title: Pondering the procephalon: the segmental origin of the labrum.
abstract: With accumulating evidence for the appendicular nature of the labrum, the question of its actual segmental origin remains. Two existing insect head segmentation models, the linear and S-models, are reviewed, and a new model introduced. The L-/Bent-Y model proposes that the labrum is a fusion of the appendage endites of the intercalary segment and that the stomodeum is tightly integrated into this segment. This model appears to explain a wider variety of insect head segmentation phenomena. Embryological, histological, neurological and molecular evidence supporting the new model is reviewed.
authors.affiliation: USDA, ARS, Grain Marketing and Production Research Center, Manhattan, KS 66502, USA.
authors.forename: M S
authors.initials: MS
authors.lastname: Haas
authors.forename: S J
authors.initials: SJ
authors.lastname: Brown
authors.forename: R W
authors.initials: RW
authors.lastname: Beeman
mesh_terms: D000818:Animals; D005075:Biological Evolution; D004331:Drosophila melanogaster / Q000033:anatomy & histology / Q000196:embryology / Q000235:genetics; D018507:Gene Expression Regulation, Developmental; D006257:Head / Q000033:anatomy & histology* / Q000196:embryology* / Q000294:innervation; D007313:Insecta / Q000033:anatomy & histology* / Q000196:embryology* / Q000235:genetics; D023421:Models, Animal; D008954:Models, Biological; D009055:Mouth / Q000033:anatomy & histology / Q000196:embryology / Q000294:innervation; D009987:Orthoptera / Q000033:anatomy & histology / Q000196:embryology / Q000235:genetics