Residents in the Seoul metropolitan area are distressed by the recent swarms of mayflies appearing every night. These large mayflies, known as Oriental Mayflies, emerge in significant numbers near the Han River during May. 

They are typically found in areas with clean water, such as the Han River bordering Gangdong-gu, Gwangjin-gu, Songpa-gu, and Seongdong-gu in Seoul, as well as Yangpyeong, Namyangju, and Hanam in Gyeonggi-do.

Oriental mayflies, measuring 2 to 3 cm in body length and up to 4 to 5 cm with spread wings, do not pose a threat to animals or plants since they do not transmit diseases like flies or mosquitoes. 

However, the sheer quantity of these insects is what is causing many issues. They are attracted to bright lights, often swarming around street lights and shopping mall illuminations, and many residents have become disgusted by the massive swarm of bugs.

Although they naturally die within 4 to 5 days, the accumulation of their bodies on the ground is also an issue due to their collective behavior. Netizens who saw the photos and videos expressed their repulsion, stating that these insects are short-lived yet appear in large numbers yearly.