Pondering the procephalon: the segmental origin of the labrum

With accumulating evidence for the appendicular nature of the labrum, the q uestion of its actual segmental origin remains. Two existing insect head se gmentation models, the linear and S-models, are reviewed, and a new model i ntroduced. 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 tigh tly integrated into this segment. This model appears to explain a wider var iety of insect head segmentation phenomena. Embryological, histological, ne urological and molecular evidence supporting the new model is reviewed.