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.