The character‐identity network of the arthropod segment and its bearing on the arthropod head problem

The arthropod head problem has been puzzling scientists for more than a century. Key to this conceptual dispute is the question if the anterior of the arthropod head is serially homologous with the rest of the arthropod body, is unsegmented, or is built of non-homologous segments. Recent work revived the latter hypothesis which would, if taken for true, provide a simple solution to most aspects of the arthropod head problem, thus being of significant importance for our understanding of structural homology and arthropod evolution. One of the key arguments supporting this hypothesis is that the segment-polarity gene (SPG) network is highly conserved in posterior segments, but varies significantly in anterior head segments. Defining the SPG network as a character-identity network (CHiN) for the arthropod segment, the anterior variability would strongly indicate a different origin of the anterior versus the posterior head segments in arthropods. Here I discuss the arthropod head problem with respect to the proposed CHiN. I come to the conclusion that careful literature analysis shows that the SPG network is more flexible than claimed, in both anterior and posterior segments, and that the CHiN argument is therefore not supporting the so-called "Non-Homology-Hypothesis."