Irish explorer and entomologist, specialising in Odonata (dragonflies), who also collected plants on her travels. A founder member of the British Dragonfly Society. Cynthia Longfield was employed by the British Museum (Natural History) in 1921, having completed a visit to South America that year. In 1924, Longfield became Keeper of the insect house at London Zoo and subsequently participated on the St. George expedition to the Pacific islands in 1924, retracing Darwin's journey. Employed as an assistant to the entomologist Cyril Collenette, the two began a long-term romantic involvement, though they never married. Longfield became an associate scientist at the Natural History Museum, resident Odonata expert and later nicknamed 'Madam Dragonfly'.The couple returned from a field expedition to Mato Grosso, Brazil in 1938 to find that Europe was rapidly building up to the Second World War. Cynthia Longfield joined the Auxiliary Fire Services as driver and supervisor of women firefighters in South Kensington. Collenette wrote: "during the war years she served with the fire fighting services and when an incendiary bomb fell on the Botany Department of the museum, she at once realised the gravity of the threat and was in a position to divert all resources to controlling the fire and her prompt action probably saved the whole museum from destruction".

Longfield continued working at the Museum through the war until her retirement in 1957. Botanical collections were generally made with travelling companions, including her sisters cited as 'The Misses Longfield' who accompanied her on horseback through the Canadian Rockies, though her visit to the Masai Mara in Uganda (1934) was made alone.