Defensive fungal symbiosis on insect hindlegs

Tympanal organs, as “insect ears,” have evolved repeatedly. Dinidorid stinkbugs were reported to possess a conspicuous tympanal organ on females’ hind legs. Here we report an unexpected discovery that the stinkbug’s “tympanal organ” is actually an external symbiotic organ. The organ’s surface is not membranous but a porous cuticle, where each pore connects to glandular secretory cells. In reproductive females, the hind leg organ is covered with fungal hyphae growing out of the pores. Upon oviposition, the females skillfully transfer the fungi from the organ to the eggs. The eggs are covered with fungal hyphae and physically protected against wasp parasitism. The fungi are mostly benign Cordycipitaceae fungi and show considerable diversity among insect individuals and populations, indicating environmental acquisition of the specific fungal associates.