| Drab stinkbug | |
|---|---|
 | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Insecta |
| Order: | Hemiptera |
| Suborder: | Heteroptera |
| Family: | Dinidoridae |
| Genus: | Megymenum |
| Species: | M. gracilicorne |
| Binomial name | |
| Megymenum gracilicorne Dallas, 1851 | |
Thedrab stinkbug (orsaw-toothed stinkbug[1]),Megymenum gracilicorne, is an insect in the familyDinidoridae. The species is found in East Asia and eats plants includingcucurbits. It has the distinctive behavior of cultivating fungus on females' hind legs, which is used to protect their eggs from parasitic wasps.[2][3] According toentomologistJohn Noyes, "this is a unique system, as far as I'm aware".[4]
The family Dinidoridae is subdivided into twosubfamilies: theDinidorinae, and theMegymeninae, with thegenusMegymenum in the latter.[5] The Megymeninae have roughdorsal surfaces, unlike the smooth surfaces of the Dinidorinae, along with a linear arrangement of thespiracles (openings for air exchange) on their abdomens, and theirtrichobothria (hair-like sensory structures) are paired.[5] Acladistic analysis placesMegymenum pratti asM. gracilicorne's closest relative, withM. mekongum as the next closest.[6]M. tauriformis is now considered to be identical toM. gracilicorne.[6]
M. gracilicorne is found in Japan, Korea, China, and Hong Kong.[6][7] It feeds on plants including theCucurbitaceae, such as pumpkins and cucumbers, as well as other kinds of crop plants, including grapes and potatoes.[3][8]
Drab stinkbugs are vulnerable to attack byparasitoid wasps of the speciesTrissolcusbrevinotaulus. The wasps insert their eggs into the eggs of the stinkbugs. When the wasp eggs hatch, thelarvae feed on the stinkbug eggs from within, killing the stinkbug eggs, after which the wasps hatch.[2][9]
Along with various other stinkbugs, the drab stinkbug has evolved a specially constricted passage in itsintestine, that separates the parts in front of the constriction from aposterior portion of the midgut, which beneficialsymbioticbacteria can reach by using theirflagella, and colonize.[10] In drab stinkbugs, these bacteria aregammaproteobacteria mostly of the genusPantoea.[1] Unlike most other stinkbugs, who characteristically transmit symbiotic bacteria from mothers to offspring, each generation ofM. gracilicorne obtains these bacteria from the environment.[1][11] The bacteria may bepathogens for some plants that the stinkbugs feed on, but areectosymbionts that provide the stinkbugs with protection against predators and pathogens of their own.[12] First-instarM. gracilicorne raised in a laboratory without gut bacteria fail to grow, indicating that the bacteria are necessary for the insect's development.[1]
Drab stinkbugs possess a distinctive structure on their hind legs, that was previously misidentified as atympanal organ used for detecting sound. However, this structure, which is present only in females, was found in 2025 to instead be a grouping of up to 2,000mycangia, small pores in which fungus can grow.[2][3][4] The fungi carried by female drab stinkbugs have been identified asCordyceps andSimplicillium.[2][3][4] It is as yet unknown how the stinkbugs collect only these types of fungus, without contamination by others.[2][3][4] These fungi are not harmful to the stinkbugs.[2][3][4]
When the female drab stinkbug lays her eggs, she uses one hind leg to scrape some fungus from the other hind leg, alternating between legs, and then applies the fungus to each egg as it is laid.[2][3][4] The fungus obtains nutrition fromcarbohydrates on the egg's surface, without harming the egg itself. Over the next three days, the fungus grows a network ofhyphae covering the egg mass, about 2 mm thick.[2][3][4] When a parasitoid wasp approaches eggs that are covered by the fungus, the fungus prevents the wasp from harming the eggs about 90% of the time.[2][3] This protection appears to be a physical barrier, rather than a chemical repellent.[2][3][4] Entomologists including John Noyes observed when these findings were published that such a use of fungus by an insect has not been reported before.[4] When the eggs hatch, some of the fungus remains on the stinkbugnymphs, but it is shed when they molt.[2][3][4]