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Arthropod head drawback – Wikipedia

Arthropod head drawback – Wikipedia

2023-05-13 07:09:55

Dispute regarding the evolution of arthropods

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.



































    = segments contributing to the top

    = physique segments

x = misplaced throughout growth

O = nephridia misplaced throughout growth

L = Leg

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.

Formation of anterior segments throughout arthropod taxa based mostly on gene expression and neuroanatomical observations,[2][3] Word the chelicera(Ch) and chelifore(Chf) arose from somite 1 and thus correspond to the primary antenna(An/An1) of different arthropods.

The (pan)arthropod head drawback[4] is a long-standing zoological dispute regarding the segmental composition of the heads of the assorted arthropod teams, and the way they’re evolutionarily associated to one another. Whereas the dispute has traditionally centered on the precise make-up of the insect head, it has been widened to incorporate different dwelling arthropods corresponding to chelicerates, myriapods, crustaceans; and fossil types, corresponding to the various arthropods recognized from exceptionally preserved Cambrian faunas. Whereas the subject has classically been based mostly on insect embryology, lately an excessive amount of developmental molecular knowledge has turn into accessible. Dozens of roughly distinct options to the issue, courting again to at the least 1897,[5] have been printed, together with a number of within the 2000s.

The arthropod head drawback is popularly often called the countless dispute, the title of a well-known paper on the topic by Jacob G. Rempel in 1975,[6] referring to its seemingly intractable nature. Though some progress has been made since that point, the exact nature of particularly the labrum and the pre-oral area of arthropods stay extremely controversial.


It might be too unhealthy if the query of head segmentation ever must be lastly settled; it has been for thus lengthy such fertile floor for theorizing that arthropodists would miss it as a area for psychological train.

R.E. Snodgrass, 1960[7]

Some key occasions within the evolution of the arthropod physique resulted from adjustments in sure Hox genes‘ DNA sequences. The trunks of arthropods comprise repeated segments, that are sometimes related to varied constructions corresponding to a pair of appendages, apodemes for muscle attachment, ganglia and (at the least embryologically) coelomic cavities. Whereas many arthropod segments are modified to a higher or lesser extent (for instance, solely three of the insect thorax and belly segments sometimes bear appendages), arthropodists broadly assume that all the segments have been practically similar within the ancestral state. Nevertheless, whereas one can normally readily see the segmental organisation of the trunks of grownup arthropods, that of the top is way much less apparent. Arthropod heads are sometimes fused capsules that bear quite a lot of complicated constructions such because the eyes, antennae and mouth elements.

The problem that the arthropod head drawback has to handle is to what extent the assorted constructions of the arthropod head might be resolved right into a set of hypothetical ancestral segments. Given the excessive compaction and complexity of grownup arthropod heads, a lot consideration has been directed in the direction of understanding the developmental processes that give rise to them, within the hope that they are going to reveal their segmental organisation extra clearly.

Head parts[edit]

Insect head elements. Legend: a, antennae; c, compound eye; lb, labium; lr, labrum; md, mandibles; mx, maxillae.

A typical insect head possesses a pair of antennae; eyes; mandibles, labrum, maxillae and labium (the latter 4 forming the cluster of “mouth elements”, no. 32. within the diagram). Mendacity above the oesophagus is the mind or supraesophageal ganglion, divided into three pairs of ganglia: the protocerebrum, deutocerebrum and tritocerebrum from entrance to again (collectively no. 5 within the diagram). Nerves from the protocerebrum result in the massive compound eyes; from the deutocerebrum to the antennae; and from the tritocerebrum to the labrum and stomatogastric nervous system. Circum-oesophageal connectives lead from the tritocerebrum across the intestine to attach the mind to the ventral ganglionated nerve wire: nerves from the primary three pairs of ganglia result in the mandibles, maxillae and labium, respectively. The place of the mouth and the circum-oesophageal connectives permits a distinction to be made between pre- and post-oral constructions; though it must be borne in thoughts that as a result of constructions can transfer round throughout growth, a pre-oral place of a construction within the grownup doesn’t essentially show that its developmental origin is from there. The myriapod head is similar to that of the bugs.

The crustacean head is broadly much like that of the bugs, however possesses, as well as, a second pair of antennae which can be innervated from the tritocerebrum. Instead of the labium, crustaceans possess a second pair of maxillae.

Chelicerate head constructions differ significantly from these of mandibulates (i.e. bugs, crustaceans and myriapods); they possess eyes and a single pair of greedy appendages innervated from the mind, plus a labrum-like construction. Behind the mouth lies one other pair of mouthparts, the pedipalps, and behind them lie the collection of strolling limbs. In chelicerates, the leg-bearing segments are fused with the anterior segments to type a prosoma, in order that in dwelling arthropods a definite head solely exists in mandibulates.

The acron idea[edit]

The arthropod head drawback has till lately been predicated on the Articulata idea, i.e. that the arthropods and annelids are shut kinfolk. Though arthropods are primarily direct builders that don’t possess a trochophore-like larva, the annelids do. Throughout annelid metamorphosis, segments are added near the posterior of the physique, behind the mouth; whereas the mind is derived from the episphere or area in entrance of the mouth. Recognition of this led to the idea of a main, non-segmental part of the physique in annelids often called the acron being developed, from which the mind is in the end derived. As a result of the arthropod and annelid heads, within the mild of the Articulata idea, have been assumed to be structurally homologous not directly, the arthropod head was additionally typically thought-about to include a non-segmental acronal part. Taking the homology between annelid and arthropod heads at face worth, Swedish employees corresponding to Hanström[8] and Holmgren[9] assumed that a big a part of the arthropod head should correspond to the acron, a view adopted later by a number of distinguished American insect employees corresponding to Butt[10] and Snodgrass. They proposed that every one pre-oral constructions in bugs have been non-segmental, though such a view is at odds with the preoral place of apparently bona fide appendages such because the antennae. A much less excessive set of theories suggest that solely the protocerebrum and related constructions must be thought-about to be acronal.

The view that the arthropod head should comprise an acronal remnant has been shaken by the comparatively current revision of protostome phylogeny, which has dismantled the Articulata and positioned the arthropods along with a bunch of unsegmented worms also known as the Cycloneuralia inside Ecdysozoa. All members of Ecdysozoa are direct builders with out a trochophore, and the cycloneuralians have terminal mouths. In consequence, the thought of the arthropods having inherited a preoral acron from their ancestors appears much less possible.

Molecular growth and the arthropod head drawback[edit]

Expression of Hox genes within the physique segments of various teams of arthropod. The Hox genes 7, 8, and 9 correspond in these teams however are shifted (by heterochrony) by as much as three segments. Segments with maxillopeds have Hox gene 7. Fossil trilobites in all probability had three physique areas, every with a novel mixture of Hox genes.

The examine of how developmental genes are expressed during embryogenesis has turn into an essential new device within the final twenty years for understanding the construction and evolution of morphology.[11] The arthropod head drawback has been tackled in three primary methods on this regard, first by utilizing genetic segmental markers to probe the obscure area in entrance of the mouth, particularly in bugs; second by Hox gene expression patterns to detect patterns of homology amongst totally different arthropods; and third, by finding out gene expression particularly options (particularly the labrum) to find out its appendiculate or different standing. As a result of all arthropods have the identical complement of 9 Hox loci, the morphological diversification noticed is brought on by heterochrony, that means that the genes are expressed at totally different occasions.

Areas of settlement[edit]

It’s broadly agreed that the insect, myriapod and crustacean heads are very comparable. The obvious lack of a second antenna in bugs and myriapods is defined by the concept that this appendage has been misplaced, leaving an appendage-less phase often called the intercalary phase. Fashionable phylogenies don’t usually help an insect-myriapod relationship, suggesting that the second antenna has been misplaced independently in every group, maybe because of a convergent adaptation to life on land. Moreover, there’s basic settlement that the mandibles, first maxillae and labium/second maxillae every signify a post-oral phase; and that the primary antenna represents a preoral phase.

Areas of disagreement[edit]

Areas of disagreement might be grouped into three classes: the character of the pre-antennal area in mandibulates; the character of the labrum; and the connection between the chelicerate and mandibulate anterior segments.

See Also

Nature of the preoral area[edit]

The diploma to which the world in entrance of the mouth is segmented stays one of many main controversies within the arthropod head drawback. As already talked about, earlier employees typically thought-about all the pre-oral area to be “acronal” and thus nonsegmental. Fashionable employees universally settle for that at the least the deuterocerebrum is segmental. Nevertheless, the character of the area in entrance of that is a lot much less sure. Some molecular growth research have given restricted help to the thought of an “ocular” phase similar to the protocerebrum; however these knowledge will not be unequivocal.[12] The concept of the protocerebrum really comprising two parts has additionally acquired help from each molecular and embryological knowledge.[13][14]

On this view, the protocerebrum contains a typical ‘phase’, the prosocerebrum, marked by the expression of engrailed at its caudal margin and a pair of appendages (in most crown euarthropods, compound eyes, that are interpreted as modified trunk appendages), and a pre-segmental area, the archicerebrum, which bore a pair of appendages that aren’t serial homologues of the trunk appendages; these are represented by the onychophoran antennae and the ‘nice appendages’ of sure stem euarthropods. The archicerebrum is in some methods equal to the ‘acron’, and could also be equal (by way of a shared equal construction within the widespread ancestor of lophotrochozoans and ecdysozoans) with the annelid prototroch; it may be acknowledged by the expression of the genes optix and six3 throughout growth, whereas the prosocerebrum is related to orthodenticle and its homologs.[15][16]

(Word that the phrases archicerebrum and prosocerebrum will not be all the time used persistently; see Composition of the protocerebrum.)

The labrum[edit]

The labrum is a flap-like construction that lies instantly in entrance of the mouth in virtually all extant euarthropods, the final exception being offered by the possible chelicerate-relatives the pycnogonids. It has proved to be by far probably the most controversial of all arthropod head constructions. It’s innervated in crustaceans and bugs from the tritocerebrum, i.e. the again of the mind. Nevertheless, in growth it typically seems on the anterior of the top, and migrates backwards in the direction of its grownup place. Moreover, it typically seems as a bilobed construction, with a set of muscle tissues, nerves and gene expression in some ways much like that of a trunk appendage.[17][4] This proof has been used to counsel that the labrum is in actual fact a extremely diminished appendage.

Its innervation from the rear of the mind has advised to some employees that, if an appendage, it’s the appendage of the tritocerebral phase; some extent disputed by others who argue that the presence of a well-developed appendage in at the least crustaceans on this phase (i.e., the second antenna, similar to the intercalary phase of bugs) guidelines this out. If the labrum is an appendage then, it appears doable that its origin is indicated by its developmentally anterior place, i.e., that it’s the appendage of a phase anterior to the primary antenna. The obvious alternative for that is the phase whose ganglion is the protocerebrum, which in extant euarthropods bears no appendage (aside from the eyes). Strausfeld finds help for this speculation within the presence of a median nerve bundle connecting the labrum to the anterior of the protocerebrum,[15] and the expression of the gene six3 within the labrum has been taken as proof for its homology with onychophoran antennae (frontal appendages borne from the anterior of the protocerebrum).[4]

If the labrum is basically an anterior appendage that has migrated to the posterior, then it might be homologous to the “antennae” of onychophorans, which, as mentioned beneath, appear to be innervated from a really anterior a part of the mind, i.e. in entrance of the eyes. It has even been advised (e.g., by Roonwal) that the labrum belongs to an much more obscure phase that lies in entrance of the ocular one. However, many employees proceed to be extremely skeptical in regards to the appendiculate nature of the labrum, preferring to see it because it seems, i.e., as an outgrowth of the physique wall simply in entrance of the mouth.

Notably in some fossil teams, corresponding to sure trilobites, the labrum is commonly coated with a sclerotised plate, the hypostome. Confusion can come up the place the 2 constructions are conflated or mistaken for each other.[4]

Mandibulate/chelicerate head homologies[edit]

Given the disagreements in regards to the construction of the insect head, on which most effort has been spent, it’s no shock that the potential homologies between it and different arthropods, notably the chelicerates, are additionally very controversial. From after the Second World Battle to the Nineteen Eighties a generally accepted mannequin of arthropod evolution was that the extant euarthropods have been polyphyletic, i.e. the principle lineages had developed independently from soft-bodied, annelid-like ancestors, following the work of Tiegs and particularly Sidnie Manton. On this view, many of the head constructions would even be convergent, and thus there was no level on the lookout for particular homologies between main teams. Nevertheless, the monophyletic idea of arthropod origins has since decisively gained the higher hand, which raises the issue of head homology as soon as extra.

The classical view was that the chelicerae have been homologous to the second antennae of crustaceans (i.e., they’re innervated from the tritocerebrum), a view based mostly partly on the truth that the chelicerae have been innervated from the identical ganglion that innervates the labrum, which is the tritocerebrum in crustaceans and bugs. On condition that there are apparently no appendages in entrance of the chelicerae, the implication was that the deuterocerebrum had been misplaced in chelicerates (the protocerebrum innervates the eyes in each teams on this view). Whereas this view nonetheless has its defenders (notably Colette and Jacques Bitsch), the choice view that the chelicerae are innervated from the deuterocerebrum has gained floor, based mostly on molecular growth in mites[18] and spiders,[19] and neuroanatomy in Limulus.[20] If so, then chelicerates merely haven’t any tritocerebrum, i.e. there is no such thing as a third supraoesophageal ganglion of the mind; the phase similar to it might be the suboesophageal pedipalp one. Such a idea doesn’t, nevertheless, instantly account for a similar ganglionic innervation of the chelicerae and labrum, though one resolution is just to assert that the labrum itself isn’t homologous between mandibulates and chelicerates (the view, for instance, of Dieter Waloszek and colleagues).

The pinnacle of onychophorans[edit]

The brains of onychophorans (velvet worms) have been lately re-investigated[21][22] and have been proven to own two uncommon options. First, though the mouth is ventral, as is the case in euarthropods, it’s innervated from three totally different locations; the perimeters, the posterior, and by a nerve that originates dorsally, and passes anteriorly all the way down to curve again to the entrance of the mouth. This set of innervation is sensible if the mouth of onychophorans was initially terminal and has been bent downwards. Second, the antennae of the onychophorans seem like innervated from in entrance of the eyes; which in euarthropod phrases implies a protocerebral (or doubtlessly much more anterior) innervation. That is supported by gene expression knowledge, which present that the jaws too are derived from a protocerebral or deuterocerebral phase.[23] As all euarthropod antennae are deuterocerebral or tritocerebral, this suggests that the onychophoran antennae will not be homologous to any euarthropod ones.

The tritocerebrum in arthropods is the primary phase to specific Hox genes; on this foundation, it may be recognised as homologous to the third head phase in onychophora, which bears the slime glands (a pair of extremely modified appendages).[23][4]

The pinnacle of pentastomids[edit]

The parasitic pentastomida hatch with 4 head segments and three trunk segments, with two extra physique segments being added throughout postembryonic growth. This variety of segments then stays fixed for the remainder of their life. There aren’t any antenna, and their mouth has no piercing, biting or sucking extremities. On all sides of the mouth there’s a pair of retractable hooks, and the mouth itself is sustained by a chitinous buccal ring. They feed by means of a pumping mechanism situated within the pharynx, consisting of two inflexible chitinous plates that’s linked to a number of related muscle tissues.[24]

The mind of tardigrades[edit]

Tardigrades bear a circumoral nerve ring which has been homologised with the nerve ring of the ancestral ecdysozoan, and the arthropod (=Euarthropoda + Onychophora) protocerebrum, suggesting that the protocerebrum is homologous with the ancestral ecdysozoan mind.[25]

On this view, the stylet equipment is homologous with the euarthropod labrum / onychophoran antennae, and the primary pairs of strolling legs correspond to the deutocerebral and tritocerebral appendages.[4]

Fossil proof[edit]

The Cambrian fossil file, above all the assorted lagerstätten such because the Burgess Shale, Sirius Passet, Chengjiang and Orsten faunas, has yielded a really wealthy file of well-preserved arthropods, together with the well-known trilobites.

Many Cambrian arthropods, together with the trilobites themselves, possess a single pair of slender antennae, which have been equated with both the primary or second antennae of the crustaceans; and both the chelicerae or the lacking appendages of the supposedly diminished deuterocerebrum in chelicerates. Nevertheless, one other group of arthropods, the so-called “nice appendage” arthropods, together with Yohoia, Leanchoilia and Alalcomenaeus, don’t possess easy antennae, however somewhat have a sturdy, branched construction, which was referred to as the “nice appendage” by Harry B. Whittington in his restudy of those taxa. One more group of arthropods might possess two differentiated head appendages, of which crucial and controversial is the Chengjiang type Fuxianhuia. Fuxianhuia was claimed to own a pair of brief antennae anterior, adopted by a sturdy pair of “sub-chelate” appendages. Nevertheless, this evaluation has been each disputed by Waloszek and colleagues, who take into account that the sub-chelate appendages are in actual fact intestine diverticulae; and supported by Graham Budd. Thus, its nature stays controversial at current. Different taxa have additionally been claimed to have a considerably comparable anterior appendage association (e.g. Fortiforceps) however, aside from the well-preserved Branchiocaris from the Burgess Shale, most of them are extremely equivocal.

In virtually all Cambrian arthropods, the post-oral limbs present little or no differentiation in comparison with the trunk limbs; the heads posterior to the mouth reveals a substantial diploma of variability, nevertheless, within the variety of segments included into the top.

Trilobites, particularly, possess a ventral sclerotised plate within the head referred to as the hypostome. Whether or not that is homologous to the labrum or not is debated; though Waloszek and others have argued that because the phosphatocopines (higher stem-group crustaceans) appear to own each, it can’t be.[26]

Theories of Cambrian arthropod head segmentation[edit]

There are at the least 4 primary theories to account for anterior head appendages in Cambrian arthropods:

Scholtz and Edgecombe[edit]

Gerhard Scholtz and Greg Edgecombe[27] settle for that the antennae of onychophorans are protocerebral, and name them “main” antennae to tell apart them from the “secondary” antennae of teams such because the bugs and crustaceans. Additionally they settle for that taxa corresponding to Fuxianhuia possess each antennae and “nice appendages”. As a result of in Fuxianhuia the antennae lie anterior to the nice appendages, they counsel that these antennae are the inherited primitive “main” antennae; and that the nice appendages are thus equal to the primary antennae of crustaceans. As a result of the secondary antennae will not be current in stem group arthropods corresponding to Fuxianhuia, nor within the extant chelicerates, they suggest that arthropods, such because the trilobites, that possess secondary antennae, belong in a monophyletic group that additionally consists of the mandibulates, referred to as the Antennata. The trilobites are thus, of their view, not stem-group chelicerates, a generally held view, however somewhat, stem-group mandibulates. The standing of the labrum isn’t resolved by this idea, however they argue that it the proof for it being appendiculate isn’t compelling; thus it doesn’t need to correspond to a well-developed appendage of any Cambrian arthropod.

The 2014 description of Lyrarapax poses a problem for this theorem: assuming that its nervous tissue is appropriately recognized as such, the nice appendages of this radiodont are innervated into the entrance of the protocerebrum, undermining the suggestion that the nice appendages are deuterocerebral.[28][4]


Graham Budd‘s idea[29] agrees with that of Scholtz and Edgecombe in accepting the protocerebral nature of the onychophoran antennae, and the 2 preoral appendages of Fuxianhuia. Nevertheless, he traces the origin of the “nice appendages” within the differentiated frontal appendages of Cambrian lobopods corresponding to Aysheaia and Kerygmachela, neither of which possess convincing antennae. Thus, in Budd’s view, the order of the 2 anterior appendages of taxa corresponding to Fuxianhuia are reversed: the antennae are the primary antennae (deutocerebral) of the mandibulates; and the nice appendages correspond to the first antennae of the onychophorans and Cambrian lobopods. Following earlier work by Dewel and colleagues, Budd accounts for his or her reversal by arguing that the mouth in basal lobopods was terminal, and that because it rotated backwards and downwards, it introduced the anterior appendage backwards with it. Given this transformation, it’s possible, below this idea, that the remnant of the nice appendage/main antenna is the labrum of extant arthropods. As a result of on this view Fuxianhuia possesses each a hypostome and an incredible appendage, the hypostome can’t be straightforwardly homologous with the labrum.

Pycnogonids and the nice appendage idea[edit]

Maxmen and others lately printed a morphologically-based paper[30] that claimed the enigmatic chelifores of extant pycnogonids (sea spiders) are innervated from the protocerebrum, and never from the trito- or deutocerebrum as beforehand claimed.[31] This is able to counsel that pycnogonids had uniquely retained a “nice appendage” homologue as an appendage, not like all different euarthropods by which it had been reworked into the labrum (pycnogonids lack a labrum). Nevertheless, expression knowledge of Hox genes that have been printed shortly afterwards[32] advised that the chelifores have been deuterocerebral and thus probably to be homologous to the chelicerae. The pycnogonids are thus impartial with regard to the nice appendage idea.


Dieter Waloszek and colleagues have supplied a somewhat totally different account[33] of Cambrian arthropod head construction. They don’t essentially settle for the first antenna idea of the onychophoran antennae; they usually reject the concept that Fuxianhuia or any of its shut kinfolk possessed an incredible appendage. Moderately, they place the “nice appendage” arthropods within the stem-group of the chelicerates, arguing that the nice appendage is homologous to the chelicerae of chelicerates, and the primary antennae of crustaceans.

Cotton and Braddy[edit]

Trevor Cotton and Simon Braddy, in a complete cladistic evaluation of Cambrian arthropods,[34] additionally proposed that the nice appendage arthropods have been stem-group chelicerates; accepting that Fuxianhuia and kinfolk possessed two preoral appendages, they defended the classical view that the nice appendage and the chelicerae have been tritocerebral in origin; i.e. that the antennae of Fuxianhuia have been deuterocerebral.


The quantity and nature of the post-oral segments within the insect head have hardly ever been questioned. A way more tough space, nevertheless, has been the character of the preoral area. The apparent contradiction between a idea that no-preoral constructions are segmental, and proof, corresponding to for the primary antennae of crustaceans, that some such constructions clearly are, led employees as way back as Lankester[35] to posit that there was ahead migration of segments in entrance of the mouth. Certainly, such a course of might be seen in ontogeny of the tritocerebrum, which might be seen emigrate ahead because the mind develops; moreover, though in most bugs and crustaceans its ganglia are a part of the mind, its commissures nonetheless loop behind it, suggesting derivation from a extra posterior place.

However, even permitting for this risk, the complexity of the anterior a part of the mind, which even when the acron idea is inaccurate should have been inherited from very basal animals;[36] untangling the brand new characters developed by the earliest arthropods from these inherited from their ancestors due to this fact nonetheless stands centrally within the arthropod head drawback.[37]

Additional studying[edit]


  1. ^ Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). Invertebrate Zoology (seventh 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 Construction & Growth. 46 (3): 395–418. 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 Construction & Growth. Evolution of Segmentation. 46 (3): 354–379. doi:10.1016/j.asd.2016.10.011. ISSN 1467-8039. PMID 27989966.
  4. ^ a b c d e f g Ortega-Hernandez, Javier and Janssen, Ralf and Budd, Graham E. (2017). “Origin and evolution of the panarthropod head – a palaeobiological and developmental perspective”. Arthropod Construction & Growth. 46 (3): 354–379. doi:10.1016/j.asd.2016.10.011. ISSN 1467-8039. PMID 27989966.{{cite journal}}: CS1 maint: a number of names: authors checklist (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 Infinite Dispute”, Quaestiones Entomologicae, 11: 7–25
  7. ^ Snodgrass, R. E. (1960), “Information and Theories Regarding the Insect Head”, Smithsonian Miscellaneous Collections, 142: 1–61
  8. ^ Hanström, B. (1928), Vergleichende Anatomie Des Nervensystems Der Wirbellosen Tiere Unter Berücksichtigung Seiner Funktion, Berlin, Heidelberg and New York: Julius Springer
  9. ^ Holmgren, N. (1916), “Zur Vergleichenden Anatomie Des Gehirns Von Polychaeten, Onychophoren, Xiphosuren, Arachniden, Crustaceen, Myriapoden Und Insekten”, Kungliga Svenska Vetenskapsakademiens Handlingar, 56: 1–315
  10. ^ Butt, F. H. (1957), “The Position of the Premandibular or Intercalary Phase in Head Segmentation of Bugs and Different Arthropods”, Transactions of the American Entomological Society, 83: 1–30
  11. ^ Angelini, D. R.; Kaufman, T. C. (2005), “Comparative Developmental Genetics and the Evolution of Arthropod Physique Plans”, Annual Overview of Genetics, 39: 95–119, doi:10.1146/annurev.genet.39.073003.112310, PMID 16285854, S2CID 35597070
  12. ^ Scholtz, G. (2001), “Evolution of Developmental Patterns in Arthropods – the Evaluation of Gene Expression And Its Bearing on Morphology and Phylogenetics”, Zoology, 103: 99–111
  13. ^ Schmidt-Ott, U.; Technau, G. M. (1992), “Expression of En and Wg within the Embryonic Head and Mind of”, Growth, 116 (111): 125, doi:10.1242/dev.116.1.111, PMID 1483381
  14. ^ Roonwal, M. L. (1938), “Some Latest Advances in Insect Embryology, with a Full Bibliography of the Topic”, Journal of the Royal Asiatic Society of Bengal, Science, 4: 17–105
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