Like us humans, insects possess sensory organs responsible for vision, hearing, smell, taste, and touch. For vision, insects primarily rely on compound eyes. But what about hearing? For example, crickets develop tympanal organs on their forelegs, which function like a human's eardrum to detect sound. They use these "ears on the legs" to listen to courtship songs and sense approaching enemies.
The tympanal organs have evolved in insects repeatedly. For example, cicadas, grasshoppers, moths and mantises have tympanal ears on their abdomen or thorax. Uniquely, stinkbugs of the family Dinidoridae, encompassing around 100 species representing 16 genera in the world, have yet been reported to possess a tympanal organ specifically on the hindlegs of adult females. However, no detailed studies have been conducted on this minor group of stinkbugs. How do dinidorid females perceive male's song or dance using their hindlegs?
To address this question, we investigated the Japanese dinidorid stinkbug Megumenum gracilicorne. Unexpectedly, we found that its so-called "tympanal organ" is not an auditory organ but instead a novel symbiotic organ.
We observed that, in reproductively mature females, the "tympanal organ" was covered with fungal hyphae which were benign Cordyceps-allied filamentous fungi selectively acquired from the environment every generation. Strikingly, while laying eggs, the females skillfully transferred the fungi from the organ to the eggs (Movie).
Then, the eggs were quickly covered with fungal hyphae. The fuzzy hyphal thickets physically prevented wasps from approaching, thereby protecting the eggs from parasitism. When hypha-covered and hypha-removed eggs were presented to parasitic wasps in both laboratory and field experiments, the fungus-covered eggs consistently showed lower parasitism rates, verifying effective defense against the natural enemy by the egg-covering hyphae.
We also examined diverse dinidorid stinkbugs collected from Taiwan and a southwestern island of Japan. We found that all of these species possess the female-specific hindleg organs and exhibit the egg-smearing behaviors. Hence, the hindleg organ and the egg-smearing behavior must have evolved in the common ancestor of the Dinidoridae.
In conclusion, our re-examination of the female-specific "tympanal organ", which has been known from dinidorid stinkbugs for decades, led to the unexpected discovery of novel external fungal symbiosis that provides physical defense against wasp parasitism. The previous interpretation that the female-specific hindleg organ might be a tympanal ear was based merely on superficial morphological resemblance. This defensive fungal symbiosis on insect hindlegs provides an impressive example of how evolutionarily novel traits for microbial symbiosis emerge, develop, and constitute the elaborate syndrome that integrates molecular, cellular, morphological and behavioral specializations.
