Assessment
The number and nature of the post-oral segments in the insect head have rarely been questioned. A much more difficult area, however, has been the nature of the preoral region. The obvious contradiction between a theory that no-preoral structures are segmental, and evidence, such as for the first antennae of crustaceans, that some such structures clearly are, led workers as long ago as Lankester to posit that there has been forward migration of segments in front of the mouth. Indeed, such a process can be seen in ontogeny of the tritocerebrum, which can be seen to migrate forward as the brain develops; furthermore, although in most insects and crustaceans its ganglia are part of the brain, its commissures still loop behind it, suggesting derivation from a more posterior position.
Nevertheless, even allowing for this possibility, the complexity of the anterior part of the brain, which even if the acron concept is incorrect may still have been inherited from very basal animals; untangling the new characters evolved by the earliest arthropods from those inherited from their ancestors therefore still stands centrally in the arthropod head problem.
Read more about this topic: Arthropod Head Problem
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