Antonie Johannes Theodorus Janse, entomologist, was the son of Antonie Johannes Janse and his wife Willemina Broekhuisen. He was trained as a missionary teacher in The Hague and in May 1899, a few months before the outbreak of the Anglo-Boer War (1899-1902), emigrated to the South African Republic (Transvaal) and became a teacher on the farm Waterval (later New Smitsdorp, near present Mokopane, Limpopo). During the war he was interned at Pietersburg (now Polokwane) and after the war settled there as a photographer. In 1905 he obtained an appointment as lecturer in the natural sciences (human physiology, biology, geography) at the newly established Normal College (a teachers' training college) in Pretoria. He was also put in charge of the Normal College Herbarium. He remained at the college until his retirement in 1937, at the age of 60.
During his first few years at the college Janse collected some plants in the northern parts of the Pretoria District, with Miss Reino Leendertz* of the Transvaal Museum. The specimens went to the Transvaal Museum and later to the National Herbarium in Pretoria. The species Delosperma Jansei was named after him by N.E. Brown*. Early in 1909 he was appointed as the first temporary lecturer in botany at the Transvaal University College in Pretoria, for that year only. Some years later he wrote a popular booklet consisting of two parts: Rambles in kloof and mountain, and A natural history reader for South African schools (1912); later also an Afrikaans textbook for physiology entitled Ons wonderskone liggaam; handboek vir menslike fisiologie (1930).
From the time of his arrival in South Africa Janse started collecting insects in his spare time. In 1902 he already had a small collection, which at that time was housed in a refugee camp at Pinetown, KwaZulu-Natal. During the next few years (1903, 1904 and 1905) he presented a number of Coleoptera (beetles) and some Orthoptera from the northern Transvaal to the South African Museum in Cape Town. However, he soon began to specialise in the Lepidoptera (butterflies and moths), a group that he continued to study for the rest of his life. His holidays were spent in the veld, travelling on foot from a suitable railway station with a wheelbarrow full of collecting and camping equipment. Later he used a donkey cart, and still later a motorcar and caravan. At his house in Gezina, Pretoria, he built a private entomological laboratory which he equipped with insect cabinets he made himself. During a year's study leave in 1920 he visited museums in London, Leiden and Berlin to study their collections and identify many of his specimens. During the first entomological congress held in Pretoria in 1924 delegates visited his home laboratory, where he eventually accumulated around 100 000 specimens of moths. Meanwhile, in 1923, he was appointed (in addition to his lecturing post at the Normal College) honorary professor of systematic entomology at the Transvaal University College (later the University of Pretoria). His collection, library and equipment was bought by the South African government for the Transvaal Museum (now the Ditsong Museum of Natural History) in 1945, and he was appointed its curator for life. When he started his work there was no extensive collection of moths in South Africa, but by the time of his death he had built up an almost complete collection.
Most of the specimens that Janse studied were very small, requiring difficult dissections and accurate drawings of their morphology and organs. From 1915 onwards he produced about 50 publications and described over 500 species, all of them with his own detailed illustrations. His more general papers included the following: "On a zoological survey from an entomological point of view" (South African Journal of Science, 1919, Vol. 16, pp. 426-428); "On collecting, preserving and packing lepidopterous insects" (Journal of the Entomological Society of South Africa, 1939 , Vol. 2, pp. 176-180); "Glimpses of the development of entomological science in South Africa" (ibid, 1940, Vol. 3, pp. 1-8); and "The methods and aims of taxonomic study in entomology, with special reference to Lepidoptera" (South African Journal of Science, 1948, Vol. 45, pp. 107-112). Years earlier he had compiled a comprehensive Check-list of the South African Lepidoptera heterocera (1917, 219 pp). However, his magnum opus was The Moths of South Africa, published from 1932 onwards. Shortly before his death he was still planning Volume 7 of this work, for which he had already completed the drawings and made photographs.
Janse was an outstanding pioneer of South African entomology whose work on moths was widely recognised both locally and internationally. In 1925 an honorary Doctor of Science (DSc) degree was conferred upon him by the University of South Africa (via the Transvaal University College). He was an early member of the Royal Society of South Africa and was elected one of its Fellows in 1931. He was a foundation member, and a regular contributor of papers and demonstrations, of the Transvaal Biological Society (founded in 1907) and its successor, the South African Biological Society (from 1916), serving as president in 1915, 1916, 1918, 1919, 1920 and 1952. In 1922 he was awarded the society's Senior Captain Scott Memorial Medal. He became a member of the South African Association for the Advancement of Science in 1915, soon served on its council, and received its South Africa Medal (gold) in 1948. In May 1933 he became the first chairman of the Pretoria Entomological Club, which lasted until 1936. The next year he became a foundation member of the Entomological Society of South Africa, served as its second president in 1938/9 and several times as vice-president thereafter, and in 1956 became a life member. The next year the society dedicated Volume 20 of its Journal to him, to mark his eightieth birthday. He was a Fellow of the Royal Entomological Society, and an honorary member of the Nederlandse Entomologische Vereniging.
Janse was a man of boundless energy and whole-hearted devotion to entomology. His thoroughness in his work is clear from his publications, which show the meticulous care with which he studied and illustrated the genitalia and wing structure of numerous species. After his retirement he undertook three long collecting expeditions, travelling all over southern Africa in a caravan adapted to his scientific needs. He was married to Francina Johanna Elise Cramer, with whom he had a daughter. After they were divorced he married Frances Minnie Lewis, with whom he had two daughters.