One of the longest-running puzzles in evolutionary biology, the so-called arthropod head problem, has just received a fresh twist from an unlikely cast of characters: harvestmen, tarantulas and cellar spiders. A new open-access study published in Discover Developmental Biology reports that a pattern of gene expression once proposed as evidence that the most anterior segments of the arthropod head evolved independently from the rest of the body is far less consistent across species than previously claimed, and therefore cannot carry the weight that has been placed upon it.
The arthropod head problem, sometimes called the “Endless dispute”, concerns the evolutionary origin and homology of the pre-gnathal segments, the three most anterior head segments that are associated with the tripartite brain of arthropods. These are the protocerebral, deuterocerebral and tritocerebral segments, which in insects are associated with the ocular region and labrum, the antennae, and the limb-less intercalary segment respectively. In chelicerates such as spiders and harvestmen, their homologs are associated with the ocular region and labrum, the cheliceral segment, and the pedipalpal segment. It has been suggested that these pre-gnathal segments might have evolved independently from the segments that build the rest of the arthropod body, a proposition known as the non-homology hypothesis.
A key line of evidence for that hypothesis came from the expression of the hedgehog gene, a segment polarity gene first identified in the vinegar fly Drosophila melanogaster. Hedgehog encodes a secreted signalling protein and is expressed in characteristic transverse stripes in each developing segment, where it is co-expressed with and positively regulated by another segment polarity gene, engrailed. In Drosophila, and reportedly also in the common house spider Parasteatoda tepidariorum, the most anterior domain of hedgehog expression was described as splitting twice during early head development, producing three stripes corresponding to the three pre-gnathal segments.
The new study, authored by Ralf Janssen, tested this idea by examining early hedgehog expression in chelicerates that had never been investigated at these stages before. In the harvestman Phalangium opilio, no hedgehog expression is detectable at the early blastoderm stage, but a broad anterior stripe appears soon after at the late blastoderm stage. This stripe then splits only once, into two stripes corresponding to the ocular segment and the cheliceral segment, the latter being the homolog of the insect antennal segment. The two resulting stripes remain dorsally and laterally connected for a while before finally disconnecting. Despite examining more than one hundred embryos covering the relevant developmental stages, Janssen never observed any additional splitting events in the harvestman, suggesting that a single splitting into ocular and cheliceral stripes may be the ground state for chelicerates.
The investigation was extended to two additional spiders, the tarantula Acanthoscurria geniculata and the cellar spider Pholcus phalangioides. Spiders possess at least two hedgehog orthologs, of which only the Group-1 genes are expressed in a segment polarity fashion, and it is these that the study followed. In Acanthoscurria, a strong anterior stripe appears when the dorsal field forms, together with several weaker posterior stripes, and this anterior stripe subsequently splits into the ocular and cheliceral stripes, which remain dorsally and laterally connected until approximately stage 8. No further splitting of hedgehog stripes could be identified in this species. In Pholcus, the most anterior stripe likewise splits into ocular and cheliceral stripes, but here the investigators documented a second, entirely unexpected splitting event: a single initial hedgehog stripe corresponding to the second and third leg-bearing segments splits into two separate stripes.
That observation in Pholcus carries particular weight for the debate. The second and third leg-bearing segments are not homologs of the pre-gnathal segments, so hedgehog splitting is clearly not a phenomenon confined to the anterior head. This finding directly weakens the argument that hedgehog splitting represents an indicative remnant of segment splitting during the evolution of the pre-gnathal segments. If the same developmental process occurs in segments far removed from the head, its occurrence in the head cannot be treated as a special signature of an independent evolutionary origin for those segments.
The study also re-examined the published evidence from other arthropod groups, and here the critical re-evaluation proved decisive. The original claim of a two-fold splitting in Drosophila, based on data from an earlier mutant-screening and expression study, does not hold up under scrutiny. The short intercalary stripe in the fly is not connected in any form with the more anterior stripes corresponding to the antennal and ocular regions, or with the posteriorly located mandibular stripe, and a later study showed that the intercalary hedgehog stripe in Drosophila develops independently from the ocular and antennal stripes. In other words, there is no two-fold splitting of anterior hedgehog domains corresponding to the three pre-gnathal segments in Drosophila at all. In other insects, including the beetle Tribolium castaneum, the cricket Gryllus bimaculatus and the true bug Oncopeltus fasciatus, an early anterior stripe may split into two stripes corresponding to the ocular and antennal segments, while the intercalary stripe forms independently and with some delay.
The picture in myriapods adds further complexity. In the millipede Glomeris marginata, splitting of a broad anterior hedgehog stripe results in an ocular and an antennal stripe, matching the pattern seen in insects and most chelicerates. In the centipede Strigamia maritima, however, an initial broad domain appears to split into three stripes, the ocular, antennal and intercalary, resembling the situation in Parasteatoda, and the mandibular stripe may additionally be dorsally and laterally connected to the others, which could indicate splitting into four stripes or simply an optical artefact. No relevant early expression data exist for any crustacean species, leaving a conspicuous gap in the comparative record. Taken together, the available data suggest that a single splitting event of hedgehog in the arthropod head represents the ancestral state, and that the splitting into three stripes in Parasteatoda, and the splitting into three or four stripes in Strigamia, must be interpreted as derived conditions.
Why should spiders show such variable splitting patterns in the first place? The author offers a speculative but grounded explanation rooted in spider embryology. In spiders, a relatively large number of segments, all of the prosomal segments, develop from the germ disc, a blastoderm-like structure, and the embryo undergoes dramatic morphogenetic changes as the radial germ disc transforms into a bilateral germ band. The area in which splitting events of segment primordia and their associated gene expression patterns can occur is therefore larger and more likely in spiders than in other chelicerates, such as harvestmen and scorpions, in which far fewer segments form from the germ disc or blastoderm. Differences in developmental timing, or heterochrony, combined with the parallel transformation of the germ disc into a germ band, may explain why different splitting events are observed among spider species. To test these ideas further, the study proposes systematic investigation of pair-rule gene ortholog expression during early spider development, together with cell-labelling experiments to reveal the early fate map of segment primordia in the spider germ disc.
The broader lesson of the study extends beyond the head itself. The author concludes that two-fold splitting of the most anterior hedgehog stripe does not represent a conserved feature of arthropod head development and must be assumed to be a derived character. Because hedgehog splitting also occurs outside the pre-gnathal segments, as demonstrated for Pholcus and discussed for Strigamia, the argument that such splitting supports the independent evolution of the anterior head segments is considerably weakened. The work also highlights the need for more comparative studies in arthropod evolutionary developmental biology, including species representing as many groups as possible, since isolated insights from a few model organisms can easily lead to misinterpretation of the available data, as the example of hedgehog expression in the early developing head of Parasteatoda demonstrates. With the re-evaluation of hedgehog splitting, one line of evidence for the non-homology hypothesis has been considerably weakened, and the study instead reveals a more variable picture of how the most anterior hedgehog domain is patterned across arthropods.
Subject of Research: Early expression of hedgehog gene orthologs in chelicerate embryos and its implications for the homology of arthropod head segments
Article Title: Early expression of chelicerate hedgehog orthologs and its bearing on the homology of arthropod head segments
Article References: Janssen, R. (2025). Early expression of chelicerate hedgehog orthologs and its bearing on the homology of arthropod head segments. Discover Developmental Biology, 235(1), Article 2. https://doi.org/10.1007/s00427-025-00727-7
Image Credits: AI Generated
DOI: 10.1007/s00427-025-00727-7
Keywords: arthropod head problem, hedgehog gene, chelicerates, spiders, harvestman, pre-gnathal segments, segment polarity genes, evo-devo, embryogenesis, Parasteatoda tepidariorum, Pholcus phalangioides, segment homology
News Source: Juliet Wilcox. (October 28, 2026). Hedgehog Gene Splitting in Spider Heads Undermines Claim That Arthropod Face Segments Evolved Independently. Scienmag.
