The evolution of air-breathing respiratory faculties in invertebrates

This chapter aims at piecing together the evolution of air breathing in invertebrates, the main conclusion here being that it evolved independently several times. In molluscs alone, air breathing has evolved several times, but almost exclusively among snails. Among crustaceans, several groups of crabs have also independently developed terrestrial representatives and transitional stages, particularly in the control of breathing, are evident. Analysis of insects shows few recognizable evolutionary progressions: air sacs and different stigmatal closure mechanisms have appeared and disappeared numerous times, even within closely related groups. But other tracheate groups such as myriapods show an interesting correlation between the presence of tracheal lungs, which end in an open circulatory system, and tracheae that invade the tissue as in insects, and the presence or reduction of respiratory proteins. In arachnids a similar tendency is seen, and the most interesting developments were the (partial) replacement of a ‘perfectly good’ air-breathing organ (book lungs) by another one (tracheae).