Alexander Henry Haliday was an Irish entomologist. He is primarily known for his work on Hymenoptera, Diptera, and Thysanoptera, but worked on all insect orders and on many aspects of entomology.
Haliday was born in Holywood, County Down, Ireland. A boyhood friend of Robert Templeton, he divided his time between Ireland and Lucca, where he co-founded the Italian Entomological Society with Camillo Rondani and Adolfo Targioni Tozzetti. He was a member of the Royal Irish Academy, the Belfast Natural History Society, the Microscopical Society of London, and the Galileiana Academy of Arts and Science, as well as a fellow of the Entomological Society of London.
Alexander Haliday was among the greatest dipterists of the 19th century and one of the most renowned British entomologists. His achievements were in four main fields: description, higher taxonomy, synonymy, and biology. He erected many major taxa including the order Thysanoptera and the families Mymaridae and Ichneumonidae.