Springtail post antennal organ. Coloured scanning electron micrograph (SEM) of the springtail post antennal organ. The postantennal organ or "PAO" is a chemoreceptor sensory organ found in some springtails. However, families like the Entomobryidae do not have it. The post-antennal organ is generally made up of a system of tiny pores (between 5 and 10 nanometers) organized on a single sensory cell, with branched external hair segments, connected to the central nervous system. These pores have an olfactory function and very probably also play the role of thermal and hygrometric ( measures humidity) probe. Magnification: x800, when printed 10 cm wide.