This week, on one of my daily walks around our Colorado Springs neighborhood, I encountered a beetle on the sidewalk that gave me reason to pause. I initially dismissed it as a species of soldier beetle that is extremely abundant at this time of year, but something looked a little "off." Sure enough, it was something else; and that got me thinking about mimicry among all these beetles.
The beetle on the sidewalk turned out to be a blister beetle, Epicauta stuarti, a species I had not seen before. Blister beetles, family Meloidae, are well known for containing high concentrations of the potent, irritating chemical cantharidin. Blister beetles exude the chemical in liquid form from leg joints and from between other body segments if the insect is squeezed or crushed. The chemical goo can raise painful, scarring blisters on sensitive skin; it can be fatal if ingested.
Ironically, many, if not most, blister beetles don't advertise their toxicity with bright "warning colors." Epicauta stuarti is one that does, but the interesting part is that its pattern of black and orange is very similar to that of our two common autumn soldier beetles. Soldier beetles are in the family Cantharidae, and they also have chemical defenses, which they secrete from abdominal glands. The Colorado Soldier Beetle, Chauliognathus basalis, is common on the plains. Its close relative, C. deceptus, replaces it in the foothills and mountains.
This kind of shared color pattern that reinforces predator deterrence is called Müllerian mimicry. Both animals can back up their colorful advertisement of toxicity with actual chemical weaponry. This is a very interesting example of mimicry, but it doesn't end here. Other local beetles have jumped on the bandwagon.
Also appearing at this time of year is the End Band Net-wing beetle, Calopteron terminale. They generally occur in far fewer numbers than the soldier beetles, but can be mistaken for them with just a passing glance. It is widely assumed that net-winged beetles (family Lycidae) are distasteful to predators, because they have colorful patterns in many cases. Whether this has been proven I do not know.
Net-winged beetles may exaggerate the effect of their wardrobe by raising and lowering their wing covers (elytra) in a unique display. As adults, they feed on nectar, and the "honeydew" secreted by aphids and related insects as a sweet liquid waste product. Larvae of netwing beetles feed on fungi or metabolic products of fungi. Whether this diet can be converted into toxic compounds is debatable.
While the jury may still be out on whether net-winged beetles are indeed toxic, there is no question that yet another mimic in this complex is pulling one over on predators. The longhorned beetle Crossidius discoideus fools us into thinking it is dangerous to eat by mimicking the pattern of the solider beetles. It can even be found on the same flowers as the soldier beetles and is difficult to easily separate from them, except for its long antennae. This brand of mimicry is called Batesian Mimicry, whereby a harmless animal masquerades as a dangerous one.
Crossidius discoideus has no common English name. As a larva, it bores in the root crowns of Broom Snakeweed, Gutierrezia sarothrae, or Jimmyweed (Isocoma spp.). The adults feed on flower pollen and nectar. Broom Snakeweed is also where the blister beetles hang out, so I may have to look more thoroughly for them now.
Yet another kind of longhorned beetle, Batyle suturalis, appears just before the populations of soldier beetles explode. Is it, too, taking advantage of a similar color pattern to gain "cover" from predators? What I would like to know is which of the truly toxic beetles started this whole complex. The blister beetle? The solider beetles? We will likely never know, and that is part of the appeal of entomology. It is an endlessly curious endeavor, seeking the answers to more puzzles than mankind will ever unravel.