Arnold W. Cooper was a solicitor in Richmond, Natal, from about 1880 to about 1911. By 1906 he was a Justice of the Peace, advocate, solicitor and notary public, counting among his clients the Bank of Africa and the London and Lancashire Fire Assurance Company. By 1909 he was chairman of the Board of Trustees of the Natal Museum. He had artistic abilities, for in 1889 he provided illustrations for the book The diamond hunters of South Africa, by the popular author Alfred W. Drayson. Three years later he also illustrated one of the many editions of Drayson's Among the Zulus; the adventures of Hans Sterk, South African hunter and pioneer.
In October 1904 Cooper was appointed as a member of the Natal Technical Education Commission, which recommended that a hybrid university-technical institute be established. Nothing came of this recommendation.
Cooper was also a naturalist. He had a lively interest in microscopy and was a Fellow of the Royal Microscopical Society. In 1896 he became a member of the South African Philosophical Society and remained a member to 1907, but did not join its successor, the Royal Society of South Africa. By 1906 he had also joined the South African Association for the Advancement of Science. He was still a member, and living in Richmond, in 1911.
Cooper's most important contribution to scientific knowledge was a paper, "Notes on a new species of Gymnoplea from Richmond, Natal, South Africa: Adiaptomus natalensis", which was published in the Annals of the Natal Museum (1906, Vol. 1, pp. 97-104). He proposed that his new species be placed in a new genus of the Gymnoplea, the latter being a group of some hundreds of species in the subclass Copepoda, which contains mainly small and parasitic crustaceans. He also published a short paper on "The Bag-worm in Natal" in the Natal Agricultural Journal (1899, Vol. 2, pp. 9-10).
In 1896, during a plague of red locusts, Cooper recognized the presence of a fungus on dead locusts near Richmond. Many of the insects had been killed by this fungal disease, which led him to start investigating the possibility of using it for locust control. Although the effects of the fungus had already been observed (and announced) by the botanist J.M. Wood* and his associate, Maurice Evans*, in April 1895, Cooper took the lead in cultivating the fungus. His initiative was described in an article by the Government Entomologist of the Cape Colony, C.P. Lounsbury* in the Agricultural Journal of the Cape Colony (1896, Vol. 8, pp. 330-331), and he was awarded a small grant by the Natal government to pursue his investigations. Requiring expert help, he went to the Colonial Bacteriological Institute in Grahamstown, where he and Dr. R.S. Black* produced a pure culture of the fungus. The Institute distributed the culture in South Africa and in several other countries for some years, but after apparent initial success doubts arose as to the purity of the culture and its effectiveness. Later cultures were shown by I.B. Pole Evans* to contain a species of fungus that is harmless to locusts.