It’s always nice to see something for the first time. This is a short video of a yet undescribed type of Batwing Slug, Siphopteron cf. quadrispinosum.
Batwing Slug are a type of Headshield Slug and have the ability to actually swim through the water, using the wing-like bits on the side of its body (the parapodia), which it can flap, up and down, like a bird.
This one was not flying by the time I saw it, though my guide said that was how he first spotted it, as it moved through the water. This explained, at least, who he spotted it in the first place, as it was only a few millimetres long!!
The Siphopterons move with a sort of jerky movement, as the pull themselves along, using their head to aid locomotion.