ocular segment While classically this structure is not considered to be a segment, but rather to be the non-segmental acron - anterior to the first segment, this ontology follows Schmidt-Ott and colleagues (Schmidt-Ott et al., 1994, and Schmidt-Ott and Technau, 1992) in considering it to be the second segment (posterior to the labral segment in the segmenting embryo). This is based on relating gene expression in the early segmenting embryo to the effects of loss of function mutations of these genes on formation of larval head structures. FBbt:00000005 UBERON:6000005 acron Head segment derived from the second embryonic segment (between the labral and antennal segments). In the larva, this segment includes Bolwig's organ. fly_anatomy.ontology procephalic segment