Unraveling the insect head: Segmentation and pancrustacean homologies.

The segmental composition of the arthropodan head is a long-standing biological question. This is particularly controversial in insects where considerable fusion and modification obfuscate the postembryonic segmentation. Here we propose a reinterpretation of the insect head morphology and segmentation, which also accords with current genetic and embryological data. The new interpretation of a generalized insect head is based on the discovery of intercalary segment remnants (the intercalates), thanks to the use of 3D-reconstruction of a Cretaceous thripidan and to a reassessment of the 'anterior tentorial arm' as the anteriormost part of a lateral segmental suture from the contact of the head lobe and gnathal segments during development. We show that the two-jointed mandibular articulation is likely symplesiomorphic for Insecta, no longer supporting the clade 'Dicondylia', the most diverse lineage of all insects. We discuss new perspectives on the 'labrum' in Pancrustacea.