A structure on the hindlegs of a stinkbug, long believed to be an organ for hearing, has been revealed as a specialized nursery for cultivating symbiotic fungi. Female stinkbugs of the Dinidoridae family use these unique organs to gather, grow, and then smear fungi onto their eggs, creating a protective shield that physically blocks attacks from parasitic wasps. This discovery overturns decades of assumptions about this anatomical feature and unveils a novel form of symbiosis in the insect world.
The research, centered on the Japanese dinidorid stinkbug Megymenum gracilicorne, found that what appeared to be a tympanal organ for sound detection is actually a complex structure designed for a surprising agricultural purpose. Instead of a thin membrane for hearing, the organ has a robust surface with thousands of microscopic pores connected to glandular cells. These glands secrete substances that nourish specific fungi, which the female collects from her environment. She then meticulously transfers this cultivated fungal growth to her eggs as she lays them, ensuring they are encased in a protective, filamentous web. This defensive layer is highly effective against parasitic wasps, a major threat to stinkbug offspring, which lay their own eggs inside those of other insects.
An Organ Reimagined
For many years, the enlarged structure on the hindlegs of female dinidorid stinkbugs was classified as a tympanal organ. Such organs are common across various insect groups and are primarily used for detecting sound waves, playing roles in communication and predator avoidance. This interpretation was based on superficial morphological similarities to known auditory structures in other insects. However, the fact that these organs were specific to females in this stinkbug family remained a biological puzzle that prompted researchers to take a closer look.
From Ear to Fungal Farm
A detailed investigation led by Takanori Nishino revealed the structure’s true nature. Instead of a vibrating membrane typical of an ear, microscopic analysis showed a hard cuticle perforated by several thousand pores. Each pore serves as a channel for glandular secretions that support the growth of fungal filaments, known as hyphae. This finding transformed the understanding of the organ from a passive sensory device into an active, symbiotic tool—a kind of external microbial farm. The researchers termed this previously unknown structure a mycangia, an organ specifically evolved to carry symbiotic microbes.
A Symbiotic Defense Strategy
The relationship between the stinkbug and the fungi is a clear example of defensive symbiosis. The female stinkbug provides the fungi with a nurturing environment and nutrients via its leg glands, and in return, the fungi provide a crucial service: protecting her offspring. As the female lays her eggs, she uses her opposing leg to scrape the fungus-laden organ and carefully applies the hyphae onto each egg. Within days, the fungi grow into a thick, fuzzy coating over the entire egg mass.
This fungal barrier acts as a physical shield. Parasitic wasps, such as Trissolcus brevinotaulus, are a significant threat, laying their own eggs inside the stinkbug eggs, which are then consumed from within by the wasp larvae. Experiments demonstrated that wasps were unable to penetrate the thick fungal web to deposit their eggs. When the fungal layer was removed or sparsely applied, the rate of successful parasitism increased dramatically. Researchers confirmed the protection is mechanical, as the fungi themselves are not toxic or pathogenic to the wasps.
The Fungal Partners
The stinkbugs are not born with their fungal partners. The symbiotic fungi are lost during the nymphal molting stages, meaning adult females must acquire new fungi from the environment in each generation. Analysis revealed that the cultivated fungi are diverse and vary between individual stinkbugs and locations, but they often belong to the Cordycipitaceae family. This family is well-known for containing potent insect-killing pathogens, such as the famous Cordyceps.
However, the stinkbugs appear to selectively cultivate species or strains that have low pathogenicity, ensuring the fungi protect the eggs without harming the developing nymphs. This ability to choose beneficial partners from a family of otherwise deadly microbes highlights a sophisticated evolutionary adaptation. The dynamic nature of this acquisition suggests a flexible and resilient symbiotic strategy, allowing the stinkbugs to adapt to different environments and fungal availabilities.
Broader Implications of the Discovery
This unexpected finding opens new avenues for research in insect biology, symbiosis, and evolution. According to Professor Takema Fukatsu, a co-author of the research, the shape and architecture of these fungus-filled organs are extraordinary and difficult to compare to anything else in the animal kingdom. It challenges scientists to reconsider the functions of other poorly understood insect structures. The discovery also raises questions about how this unique symbiotic organ evolved. Researchers speculate it may have originated from a simpler behavior of smearing defensive chemicals on eggs, with environmental fungi later colonizing the organ and being co-opted for their protective qualities.
Beyond its fundamental biological interest, the discovery could have practical applications. Stinkbugs are significant agricultural pests, and parasitic wasps are often used as a form of biological control. Understanding this natural fungal defense could lead to new, environmentally friendly pest management strategies. By learning how to disrupt this protective symbiosis, it might be possible to enhance the effectiveness of natural predators without resorting to chemical pesticides. The study serves as a powerful reminder of nature’s ingenuity and the vast number of discoveries still waiting to be made.