Digestive tract
Along with various other stinkbugs, the drab stinkbug has evolved a specially constricted passage in its intestine, that separates the parts in front of the constriction from a posterior portion of the midgut, which beneficial symbiotic bacteria can reach by using their flagella, and colonize. [10] In drab stinkbugs, these bacteria are gammaproteobacteria mostly of the genus Pantoea . [1] Unlike most other stinkbugs, who characteristically transmit symbiotic bacteria from mothers to offspring, each generation of M. gracilicorne obtains these bacteria from the environment. [1] [11] The bacteria may be pathogens for some plants that the stinkbugs feed on, but are ectosymbionts that provide the stinkbugs with protection against predators and pathogens of their own. [12] First-instar M. gracilicorne raised in a laboratory without gut bacteria fail to grow, indicating that the bacteria are necessary for the insect's development. [1]
Fungal defense
Drab stinkbugs possess a distinctive structure on their hind legs, that was previously misidentified as a tympanal 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 2,000 mycangia, small pores in which fungus can grow. [2] [3] [4] The fungi carried by female drab stinkbugs have been identified as Cordyceps and Simplicillium. [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 apply the fungus to each egg as it is laid. [2] [3] [4] The fungus obtains nutrition from carbohydrates on the egg's surface, without harming the egg. Over the next three days, the fungus grows a network of hyphae covering the egg mass, about 2 mm thick. [2] [3] [4] When a parasitoid wasp approaches the eggs, the fungus prevents the wasp from harming the eggs. This protection appears to be a physical barrier, rather than a chemical repellent. [2] [3] [4] When the eggs hatch, some of the fungus remains on the stinkbug nymphs, but it is shed when they molt. [2] [3] [4]
