The kind of physical gill known as a plastron is a gas film of constant volume and an extensive water-air interface. Such films are held by hydrofuge structures, and they resist wetting at the hydrostatic pressures to which they are normally subjected in nature. In well-aerated water a plastron enables the insect to be immersed indefinitely. A plastron is here recorded for the first time in adult beetles of the suborder Myxophaga. In both known genera of the family Torridincolidae a plastron is present on the abdomen. In Ptyopteryx of Brazil the plastron forms a diffraction grating that is responsible for the iridescence oi the abdomen. The structure of the plastron of Torridincola of Rhodesia is like that of some other beetles, and it is not iridescent. Among adult beetles plastron respiration has been independently evolved in at least eight quite different groups, and within some of these groups it has been evolved on more than one occasion.
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