Little Transformers: Bolitotherus cornutus – the first dinobeetle?

Little Transformers are back with another coleopteran representative. I usually use this platform to present insect adaptations from the tropics, however this time I am focusing on a local species with a wide distribution in central and eastern North America: the forked fungus beetle (Bolitotherus cornutus). It is one of the most iconic North American beetle species, and I remember that flipping through pages of insect books as a kid, there was always an image of a forked fungus beetle under the darkling beetles section. In fact, as soon as I arrived to Canada this was the first species I sought after. And as much as I hate to admit, I looked for it in all the wrong places. I thought it was associated with wood (it is, but in a more indirect way), and cracked open fallen logs in search for adults. Of course I found nothing. Eventually the first fungus beetles I found were under a huge woody bracket mushroom in a conservation area near Price Edward, Ontario. Today this makes me laugh because back then we drove so far, and a year later I found out that I can find the beetles within just a mere 5 mins bus ride from my house.

I must say I am puzzled why this beetle is shown as an example for darkling beetles in books. Family Tenebrionidae is big and diverse, but there are some common characteristics that stay uniform across different genera. Bolitotherus cornutus, however, is not exactly a “typical” darkling beetle. And even though this beetle is widespread and common, it is often hard to find. When I presented this beetle in a talk to a group of local naturalists and asked how many people have seen it in the wild, only one hand was raised, surprisingly or not it came from a mushroom expert.

A pair of forked fungus beetles (Bolitotherus cornutus), dorsal view

A pair of forked fungus beetles (Bolitotherus cornutus), dorsal view

At first glance, forked fungus beetles look like they were designed by a drunk military engineer. Like most members of tribe Bolitophagini, they are built like small tanks, and to some extent they also look like ones. A compact and rugged body, sealed to the outside thanks to the tight elytra forming a protective shell. The body surface is heavily granulated to provide further shock protection in case of falling to the ground, as well as camouflage against tree bark and dried bracket mushrooms that the beetles feed on. Male beetles have two sets of horns, each with a different function.

Male forked fungus beetle (Bolitotherus cornutus)

Male forked fungus beetle (Bolitotherus cornutus)

The curved thoracic horns are hairy and used for pushing an opponent off the surface while fighting for territory and mates. The length of these horns is variable depending on various conditions (both genetic and environmental), with two extreme male morphs: major with long arching horns, and minor with short stout horns.

Male forked fungus beetle (Bolitotherus cornutus), frontal view. The thoracic horns can be long!

Male forked fungus beetle (Bolitotherus cornutus), frontal view. The thoracic horns can be long!

The other set of horns are found on the beetle’s head. These are called cephalic horns and they are sometimes missing. Their function is very peculiar: males use them as a pitchfork to scrape, lift, and throw off minor individuals that cling tightly to females. By the way, other members of Bolitophagini have horns as well, for example genus Byrsax has impressive horns that make it look like a perfect samurai helmet!

Another frontal view of a male forked fungus beetle (Bolitotherus cornutus), showing its orange pom-poms.

Another frontal view of a male forked fungus beetle (Bolitotherus cornutus), showing its orange pom-poms.

Ok, but what does Bolitotherus cornutus have to do with Little Transformers? Sure, touch the beetle and it folds its legs tightly close to its body, creating an impenetrable structure. We have seen similar defense behavior in other beetle transformers, like the Ceratocanthinae pill scarab and the shiny leaf beetle. In addition, the fungus beetles also secrete a smelly mixture of chemicals when disturbed. But the reason I am mentioning it here as a transformer is because of its horns. You see, many phylogenetically distant species share similar morphological adaptations. Studying these cases of convergent evolution can teach us something about the processes these adaptations go through, as well as their function. To be more specific, how is this…

Portrait of a male forked fungus beetle (Bolitotherus cornutus)

Portrait of a male forked fungus beetle (Bolitotherus cornutus)

…any different from this?

Portrait of Machairoceratops cronusi. Art by Andrey Atuchin, used with permission.

Portrait of Machairoceratops cronusi. Art by Andrey Atuchin, used with permission.

This fabulous artwork by Andrey Atuchin shows Machairoceratops cronusi, a recently described member of the rhino-like dinosaurs, and a relative of the famous triceratops. Yes, Bolitotherus cornutus is basically a miniature six-legged dinosaur in disguise. Now I know what you are thinking. The beetle’s horns are hairy, and the dinosaur’s aren’t. That is probably true. The Machairoceratops dinosaur might have had hairy horns. We don’t know for sure (ask yourself why). But regardless, you have to agree that there is some uncanny resemblance between the two animals’ head structure. A set of flat horns arching over the head, another pair of spiky horns pointing upwards from the head, a granular neck shield… Of course, we don’t know how the dinosaurs used their horns, but we can speculate. Maybe observing the forked fungus beetles fighting can help us understand a behavior in an animal that no longer exists. The relationship between form and function in animal horns is a fascinating topic for discussion and hopefully I will write about it in more depth in the future. But I cannot help it, the more illustrations of Machairoceratops cronusi I look at, the more I see forked fungus beetles in them. It is almost as if someone placed an enormous beetle on top of the dinosaur’s skull.

Bracket mushrooms (Fomitopsis betulina) growing on birch. Bolitotherus cornutus beetles prefer to feed on old mushrooms (dark-colored, coated with moss and algae in the photo) rather than fresh ones.

Bracket mushrooms (Fomitopsis betulina) growing on birch. Bolitotherus cornutus beetles prefer to feed on old mushrooms (dark-colored, coated with moss and algae in the photo) rather than fresh ones.

The diet of forked fungus beetles is unique and restricted to bracket mushrooms (such as Fomitopsis, Ganoderma, Ischnoderma) growing on weak standing trees as well as fallen logs (by the way, they are not the only darkling beetles feeding on mushrooms). They prefer old, hardened bracket mushrooms.

Major male forked fungus beetles (Bolitotherus cornutus) fighting on top of a bracket mushroom. Notice that their granular body surface often attracts mites and tiny springtails.

Major male forked fungus beetles (Bolitotherus cornutus) fighting on top of a bracket mushroom. Notice that their granular body surface often attracts mites and tiny springtails.

On spring and summer nights males gather on the mushroom surface, where they engage in fighting tournaments to win territories (=food for the them and their offspring) and matings with the females waiting nearby. What is even more interesting is that while major males with impressive horns are distracted fighting and showing off their capabilities, the minor males sneak up on them and mate with some of the females.

A minor male forked fungus beetle (Bolitotherus cornutus) guarding a female after mating

A minor male forked fungus beetle (Bolitotherus cornutus) guarding a female after mating

The courtship process is long and elaborate, and includes climbing over the female and stridulating (acoustic communication). Males also tend to stay and guard the female to prevent other males from mating with her. After mating, females lay their eggs separately on the mushroom surface, and cover each egg with frass. This protects the eggs from desiccation as well as from predators and parasitoids.

Bolitotherus cornutus eggs appear as dark bumps on the surface of a bracket mushroom (there are 4 eggs in this photo)

Bolitotherus cornutus eggs appear as dark bumps on the surface of a bracket mushroom (there are 4 eggs in this photo)

Within 1-2 weeks the larvae hatch and immediately burrow into the mushroom. They are not the typical darkling wireworms, but instead look like hairy, soft-bodied grubs.

Young Bolitotherus cornutus larvae

Young Bolitotherus cornutus larvae

They spend their entire life inside their feeding substrate. The mushroom fruit body protects them from the elements, so they also use this space for pupation. Surprisingly, some larvae grow faster than others, and complete their metamorphosis before winter. This means that the beetles can overwinter inside the mushroom as larvae, pupae or fresh adults.

Male forked fungus beetle (Bolitotherus cornutus) emerging from a bracket mushroom

Male forked fungus beetle (Bolitotherus cornutus) emerging from a bracket mushroom

Male forked fungus beetle (Bolitotherus cornutus) burrowing into decomposing wood

Male forked fungus beetle (Bolitotherus cornutus) burrowing into decomposing wood

If you live in North America within the distribution range of this species I encourage you to get out there and look for these magnificent creatures. First of all, it is fun, and you might find other cool stuff while searching. And second, these beetles are really cool, and they can teach us a lot. They are also embarrassingly easy to keep, all they need is some pieces of the mushrooms they were collected on, the slightest humidity, and that’s it. They live for a few years as adults and readily breed in captivity, displaying all the behaviors mentioned above and more!

An active captive colony of forked fungus beetles (Bolitotherus cornutus)

An active captive colony of forked fungus beetles (Bolitotherus cornutus)

Adult forked fungus beetles (Bolitotherus cornutus) aggregating on the mushroom underside

A closeup on adult forked fungus beetles (Bolitotherus cornutus) aggregating on the mushroom underside

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