Option A: The principle of countercurrent flow facilitates efficient respiration in the gills of fishes.

This statement is correct. In fish, water flows over the gills in one direction while blood flows in the opposite direction. This countercurrent flow mechanism maximizes oxygen uptake and carbon dioxide removal, making respiration more efficient.

Option B: The residual air in lungs slightly decreases the efficiency of respiration in mammals.

This statement is also correct. In mammals, some air always remains in the lungs after exhalation (residual volume). This air is less oxygenated compared to fresh air, which slightly decreases the overall efficiency of gas exchange in the lungs.

Option C: The presence of non-respiratory air sacs increases the efficiency of respiration in birds.

This statement is correct as well. Birds have a unique respiratory system that includes air sacs in addition to lungs. These air sacs do not participate directly in gas exchange but help to ensure a continuous flow of air through the lungs, leading to highly efficient respiration.

Option D: In insects, circulating body fluids serve to distribute oxygen to tissues.

This statement is incorrect. In insects, oxygen is distributed directly to tissues through a network of small tubes called tracheae. The tracheae deliver oxygen directly to the cells, and the circulatory system (haemolymph) does not play a significant role in oxygen transport.

Therefore, the incorrect statement is Option D.