Arthropod Head Problem - The Acron Concept

The Acron Concept

The arthropod head problem has until recently been predicated on the Articulata theory, i.e. that the arthropods and annelids are close relatives. Although arthropods are essentially direct developers that do not possess a trochophore-like larva, the annelids do. During annelid metamorphosis, segments are added close to the posterior of the body, behind the mouth; whereas the brain is derived from the episphere or region in front of the mouth. Recognition of this led to the concept of a primary, non-segmental component of the body in annelids known as the acron being developed, from which the brain is ultimately derived. Because the arthropod and annelid heads, in the light of the Articulata theory, were assumed to be structurally homologous in some way, the arthropod head was also often considered to incorporate a non-segmental acronal component. Taking the homology between annelid and arthropod heads at face value, Swedish workers such as Hanström and Holmgren assumed that a large part of the arthropod head must correspond to the acron, a view followed later by several prominent American insect workers such as Butt and Snodgrass. They proposed that all pre-oral structures in insects were non-segmental, although such a view is at odds with the preoral position of apparently bona fide appendages such as the antennae. A less extreme set of theories propose that only the protocerebrum and associated structures should be considered to be acronal.

The view that the arthropod head must contain an acronal remnant has been shaken by the relatively recent revision of protostome phylogeny, which has dismantled the Articulata and placed the arthropods together with a group of unsegmented worms often referred to as the Cycloneuralia in the so-called Ecdysozoa. All members of the Ecdysozoa are direct developers without a trochophore, and the cycloneuralians have terminal mouths. As a result, the idea of the arthropods having inherited a preoral acron from their ancestors seems less likely.