Arthropod Head Problem - Background

Background

It would be too bad if the question of head segmentation ever should be finally settled; it has been for so long such fertile ground for theorizing that arthropodists would miss it as a field for mental exercise. R.E. Snodgrass, 1960

According to recent research, it has been suggested that some key events in the evolution of the arthropod body resulted from changes in certain Hox genes' DNA sequences. The trunks of arthropods are built out of repeated segments, which are typically associated with various structures such as a pair of appendages, apodemes for muscle attachment, ganglia and, at least embryologically, coelomic cavities. While many arthropod segments are modified to a greater or lesser extent (for example, only three of the insect thorax and abdominal segments typically bear appendages), it is widely assumed that the ancestral state was for all of the segments to be nearly identical. However, while the segmental organisation of the trunks of adult arthropods can be usually readily seen, that of the head is much less obvious. Arthropod heads are typically fused capsules that bear a variety of complex structures such as the eyes, antennae and mouth parts. The challenge that the arthropod head problem has to address is to what extent the various structures of the arthropod head can be resolved into a set of hypothetical ancestral segments. Given the high compaction and complexity of adult arthropod heads, much attention has been directed towards understanding the developmental processes that give rise to them, in the hope that they will reveal their segmental organisation more clearly.