Sentence:  Total independence of atmospheric air is possible only if insects have a permanent gas store or incompressible gas gill, called a <WSD>plastron</WSD>. Unlike compressible gas stores, the volume of a <WSD>plastron</WSD> remains constant and it is incompressible. 
Meaning of the word marked with <WSD> tags:  A film of air trapped by specialized hairs against the body of an aquatic insect, and which acts as an external gill.