Longfield, Cynthia Evelyn (1896–1991), entomologist and traveller, was born 16 August 1896 at 20 Pont St., Belgravia, London, youngest of three daughters of Lt-col. Mountifort John Courtenay Longfield and his wife Alice Elizabeth (née Mason) of Castle Mary, Cloyne, Co. Cork, the Longfield residence since the end of the seventeenth century. The once vast estate had been gradually reduced to less than 1,000 acres by 1890. However, her father farmed it successfully and the family divided their time between Cork and a house in London. James Mason, father of Alice, was a professional scientist and a Sorbonne graduate of chemistry and engineering. He made his fortune from mining in Portugal, and as well as inventing a technique for copper extraction from ore, had built his own chemistry laboratory in the grounds of his house in Oxford. Alice often used to help him, but her special interest was in archaeology.
Cynthia was educated at home by governesses. She was often impatient with the lessons, preferring the times spent in Ireland where she could roam the grounds and countryside and indulge her interest in science and nature, no doubt influenced by her mother and grandfather. She read widely and regarded Charles Darwin's voyage and theory as an inspiration. The outbreak of the first world war ended her idyllic childhood. At the age of 18 she joined the Royal Army Service Corps as a driver, going on to work in an aeroplane factory. In 1920 Castle Mary was burnt down during the ‘troubles’, although Park House was rebuilt in the grounds. As she had missed her societal coming-out, her parents encouraged her to go on a grand tour to South America with family friends in 1921, which opened her eyes to the range and diversity of wildlife. Her journals show a fascination with the birds, plants, and insects observed on her travels. In 1923 she made another tourist trip to Egypt with her mother and sister, where they visited the newly opened tomb of Tutankhamen.
In 1924 she was accepted on the St George expedition to the Pacific as a self-funded participant, joining a team of naturalists on an eighteen-month replication of Darwin's voyage. She fell in love with Cyril Collenette, one of the entomologists on board ship. Although they never married they sustained a lifelong friendship. She helped in the collection of insect specimens for the Natural History Museum, and on her return to London worked as an unpaid associate cataloguing the dragonflies, the Odonata. Thus began her lifetime work with this poorly studied group, on which she later became an international authority.
She made several further trips abroad, continually adding to the museum's collection of dragonflies and butterflies. In 1927 she joined Cyril Collenette's expedition to the Mato Grosso in Brazil, and after the death of her father in 1929 she used her legacy to fund a trip to south-east Asia with a female companion. She crossed Canada in 1932 as part of a family holiday. Her next major expedition was to east Africa in 1934, when, unusual for the time, she travelled alone through Kenya, Uganda, Rhodesia, and South Africa, visiting government entomologists and all the time collecting specimens. She kept a journal and took notes on her wide range of interests: archaeology, anthropology, politics, and natural history. Her second African trip, from Capetown to Zimbabwe, was made with a female friend in 1937. She bought a small coffee farm in Uganda, with the hope of spending future time there. However, malaria prompted a return to England, and as the second world war broke out she volunteered to join the Auxiliary Fire Service.
After the war she resumed her work with the museum until her retirement in 1957, spending her time identifying specimens from all over the world, publishing papers annually, and becoming increasingly involved in the museum nomenclature committees. She was generous with her time and particularly encouraged young researchers. In her quest for new records she travelled extensively in Britain and Ireland as well as Europe, always with net in hand. In 1935 she co-wrote The generic names of the British Odonata with a check list of British species. Her popular 1937 book, The dragonflies of the British Isles, became the standard handbook. As a result of correspondence with an increasing number of observers, the distribution of dragonflies was more accurately recorded, leading to a revised edition of the book (1945). In 1960 Dragonflies was published, one of the Collins New Naturalist Series, co-authored with Philip Corbet and Norman Moore. Some of her Irish records were published in the Irish Naturalists Journal.
She was a member of several societies: the Entomological Society of London, the Royal Geographical Society, and the London Natural History Society. The British Museum made her an honorary member in 1948, as she had worked there voluntarily since 1927. She was made first fellow of the British Dragonfly Society set up in 1983. In Ireland she became a patron of the Cork Ornithological Society in 1967. Two dragonfly species were named after her, Corphaeschna longfieldae Kimmins from Brazil and Agrionopter insignis cynthiae Lieftinck from the Tanimbar islands in Asia.
On retiring in 1956 she returned to Park House, in the grounds of Castle Mary, Cork. She continued her field trips around Ireland in her Austin, complete with net, binoculars, and walking stick, travelling and attending conferences in Europe and overseas until her seventy-sixth year. In 1979 she donated her personal library of natural history books and her microscope to the RIA, and many of her Irish specimens are preserved in the Natural History Museum, Dublin. She died on 27 June 1991 aged 94, and was the last of the Longfields to be buried in St Colman's Church of Ireland cathedral, Cloyne, Co. Cork. A stone plaque commemorates her.