He migrated to South Africa in 1889 and taught as a missionary in schools in northern Transvaal Waterval (Nuwe Smitsdorp).
He was widely respected as an authority on South African moths and was a botanist, collecting alongside Reino Leendertz.
In 1921-1922, he visited Europe and worked in museums in London, Leiden and Berlin, comparing and identifying many hundreds of specimens he collected.
In 1945, the government of South Africa purchased his collection, equipment and library and placed it in the care of the Transvaal Museum, where he himself was appointed Honorary Curator of Heterocera.
[2] Janse was a founding member of the South African Biological society where he was awarded the Senior Captain Scott Medal in 1922 and was elected five times president.
He continued his research with his involvement with Transvaal Museum, and made three extensive collecting trips by car and trailer, despite being over 75.