(en) Posnien, N, Bashasab F, Bucher G. (2009). The insect upper lip (labrum) is a nonsegmental appendage-like structure. Evolution & Development11 (5): 480–488. PMID19754705. DOI: 10.1111/j.1525-142X.2009.00356.x.
(en) Haas M, Brown S, Beeman R. (2001). Pondering the Procephalon: the Segmental Origin of the Labrum. Development, Genes and Evolution211 (2): 89–95. DOI: 10.1007/s004270000129.
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(en) Posnien, N, Bashasab F, Bucher G. (2009). The insect upper lip (labrum) is a nonsegmental appendage-like structure. Evolution & Development11 (5): 480–488. PMID19754705. DOI: 10.1111/j.1525-142X.2009.00356.x.