Arthropod head problem

Four types of arthropods showing the acron and 9 head and/or body segments. Trilobites and chelicerates are shown with 7 head segments, and crustaceans and tracheates with 5 head segments. Of these, the first head segment of chelicerates and the second head segment of trachates is lost in development. All four start with an acron at the anterior end bearing compound eyes. All have nephridia on some or all head segments, some of which are lost in development in chelicerates. All—other than chelicerates—have antennae on the first head segment, and crustaceans also have antennae on the second head segment. Only chelicerans have chelicera, on the second head segment and first body segment, and pedipalps, on the third body segment. Crustaceans have mandibles on the third head segment and maxillae on each of the fourth and fifth head segments. Trilobites and chelicerates bear legs on all remaining head segments, but crustaceans and tracheates have legs on the anterior body segments.
A
L
L
L
L
L
L
x
C
P
L
L
L
L
Ci
A
A
Mnd
Mx
Mx
L
L
L
L
L
A
x
Mnd
Mx
Mx
L
L
L
L
    = acron
    = segments contributing to the head
    = body segments
x = lost during development
    = eyes
    = nephridia
O = nephridia lost during development
L = Leg
Mnd = Mandible
Mx = Maxilla
Four types of arthropods showing the acron and 9 head and/or body segments. Trilobites and chelicerates are shown with 7 head segments, and crustaceans and tracheates with 5 head segments. Of these, the first head segment of chelicerates and the second head segment of trachates is lost in development. All four start with an acron at the anterior end bearing compound eyes. All have nephridia on some or all head segments, some of which are lost in development in chelicerates. All—other than chelicerates—have antennae on the first head segment, and crustaceans also have antennae on the second head segment. Only chelicerans have chelicera, on the second head segment and first body segment, and pedipalps, on the third body segment. Crustaceans have mandibles on the third head segment and maxillae on each of the fourth and fifth head segments. Trilobites and chelicerates bear legs on all remaining head segments, but crustaceans and tracheates have legs on the anterior body segments.
The Arthropod head problem[1]
Formation of anterior segments across arthropod taxa based on gene expression and neuroanatomical observations,[2][3] Note the chelicera(Ch) and chelifore(Chf) arose from somite 1 and thus correspond to the first antenna(An/An1) of other arthropods.

The (pan)arthropod head problem[4] 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 chelicerates, myriapods, and crustaceans, as well as 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,[5] 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,[6] 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.

  1. ^ Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). Invertebrate Zoology (7th ed.). Brooks / Cole. p. 520. ISBN 0030259827. who cite Ax, P. (2000). Multicellular Animals: The Phylogenetic System of the Metazoa. Vol. 2. Berlin: Springer. p. 396.
  2. ^ Dunlop, Jason A.; Lamsdell, James C. (2017). "Segmentation and tagmosis in Chelicerata". Arthropod Structure & Development. 46 (3): 395–418. Bibcode:2017ArtSD..46..395D. doi:10.1016/j.asd.2016.05.002. ISSN 1467-8039. PMID 27240897.
  3. ^ Ortega-Hernández, Javier; Janssen, Ralf; Budd, Graham E. (2017-05-01). "Origin and evolution of the panarthropod head – A palaeobiological and developmental perspective". Arthropod Structure & Development. Evolution of Segmentation. 46 (3): 354–379. Bibcode:2017ArtSD..46..354O. doi:10.1016/j.asd.2016.10.011. ISSN 1467-8039. PMID 27989966.
  4. ^ Ortega-Hernandez, Javier and Janssen, Ralf and Budd, Graham E. (2017). "Origin and evolution of the panarthropod head – a palaeobiological and developmental perspective". Arthropod Structure & Development. 46 (3): 354–379. Bibcode:2017ArtSD..46..354O. doi:10.1016/j.asd.2016.10.011. ISSN 1467-8039. PMID 27989966.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ^ Goodrich, E. S. (1897), "On the Relation of the Arthropod Head to the Annelid Prostomium", Quarterly Journal of Microscopical Science (s2–40): 247–268
  6. ^ Rempel, J. G. (1975), "The Evolution of the Insect Head: the Endless Dispute", Quaestiones Entomologicae, 11: 7–25