1. The oxygen consumption of normal and gill-less nymphs of the mayflies Baetis sp., Cloeon dipterum and Ephemera vulgata has been measured at various oxygen concentrations.

2. It has been found that over the complete range of oxygen concentrations studied, the tracheal gills do not aid oxygen consumption in Baetis sp. In Cloeon dipterum, at all oxygen concentrations tested, no gaseous exchange takes place through the gills; at low oxygen concentrations, however, the gills function as an accessory respiratory mechanism in ventilating the respiratory surface of the body and so aid oxygen consumption. In Ephemera Vulgata the gills aid oxygen consumption even at high oxygen concentrations. In this species the gills may function both as true respiratory organs and as a ventilating mechanism.

3. It is shown that the differences in gill function can be related to the oxygen content of the habitat of each species.

Keddey Fletcher Warr Student of the University of London.