Mimicry in Beetles

Doug Von Gausig (see comment) asks an interesting question about Tenebrionids. He asks whether there are any Carabid beetles that do a head-stand display like the Tenebrionids, or if it is only the Tenebrionds that do this.

The situation among these beetles is complex. There are a number of Tenebrionid beetles (many in the genus Eleodes) that do the headstand display and have defensive sections. Many of these species look very similar to one another, in an apparent display of Mullerian mimicry, where all of the species converge on a similar phenotype so that predators can learn a general color and body type and know that everything that looks like that is either toxic or distasteful. Some of these beetles have smooth elytra (the wing coverings, which in the case of these beetles are fused together because the beetles are incapable of flying), while others have grooves on the elytra.

These beetles need an ample supply of moisture (that they get from eating moist vegetation or drinking dew) to manufacture the defensive secretions. When they do not get the moisture that they need, they do not do the headstand display, but attempt to run away from a potential predator.

There are other Tenebrionid beetles that will also do a headstand display, but lack any capability of producing defensive sections. These are beetles in such genera as Gonasida and Stenomorpha. They look just like the beetles with defensive secretions, have the same behavior, and are found in the same places. As such, they are Batesian mimics. Batesian mimicry is where there is a model that is dangerous or distasteful, and a mimic that is edible but looks and acts like the model.  Some of these species have grooved elytra while others have smooth ones, just like the species of beetle with defensive sections that they are mimicking.

Then there are Cerambycid beetles, or long-horn beetles, in the genus Moneilema that mimic the Tenebrionid beetles with defensive secretions.  These beetles are shiny-black, with smooth elytra, and do a headstand just like the Eleodes, but lack any defensive secretions. Unlike many long-horn beetles (who are called that because most members of that family have long antennae), these beetles have short antennae. However, their antennae are still longer than the Tenebrionids, who have really short filamentous antennae. So the Moneliema have an interesting adaptation. They have a groove running along the length of their body into which they can insert their longer antennae, making the antennae seem to disappear. One potential hitch about doing this is that there is a gray area along their body where the thorax is connected to the abdomen, and a black antenna would be visible against this gray background. To counteract this, the Moneilema have a gray segment in an otherwise black antenna, and this gray segment exactly matches the color and position of the gray area between the thorax and the abdomen. When the Moneilema do their headstand, they look just like the Tenebrionids with defensive secretions.

Finally, there are the Carabid beetles. Some of these beetles are also black, and are roughly the same size and shape as the Eleodes. For example, the beetle Scaphinotus petersi looks superficially like some of the Eleodes, but runs quite fast and low along the ground, instead of walking relatively slowly like the large Tenebrionids described above. I am not aware of any of the Carabid beetles doing a headstand display. although many Carabids have defensive secretions. They seem to rely on speed to get away from their predators. Some Carabids such as the bombardier beetles (Brachinus) have defensive secretions that mix in an internal chamber and literally explode out of the rear of the beetles in a hot burst (100 degrees Centigrade) of fluids.

An interesting question is how this mimicry might have evolved.  For a predator to learn that a beetle is distasteful, the predator has to first taste the beetle, something that often proves to be fatal to the beetle. We have to assume that a small mutation that makes a non-toxic beetle look slightly more like a toxic one is enough to lower predation pressure on that beetle, and over millions of years the mutations that result in closer convergence are those that survive in the gene pool through natural selection. Personally, I have to marvel at how good that convergence can be in this beetle mimicry system and in other insects with Batesian and Mullerian mimicry.

Comments

[Chris Wemmer]

I find Eleodes scent kind of pleasing, but suppose I should try to eat one to test its effectiveness as a predation deterrent. Which reminds me of Doc Rickett's test of the nematocyst as the predatory weapon of sea anemones. The stings can barely be detected on the hand, so John Steinbeck's hero is said to have used his tongue as a more sensitive organ. He learned how deadly the nematocysts are. That's why I'll never taste an Eleodes.