Theodore D.A. Cockerell, zoologist, was educated at the Middlesex Hospital Medical School in England. In 1887 he went to Colorado, United States, where he studied entomology and botany in the field. In 1891 he became curator of the public museum of Kingston, Jamaica, but two years later was appointed professor of entomology at the New Mexico Agricultural College (1893-1896) and consulting entomologist to the New Mexico Agricultural Experiment Station (1893-1901). From 1900 to 1909 he was consulting entomologist to the Arizona Agricultural Experiment Station. At the same time he taught biology at New Mexico Normal University (1900-1903), was curator of the Colorado College Museum (1903-1904), and lectured in entomology at the University of Colorado (1904-1906). There he participated in the establishment of the University of Colorado Museum of Natural History. He became professor of systematic zoology at the University of Colorado in 1906 and subsequently professor of zoology there from 1912 to 1934.
Cockerell was a Fellow of the American Association for the Advancement of Science and the author of over 3000 articles and notes in scientific publications and other works. These dealt with the taxonomy of insects (particularly Hymenoptera and Hemiptera), molluscs and other groups; palaeontology (particularly fossil insects); the natural history of various regions and the geographical distribution of life; evolution; social reform; and education. As one of the most prolific taxonomists in history he published descriptions of over 9000 species and varieties of insects and about 1000 species of molluscs and other groups, including fossil species. Of the insect taxa he described, about 70% were bees. In one journal alone, the Annals and Magazine of Natural History, he published some 200 papers on bees. His research took him to Siberia, Japan, South America, Madeira, Russia, Australia, Morocco, Central Africa, and South Africa. At the time of his death, at the age of 81, he was working on a monograph dealing with the bees of Honduras. His publications included a textbook on Zoology (1920, 568 pp) of which revised editions were published to 1943.
Cockerell's contributions to South African science consisted mainly of a series of papers on local bees in the Annals of the Durban Museum. His first paper, "Report on a collection of South African bees chiefly from Natal" (1914-1917, Vol. 1, pp. 188-216) was followed by six others with new records of Natal bees (Vol. 1, pp. 460-468; 1917-1920, Vol. 2, pp. 39-46, 117-118, 189-196, 247-262, and 286-318). In later years he reported on bees collected in the Kalahari, in "Scientific results of the Vernay-Lang Kalahari Expedition, March-September 1930: Hymenoptera (Apoidea)" (Annals of the Transvaal Museum, 1935, Vol. 17(1), pp. 63-94). The South African non-marine mollusc Edouardia cockerelli was named after him.