All photos are in the hand as it was in the process of being removed from someone's hotel room. The flying snakes are all in the genus Chrysopelea. Usually when people talk about these they talk about Chrysopelea ornata (the big golden tree snake) or Chrysopelea paradisi (the Paradise tree snake, which is the one I've seen before in, and was looking for unsusccessfully in, Singapore Botanic Garden).
This however is something a bit different, the Sri Lankan flying snake - Chrysopelea taprobanica
It's pretty, no? Obviously it doesn't have limbs to spread membranes across to fly with so what these snakes do is flatten out their rib cages to create a kind of lateral wing and then swim through the air. If that sounds hard to visualise then this NatGeo footage of the Paradise tree snake (also shot in Singapore BG) should help.
To be honest I'm not sure of the extent to which the smaller Sri Lankan species "flies" compared to C. paradisi but it is certainly very, very quick and seemed quite keen to throw itself away from people as we removed it to a safe location. My understanding is that this is one of those technically venomous species which doesn't affect people. I wouldn't know as it wasn't remotely bitey or strikey (unlike the Boiga around in the same area).
Incidentally (and I imagine off topic for most of you), whilst I was in Sri Lanka taking this photo amongst other things, test match no. 1966 was happening between Sri Lanka and India. Being as I was in the Asian tv region I had the pleasure of watching (play happily largely coincided with the down time post-morning and pre-afternoon safari drives) the ridiculous hype put upon a team of ageing great batsmen and overrated and frankly rather average young bowlers by the Indian media. This India aren't a great test team, frankly they never were, the system just happened to rank them better than England and South Africa due to a freak of the algorhythm. As evidence hereof I present the captaincy records of England under Strauss: last 30 tests, 19-7-4 (63.33% win rate) and India under Dhoni: 30 tests, 15-9-6 (50% win rate). The monumental beating they took this week and earlier this week has been coming a long, long time and the Indian cricket establishment and India's fans would do well to look past the hype and assess why.