Why stinkbugs grow 'fungus farms' on their legs

Scientists once thought stinkbugs had tiny ears on their legs. Turns out, those “ears” are actually fungus farms.

In female Megymenum gracilicorne, two fuzzy patches on the hind legs hold thousands of pores that grow protective fungi. When it’s time to lay eggs, the mother smears those spores like a living shield – a homemade defense against parasitic wasps.

The strategy, identified in wild populations in Japan, shows how an insect can transform a body part into a next-generation safeguard – turning sound into survival.

Mother stinkbugs raise fungi

The research was led by Takanori Nishino at Japan’s National Institute of Advanced Industrial Science and Technology (AIST). The team examined the organ structure, which resembles an ear at first glance.

The results confirmed that it is not a tympanal organ, an ear-like sound detector, because the cuticle is porous and lacks sensory nerves. The researchers found that each pore connects to secretory cells that support friendly fungi.

Females scratch the tufted organ while laying eggs, then smear strands across each egg. Within days, the hyphae knit into a coat that blocks wasp attacks, quite effectively in tests.

Field and lab tests showed that stinkbug eggs kept their protection only when the fungus stayed intact. Cleaned eggs suffered more parasitism, while thickly coated eggs resisted oviposition in controlled arenas.

A case of mistaken hearing

Insects have evolved ear-like organs many times – on their legs, thoraxes, and abdomens. A classic review documents how hearing has arisen repeatedly across lineages.

The stinkbug organ fooled earlier researchers because it looks so much like a tympanum. True hearing membranes, however, are thin and backed by an air pocket – features that are absent in the stinkbug structure.

The new analysis adds a twist: a leg plate that mimics an acoustic membrane is actually home to microbes, renewed by adult females each generation. The pores and secretions fit that role perfectly.

According to the researchers, the hindleg plate represents a previously unknown type of external symbiotic organ.

Fungi protect stinkbugs eggs

The fungi belong mostly to Cordycipitaceae, a family of insect-affecting fungi, including familiar Cordyceps and Beauveria. The team noted that females tend to cultivate milder species on the organ.

When the researchers tested pathogenicity by injection, a Beauveria strain killed hosts readily. By contrast, Simplicillium and Lecanicillium isolates were far less lethal. That pattern favors a protective surface, not a pathogen within the mother.

The egg coat worked by blocking access. Parasitic wasps touched both cleaned and coated eggs, but they oviposited only on cleaned ones. The fungus shield changed the mechanics at the egg surface.

In the field, the team tracked egg clusters exposed for several days. Egg masses with intact hyphae hatched more true stinkbugs, while cleaned masses showed higher failure because of wasp attack.

Stinkbugs are fungus farmers

Specialized fungal-carrying pockets known as mycangia are small cuticular cavities that store symbiotic fungi. Ambrosia beetles use them to carry food fungi and seed them into wood. A recent review outlines how these pockets evolved and diversified across beetle lineages.

The stinkbug’s organ differs in placement and scale, but the logic is similar. Thousands of tiny pores concentrate in one field, where secretions feed selected hyphae.

Other insects also enlist microbes for defense. Leafcutter ants, for instance, harbor actinobacteria that produce antibiotics against garden blights – a relationship studied across many species.

In the stinkbug’s case, the defense is physical, not chemical. The result broadens the catalog of insect–microbe partnerships beyond toxins and odors.

From ears to armor

Natural selection often repurposes existing structures for new tasks. Here, a plate once labeled as an ear now serves as a farm for bodyguard fungi.

That switch helps explain why female legs are remodeled in these bugs. The organ appears in several relatives, suggesting the trait arose in a shared ancestor and persisted because it saves offspring.

Parasitic wasps are parasitoids, insects whose larvae eventually kill hosts, and they are major checks on many crop pests. A physical barrier on eggs could tip that arms race in favor of the bug, at least for a short window.

Understanding the timing and growth of the hyphal coat might inform gentle deterrents. Farmers sometimes struggle with egg parasitism dynamics, and this system reveals a new way insects solve the same problem.

Ears, genes, and generations

How do females choose which fungus to farm each generation? The organ secretes sugars and proteins, but the recognition cues remain unclear across seasons and sites in the wild.

What is the cost for mothers that host benign strains on exposed cuticles? The balance between protection and infection likely shapes which fungi are allowed to grow.

Repeated evolution of insect hearing set the stage for confusion here. Ear-like plates appear across insect orders, and the historical review explains why similar looking parts can do different jobs.

Future work could map genes active in the organ as females mature. Experts could also test whether wasps adapt by compressing sparse hyphae with their antennae to reach the egg shell.

The study is published in the journal Science.

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