Both A. atosia & lurida were recorded in the past in Singapore.
Urbanisation & decreasing greenery mean small colonies soon disappear except in the central forested area.
The butterfly is very similar to A. pseudomuta .
The key is that UnH spot 7 is inwards (towards the base) of spot 6 so that its inner margin is farther in.
In A. pseudomuta spot 7 is directly above spot 6, its margins lies within that of spot 6. https://www.butterflycircle.com/chec...1---Horace.jpg
Although going by the key this comes out as Arhopala lurida, mainly because of the position of HW spot 7, a number of features suggest this may be merely a variant of Arhopala pseudomuta.
These features are.
1. It is a common feature in pseudomuta for the FW spot 4 to be strongly dislocated out.
This much less in lurida.
2. UnH spot 6 is rounded in pseudomuta but more angular in lurida.
3. The green metalmark in pseudomuta is less sharp & flanked by a fuzzy strip.
It is well-defined in lurida.