Objective To determine the real cause of this biting accident and reveal the truth of so-called poisonous mosquitoes, providing scientific evidence for control and prevention. Methods Means of epidemiology such as inquiry of medical history, field investigation were used to understand and follow up the bitten patient,and man-induced landing and catching method and identification for insect body morphology were also carried out for qualitative determination. Results The so-called mosquitoes were finally identified as members of Diptera gnats; and no blackfly activities were found anywhere near the scenic spot of Western palace; Besides, the rice field next to a blueberry garden was found to be the breeding environment of them. Actually, the inspecting density of man-induced landing method could get to the level of 13 blackflies/5 min, which caused 11 catching workers were bitten mostly on skin of lower limbs from 3-20 positions by blackflies in breeding environment, and parts of being bitten were nearly at lower limbs, ankles and acrotarsiums, symptoms were mainly characterized by severe itching, pain, redness and swelling, and it dismissed after 2 weeks. Thus, local people could prove the actual existence of blackflies and it was defined that the more lower limbs skin were exposed, the more bitten positions could be. Conclusion It is certain thing that the blackflies are definitely distributed in Qingyuan county of Lishui city, and this biting accident is clearly caused by exposure skin bites in blackfly breeding environment. In accordance, measures like strengthening publicity and education, improving individual protection and avoiding the peak season of insect activities when playing and working should be taken seriously. Moreover, further research on blackfly population, density, seasonal fluctuation, distribution, pathogen infection and understanding the season density of growth and decline, strengthening inspection and establishing comprehensive preventive measures will effectively pave the way for biting reduction by blackflies.