I think the spiracles ["operculums"] of the creatures from James Cameron's Avatar are really interesting and am thinking on using the idea for the fauna of a planet. In the movies, the humanoid species of Pandora, the Na'vi (and their possible evolutionary ancestor, the prolemuris), breathe through their noses and consequently don't have breathing holes. I want my humanoid species to have them, though.
I saw this question here so I know that most vertebrates have separate smelling and breathing systems, but I'm wondering where would it make the most sense for a bipedal creature, in terms of evolution and efficiency. My first thought would be at the sides of the neck, but wouldn't it make more sense to have them closer to the lungs, so the chest?
In case it is relevant: I envision them to smell through nostrils, so some sort of connection between the nose-equivalent and the breathing holes would be beneficial, I think? But I can scrap the 'nose' if it makes no sense. The species is bipedal and human-like except for the breathing mechanism and possibly noses, at least for now. Let's assume the world is like earth, in terms of its air composition and the make-up of things.
I hope this is not opinion-based or a debate, but can edit and clarify if needed.
Description: "The operculum is a nostril-like opening found on many Pandoran animals that is used to take in air for respiration. [...] Operculum are typically placed near the animal's neck or chest in either pairs or columns. They tend to be in close proximity to the lungs, allowing animals to breathe more efficiently. During intense movement (i.e running or flying), air flows through the lungs in a continuous stream, providing a more active gas exchange, thus increasing efficiency. Despite the presence of this duct system, most animals tend to breathe in and out at rest, probably by closing the back openings with valves and allowing air to enter by expanding the chest. Since the animal's head is not directly involved in breathing, many Pandoran animals have a special muscular tube that conducts air to the vocal cords and allows them to make rather loud sounds."
EDIT 2: Added two close up pictures from the new game, one closed, one opened.