Which one of the following statements is incorrect ?

Insects respire through a network of air tubes called tracheae, which open to the outside through spiracles.
The tracheae branch repeatedly into fine tracheoles that reach every tissue and carry oxygen directly to the cells.
The circulating body fluid of insects (haemolymph) has no respiratory pigment and plays no significant role in the transport of oxygen.
So the statement that in insects the circulating body fluids serve to distribute oxygen to tissues is incorrect.
In mammals some air always remains in the lungs after expiration (residual air) and mixes with the fresh air, slightly reducing the efficiency of respiration; this is true.
In birds the non-respiratory air sacs keep air flowing through the lungs in one direction during both inspiration and expiration, increasing the efficiency; this is true.
In the gills of fishes, water and blood flow in opposite directions (countercurrent flow), which allows maximum uptake of oxygen; this is true.