Evolution has no rule stating that only one survival strategy can be employed at a time, and if, as it seems to me, the beetle is utilizing both crypsis and mimicry—the first to avoid detection and, failing that, the second to give the potential predator pause, then there is no reason why the mimicry portion of its defense couldn’t be modeling both ants and wasps as a way to maximize an overall “nasty hymenopteran” appearance.

© Ted C. MacRae 2015