I am not sure if these divisions are still in existance. But the Madura country manual written in 1860s affirms that they were very much alive - after nearly a 1000 years of their creation
Nobody knows how this division came into existance. No solid theories. But it represents a drive on the part of some groups to stand united.
The madura manual lists many many interesting practices on specific castes of Idangai / valangai. Most of these customs are not prevelant to my limited knowledge. One of these is that in a family, the wife belongs to Idangai whereas husband belongs to valangai. Meaning the females and males of this particular caste belong to two different divisions.