For performance reasons, bugs should be rendered as mere tiny particles, not 3D models, and visible only when relatively close enough (same range as for seeing dropped items). They should be so tiny as to be just "generic bugs", not distinctive multi-pixels textures for all kinds of bugs from mosquitoes to bumblebees etc. Personally i have not much interest in adding such bugs. Birds and fishes seem much more interesting.
Fireflies however seem very cool, but they pose another layer of complexity because they'd radiate light.
They probably should work like this:
- One pure-Air block becomes the "center", and that it the block emitting light.
- The fireflies, maybe looking like yellow-white XP orbs, would roam about randomly in the 3x3x3 area that is centered on the light emitting central air block. The fireflies particles themselves just look bright, but don't actually radiate light. Sometimes the particles might roam a tiny bit further, but will quickly come back within the 3x3x3 area.
- More rarely, the "lighted up center" block will move 1 block to another adjacent air block, and never more than 3 blocks away from the ground.
- They could be slightly attracted to torches, tending to have their center stick right adjacent to a torch.
- They disappear come morning.
This would allow only a few simple particles to move reasonably without constant and numerous light levels updates.