What do Laboulbeniales Look Like?

Have you ever gone mushroom picking with friends or family? Or perhaps enjoyed a delicious mushroom omelet? Fungi, the biological kingdom to which mushrooms belong, are more than just those organisms with a stem and a cap that you see in the forest or on your plate. The fungal kingdom has more than 2 million species. Some, like mushrooms, can be seen with the naked eye, while others can only be seen with a microscope. Amongst these microscopic fungi is a mysterious group called the Laboulbeniales (la-bools, for short)!

Most fungi are made up of microscopic filaments—tiny chains of cells that look a bit like threads, sometimes twisted so tightly together that you can see them with the naked eye. Mushrooms are built from these filaments, but Laboulbeniales are instead made up of a fixed number of cells arranged in a unique shape, which scientists call a thallus (thalli in plural). These tiny fungi attach themselves tightly to the bodies of arthropods, including insects, mites, and millipedes. Because they hang from arthropods’ bodies, they have earned another nickname: beetlehangers! The tiniest species measure less than 100 μm across—that is 10,000 times smaller than a meter. The biggest ones could reach 2 or 4 mm in length, which is still smaller than a typical pencil-top eraser. Early naturalists completely misidentified them, thinking they were bizarre hairs, parasitic worms, or even algae. It took some time before they realized these structures were fungi!

One of the most famous Laboulbeniales species is Hesperomyces harmoniae [1], a common banana-shaped parasite of the harlequin ladybug (or ladybird in UK English) (Figure 1). At its base, it has a foot that anchors it to the ladybug’s body. This foot is black because it contains melanin, the same pigment that gives color to human skin and hair. But melanin is not just about looks. It also makes the foot extra strong, ensuring it stays firmly attached no matter how fast the ladybug moves. Many Laboulbeniales also have melanin in their cell walls, making the whole thallus very tough. Whether facing wind, dust, or friction, the parasite hangs on for dear life!

La-bools reproduce by making spores in a cylinder-shaped structure called the perithecium that is located above a few small cells linked to the foot. Each spore can grow into a full new fungus, and they are spread between ladybugs. On the side of adult Laboulbeniales are little structures called appendages. These are simple and barely visible in some species and can be long and tentacle-like in others. Scientists are not exactly sure what the appendages are used for, but they differ a lot between species and are often eye-catching.

The harlequin ladybug-associated parasite is yellowish, but many Laboulbeniales species have much darker colors. Some are so strange looking that they seem like they come from another planet. One species looks like fireworks bursting in every direction, while another resembles a plesiosaur straight out of Jurassic Park! So far, more than 2,400 species of Laboulbeniales are known [2].

How do They Live?

What makes Laboulbeniales truly fascinating is their dependency on arthropods. They cannot survive or reproduce without a host to cling to. Laboulbeniales are found on many types of hosts, including millipedes, mites, termites, beetles, flies, crickets, and ants. But two groups are their absolute favorites: ground beetles and rove beetles. Scientists do not know why Laboulbeniales like these groups so much. Indeed, one of the most interesting (and mysterious) things about Laboulbeniales is that they are extremely picky about where they live. Many species are only found on a single species of arthropod. Some go even further by choosing a precise spot on the arthropod’s body. Laboulbeniales are actually pretty rare in nature. Arthropods have low chances of being infected, which means you could examine hundreds—or even thousands—of potential hosts before spotting a thallus.

What is a day like in the life of a Laboulbeniales? Let us take the harlequin ladybug parasite as an example (Figure 2). Spores are key to completing their life cycle. But how do these spores find a new host? One answer probably has to do with host mating seasons. When ladybugs get up close, their bodies touch, giving Laboulbeniales spores the perfect opportunity to pass from one host to another, which is helped by the stickiness of the spores. Once a spore lands on its new host, it springs into action. It germinates and goes through a series of divisions, growing into an adult thallus. At early stages, the young thallus may develop a simple, root-like hidden structure called a haustorium from the foot. The haustorium penetrates the host’s body and absorbs essential nutrients to help the fungus grow. Once fully adult, the thallus produces new spores, and the cycle starts all over again.

Sounds pretty simple? Well, not exactly. Scientists still do not fully understand how all these stages unfold. For example, they are not completely sure how and when hosts pass these parasites to each other or even whether Laboulbeniales are harmful to their hosts. At first glance, they seem harmless because they do not kill their hosts. Some evidence suggests that such parasites may influence how their hosts behave, communicate, and reproduce, although this remains unproven.

Believe it or not, Laboulbeniales are not alone in their unusual lifestyle. They belong to a larger group called Laboulbeniomycetes, which includes two other groups of very close relatives: Herpomycetales and Pyxidiophorales. Herpomycetales are fond of cockroaches. They exclusively infect this group of insects and no others. Pyxidiophorales, on the other hand, are clearly different from the other groups. They form filaments instead of thalli and have a very complex life cycle. Like their relatives, they are strictly dependent on arthropods—either as a mode of transportation or as a place to live.

How Can We Study Them?

Studying Laboulbeniales is not as simple as walking into a forest and picking mushrooms off the soil or from dead wood like some other fungi. Step one for any curious minds is collecting their hosts. Scientists can search for them in nature, by placing traps or simply by picking up ladybugs that pass by. Another way is to examine old collections of arthropods stored in natural history museums.

Once scientists have arthropods in hand, the real detective work begins. Using a powerful, tiny magnifying glass, they visually scan the arthropod’s surface, looking for signs of thalli. If they get lucky enough, the next step is to carefully remove the thalli. This is a delicate operation that requires a sharp needle—just like the ones used by doctors for taking blood—sometimes coated with a special sticky solution. Once the thalli are detached, scientists place them on a slide for closer inspection with a high-powered microscope.

If you see a koala, you immediately know it is a koala. But for Laboulbeniales species, this is not so easy! Often, what looks like a single species might be multiple species that are very similar in appearance but different in their DNA.

Why the Unknown Deserves Protection

Laboulbeniales are tricky to spot and identify. But that is not the only reason they have stayed under the radar. Unlike popular species like pandas, which capture the public’s heart with their “cuteness”, Laboulbeniales are unknown to most people. Humans tend to care more about species they are familiar with, especially ones that show emotions or behaviors we relate to. This makes it hard for scientists to find money to study overlooked organisms like Laboulbeniales. But just because they are not famous does not mean they do not need protection! They are part of biodiversity, too.

There is another major factor in the equation. Even if Laboulbeniales were easy to find, easy to identify, and there was unlimited funding to research them, they would still remain unknown if there was no one to study them! The reality is that very few scientists study these fungi [3]. That means even the most basic questions—like how many species exist, how they interact with their hosts, and how they respond to environmental changes—are still unanswered. This lack of basic knowledge prevents us from knowing whether they are in danger and what can be done to better protect them and prevent them from going extinct. As a consequence, the first and foremost goal of scientists studying Laboulbeniales is clear: to gain good fundamental knowledge about their diversity and ecology!

Scientists have found a great way to speed up discoveries: citizen science [4]. This means anyone, regardless of their age and background, can help scientists by observing these microfungi in nature. And why not you?! The next time you see a ladybug, take a closer look. It might be carrying a hidden world of mysterious fungi on its back! If that is the case, you can take a photograph and upload your observation on the online platform iNaturalist. Scientists have even created a website to collect all the observations of the harlequin ladybug’s common parasite from citizen science [5]. Check it out to see amazing photos and explore a database showing where this parasite has been spotted worldwide!

Glossary

Thallus: ↑ (thalli in plural): The body of a la-bool, made up of a fixed number of cells arranged in a unique shape.

Arthropods: ↑ A huge group of organisms belonging to the animal kingdom that includes, among many other species, insects.

Parasite: ↑ A species that needs to live on another species (a host) to survive.

Melanin: ↑ A dark pigment found in many living organisms. In Laboulbeniales, it makes the foot extra strong and resistant.

Spores: ↑ Microscopic particles that many fungi use to reproduce. For la-bools, when a spore lands on a suitable host, it can grow into a new fungus.

Perithecium: ↑ A cylinder-shaped structure sitting at the top of the thallus where spores are produced.

Haustorium: ↑ A structure that some parasites use to penetrate their host and absorb nutrients directly from it.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We are thankful to Milva Druguet Dayras for revising this text and making sure it is understandable for young minds. This study was supported in part by the Fonds de Recherche du Québec - Nature et Technologies (B2X doctoral research fellowship to JC), Czech Science Foundation (Grants 21-06446S, 26-23464M), and Czech Academy of Sciences (Lumina Quaeruntur Fellowship LQ200962501 to DH).

AI Tool Statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

Any alternative text (alt text) provided alongside figures in this article has been generated by Frontiers with the support of artificial intelligence and reasonable efforts have been made to ensure accuracy, including review by the authors wherever possible. If you identify any issues, please contact us.