develops from external sensory organ precursor cell external sensory organ precursor cell I external sensory organ precursor cell IIb Second-order precursor cell that develops from an external sensory organ precursor cell I. In the microchaete lineage, it divides to give rise to the thecogen cell and the neuron of an external sensory organ (Gho et al., 1999; Hartenstein and Posakony, 1989). In the development of antennal olfactory sensilla, it produces two daughters, pNa and pNb, which each divide asymmetrically to produce up to four types of olfactory neuron per sensillum (Endo et al., 2012; Chai et al., 2019). fly_anatomy.ontology SOPIIb pIIb FBbt:00005167