Arthropod head problem
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.
The trunks of arthropods comprise 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.
Arthropod heads are typically fused capsules that bear a variety of complex structures such as the eyes, antennae and mouth parts.
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.
A typical insect head possesses a pair of antennae; eyes; mandibles, labrum, maxillae and labium (the latter four forming the cluster of "mouth parts", no.
Lying above the oesophagus is the brain or supraesophageal ganglion, divided into three pairs of ganglia: the protocerebrum, deutocerebrum and tritocerebrum from front to back (collectively no.
Nerves from the protocerebrum lead to the large compound eyes; from the deutocerebrum to the antennae; and from the tritocerebrum to the labrum and stomatogastric nervous system.
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.
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.
The study of how developmental genes are expressed during embryogenesis has become an important new tool in the last twenty years for understanding the structure and evolution of morphology.
[11] The arthropod head problem has been tackled in three main ways in this regard, first by using genetic segmental markers to probe the obscure region in front of the mouth, especially in insects; second by looking at Hox gene expression patterns to detect patterns of homology among different arthropods; and third, by studying gene expression in particular features (especially the labrum) to determine its appendiculate or other status.
Because all arthropods have the same complement of nine Hox loci, the morphological diversification observed is caused by heterochrony, meaning that the genes are expressed at different times.
Modern phylogenies do not in general support an insect-myriapod relationship, suggesting that the second antenna has been lost independently in each group, perhaps as a result of a convergent adaptation to life on land.
The labrum is a flap-like structure that lies immediately in front of the mouth in almost all extant euarthropods, the general exception being provided by the probable chelicerate-relatives the pycnogonids.
Furthermore, it often appears as a bilobed structure, with a set of muscles, nerves and gene expression in many ways similar to that of a trunk appendage.
Nevertheless, many workers continue to be highly skeptical about the appendiculate nature of the labrum, preferring to see it as it appears, i.e., as an outgrowth of the body wall just in front of the mouth.
[4] Given the disagreements about the structure of the insect head, on which most effort has been spent, it is no surprise that the potential homologies between it and other arthropods, notably the chelicerates, are also very controversial.
From after the Second World War to the 1980s a commonly accepted model of arthropod evolution was that the extant euarthropods were polyphyletic, i.e. the main lineages had evolved independently from soft-bodied, annelid-like ancestors, following the work of Tiegs and especially Sidnie Manton.
However, the monophyletic theory of arthropod origins has since decisively gained the upper hand, which raises the problem of head homology once more.
Given that there are apparently no appendages in front of the chelicerae, the implication was that the deuterocerebrum had been lost in chelicerates (the protocerebrum innervates the eyes in both groups in this view).
[25] On this view, the stylet apparatus is homologous with the euarthropod labrum / onychophoran antennae, and the first pairs of walking legs correspond to the deutocerebral and tritocerebral appendages.
However, this assessment has been both disputed by Waloszek and colleagues, who consider that the sub-chelate appendages are in fact gut diverticulae; and supported by Graham Budd.
Other taxa have also been claimed to have a somewhat similar anterior appendage arrangement (e.g. Fortiforceps) but, with the exception of the well-preserved Branchiocaris from the Burgess Shale, most of them are highly equivocal.
The status of the labrum is not resolved by this theory, but they argue that it the evidence for it being appendiculate is not compelling; thus it does not have to correspond to a well-developed appendage of any Cambrian arthropod.
[28][4] Graham Budd's theory[29] agrees with that of Scholtz and Edgecombe in accepting the protocerebral nature of the onychophoran antennae, and the two preoral appendages of Fuxianhuia.
