Arachnids occupy a pivotal position in the history of terrestrial life. As descendants of some of the earliest animals to colonize land, they represent an ancient radiation whose origins predate the dinosaurs by hundreds of millions of years. The class encompasses an extraordinary range of body forms and ecological roles, from the aerial dispersal of tiny ballooning spiders to the armored detritivory of oribatid mites, from the ambush predation of whip spiders to the parasitic lifestyles of many tick and mite lineages. This ecological breadth means that progress on the systematic challenges outlined above reverberates far beyond arachnology itself, informing questions about terrestrial ecosystem functioning, the evolution of venom and silk as biomaterials, and the general principles governing diversification on land.

The scale of the descriptive task ahead is difficult to overstate. Estimates of arachnid diversity consistently suggest that the roughly 120,000 formally described species represent only a fraction of the true total, with some projections for spider diversity alone exceeding 50,000 undescribed species and mite diversity plausibly running into the millions. Much of this undocumented diversity is concentrated in tropical and subtropical regions, in soil and canopy habitats that remain poorly sampled, and in specialized microhabitats such as caves, leaf litter, and the burrows of other animals. Because many arachnid lineages have restricted distributions and limited dispersal abilities, they are particularly informative for biogeographic reconstruction and particularly vulnerable to habitat loss, making accelerated species discovery a matter of some urgency as well as academic interest.

The tension between molecular and morphological evidence that pervades deep arachnid phylogenetics deserves particular attention. Conflicts over the placement of horseshoe crabs relative to terrestrial arachnids, the monophyly or paraphyly of mites and ticks, and the relationships among spider families illustrate a broader phenomenon in systematics: different data types can carry genuinely different historical signals, whether because of convergent morphological evolution, incomplete lineage sorting, introgression, or heterogeneous rates of molecular evolution. Rather than treating such conflict as an obstacle, many researchers now view it as a source of biological insight, since reconciling conflicting signals often reveals processes—such as ancient rapid radiations or bursts of morphological innovation—that would otherwise remain hidden. The arachnid tree, with its combination of deep divergence times and dramatic morphological disparity, is an ideal system for developing and testing such integrative approaches.

Advances in genomic methodology are rapidly changing what is feasible for research groups of modest size. Ultra-conserved elements and other anchored hybrid enrichment approaches allow hundreds to thousands of loci to be collected from both fresh and museum-preserved specimens, dramatically expanding the taxonomic scope of phylogenomic studies. Low-coverage genome skimming offers a complementary path, providing organellar genomes and large numbers of nuclear markers at a fraction of the cost of complete genome assembly. As reference genomes accumulate for key arachnid lineages, these methods become increasingly powerful, and the prospect of a genuinely comprehensive arachnid tree of life—spanning all recognized families and, eventually, genera—moves from aspiration toward realistic research program. Achieving this will require coordinated community effort, standardized sampling, and shared analytical pipelines, but the payoff would be a backbone hypothesis that stabilizes comparative work across the entire field.

The molecular clock problem in arachnids illustrates how much remains to be learned about the timing of evolutionary events. Divergence estimates for major arachnid lineages have varied by tens or even hundreds of millions of years across studies, reflecting both the scarcity of informative fossils and the difficulty of modeling rate variation across such deep timescales. Arachnid fossils themselves are remarkable when they occur—amber specimens preserving delicate silk and behavior, compression fossils revealing fine anatomical detail—but the record is unevenly distributed across lineages and geological time. Improved integration of paleontological data with genomic clocks, including the careful vetting of calibration points and the use of fossilized birth-death approaches, represents one of the most promising avenues for establishing a reliable temporal framework for arachnid evolution. Such a framework would, in turn, allow researchers to test whether diversification rate shifts coincide with major geological or climatic events, with the origin of key innovations, or with episodes of geographic dispersal and isolation.

The species problem, as it plays out in arachnids, offers a window into one of the deepest conceptual debates in biology. Cryptic diversity—the existence of reproductively independent lineages that defy morphological diagnosis—appears to be widespread in groups as different as trapdoor spiders, harvestmen, and mites. In some well-studied mygalomorph lineages, what was once considered a single widespread species has been revealed as a complex of narrowly endemic relatives, each with its own evolutionary history and conservation status. These discoveries have practical consequences: conservation assessments based on morphology alone can dramatically underestimate both the number of evolutionary lineages at risk and the degree to which local populations represent unique and irreplaceable branches of the tree of life. At the same time, purely molecular delimitation methods can fragment real biological diversity when applied without regard to morphology, ecology, and geography, underscoring the need for the integrative, multi-evidence frameworks that the field is actively developing.

The taxonomic impediment—the persistent shortage of trained taxonomists and the funding to support their work—compounds every other challenge in arachnid systematics. Describing species well requires deep familiarity with a lineage’s morphology, natural history, and literature, expertise that takes years to develop and that is increasingly scarce as retirements outpace replacements. Digital tools offer partial remedies: high-resolution imaging, openly shared specimen data, online species pages, and machine-assisted identification can each lower barriers to taxonomic work and connect dispersed expertise. Yet these tools supplement rather than replace the comparative judgment of trained specialists, and sustaining that expertise remains a community-level responsibility. Journals, museums, and universities all have roles to play in ensuring that the next generation of arachnid taxonomists is trained, employed, and equipped to tackle the diversity still awaiting discovery.

Linking phenotype to genotype in arachnids is advancing on multiple fronts simultaneously. Reference genomes for representative species across the class now make it possible to identify gene families underlying signature arachnid traits, from the silk proteins of spiders to the venom cocktails of scorpions and the sensory apparatus of harvestmen. Comparative genomics can reveal whether such innovations arose once and were elaborated, or evolved independently in parallel across lineages, questions that speak to the predictability of evolution at the genomic level. Meanwhile, three-dimensional imaging methods—micro-computed tomography in particular—are transforming morphology by making internal anatomy accessible, quantifiable, and shareable, enabling the construction of anatomical ontologies that standardize character descriptions across research groups. When these morphological data are combined with phylogenies and genomic information, they support powerful comparative analyses of how form, function, and ecological performance have co-evolved.

The concept of the extended phenotype is especially rich in arachnids. Spider webs, burrow traps, eggcase architectures, and mating displays are all products of genes expressed far beyond the body itself, and their diversity across lineages provides a natural experiment in the evolution of complex behavior. Understanding how genomic variation maps onto these extended traits—how spinning behavior relates to silk gland complement, how burrow architecture relates to sensory ecology—requires integrating behavioral observation, functional testing, and evolutionary analysis in ways that few other systems allow. Sexual selection adds another dimension, as many arachnid groups exhibit striking sexual dimorphism, elaborate courtship rituals, and, in some lineages, sexual cannibalism and genital mechanics that have driven rapid and sometimes antagonistic coevolution between the sexes.

Machine learning and artificial intelligence are beginning to permeate every stage of the arachnid research workflow. Automated image analysis can accelerate species description, quantify morphological variation at scales previously impractical, and even assist with identification in the field. Deep learning approaches applied to acoustic, vibrational, or visual signals open new windows on communication and mating behavior. In phylogenetics, machine-assisted methods are being explored for character coding, fossil placement, and model selection. These tools do not eliminate the need for biological expertise, but they change the scale at which questions can be asked, making it feasible to analyze thousands of specimens or hundreds of traits in ways that were inconceivable a generation ago. Realizing this potential will require attention to training data quality, reproducibility, and the interpretability of results—challenges that the field is only beginning to confront systematically.

Ultimately, the three grand challenges articulated for arachnid morphology, systematics, and evolution are deeply interconnected. A well-resolved, time-calibrated tree of life provides the framework upon which species delimitation and trait evolution studies depend; robust species boundaries are the units upon which comparative analyses rest; and understanding trait evolution feeds back into phylogenetics by revealing homology and convergence that inform character interpretation. Progress on any one front strengthens the others, and the methodological tools—genomic sequencing, advanced imaging, computational analysis—are increasingly shared across all three. For a lineage that has been evolving on land for hundreds of millions of years, arachnids remain surprisingly understudied relative to their diversity and ecological importance, and the coming decade offers an exceptional opportunity to close that gap through coordinated, integrative, and technologically sophisticated research.

Subject of Research: …

Article Title: Grand challenges in morphology, systematics, and evolution

Article References: Grand challenges in morphology, systematics, and evolution. (n.d.). https://doi.org/10.3389/frchs.2025.1713046

Image Credits: AI Generated

DOI: 10.3389/frchs.2025.1713046

Keywords: Grand, challenges, morphology, systematics, evolution, scientific research