Arthropod head problem

The arthropod head problem is a long-standing zoological dispute concerning the segmental composition of the heads of the various arthropod groups, and how they are evolutionarily related to each other. While the dispute has historically centered on the exact make-up of the insect head, it has been widened to include other living arthropods such as the crustaceans and chelicerates; and fossil forms, such as the many arthropods known from exceptionally preserved Cambrian faunas. While the topic has classically been based on insect embryology, in recent years a great deal of developmental molecular data has become available. Dozens of more or less distinct solutions to the problem, dating back to at least 1897, have been published, including several in the 2000s.

The arthropod head problem is popularly known as the endless dispute, the title of a famous paper on the subject by Jacob G. Rempel in 1975, referring to its seemingly intractable nature. Although some progress has been made since that time, the precise nature of especially the labrum and the pre-oral region of arthropods remain highly controversial.

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.

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