Arthropods are invertebrate animals with a segmented body, jointed legs, and an external skeleton made of a tough material called chitin. They include insects, spiders, crabs, centipedes, scorpions, shrimp, ticks, and millions of other species. In terms of sheer number of species, arthropods dominate life on Earth, accounting for roughly 80 percent of all known animal species. Their fossil record stretches back over half a billion years, and their success story involves a set of biological innovations that no other animal group has matched for versatility.
When Arthropods First Appeared
The oldest evidence of arthropods comes from trace fossils, the tracks and burrows left by animals moving through sediment, dating to around 537 million years ago during the early Cambrian Period.1PubMed Central. Early fossil record of Euarthropoda and the Cambrian Explosion Trilobites, among the most recognizable early arthropods, show up in the fossil record by about 521 million years ago. Remarkably, analysis of trilobite evolution suggests that by the time those body fossils appear, the explosive diversification of arthropods had already largely concluded. Evolutionary rates across their preserved anatomy stayed constant throughout the rest of the Cambrian, pointing to a rapid burst of change in perhaps a 20-million-year window during the latest Ediacaran and earliest Cambrian.2PubMed Central. Trilobite evolutionary rates constrain the duration of the Cambrian explosion
Deposits from the latest Ediacaran Period, around 550 million years ago, preserve abundant algae but no animals at all. That absence sets a hard floor: the arthropod lineage is no older than about 550 million years.3PubMed Central. Early fossil record of Euarthropoda and the Cambrian Explosion The famous Burgess Shale and similar fossil sites from around 518 million years ago preserve animals like Anomalocaris, a large predator that sits on the stem lineage leading to modern arthropods. Fossils like these reveal how arthropod features were assembled step by step: the exoskeleton, paired branching limbs, body segments, and a distinct head region with specialized appendages. The early Cambrian ocean was essentially the proving ground for the arthropod body plan, and plankton-feeding lifestyles along with hard outer coverings appear to have been critical to their early diversification.4PubMed. The origin and early evolution of arthropods
The Four Major Living Groups
Modern arthropods split into two great branches early in their history. One branch, Chelicerata, includes spiders, scorpions, horseshoe crabs, ticks, and mites. The other branch, Mandibulata, includes everything with mandibles: insects, crustaceans, and centipedes and millipedes. Molecular studies consistently and strongly support this basic split.5PubMed Central. Arthropod Phylotranscriptomics With a Special Focus on the Basal Phylogeny of the Myriapoda
Within Mandibulata, one of the bigger surprises from modern genetics is that crustaceans are not a natural group unto themselves. Insects actually nest within the crustacean family tree, making “Crustacea” in the traditional sense an outdated category. The correct grouping is Pancrustacea, which combines all crustaceans and all insects into one clade. The closest living relatives of insects appear to be remipedes, a small group of blind, cave-dwelling crustaceans.6Current Biology. The Evolution of Arthropods Within Pancrustacea, the relationships are complex: barnacles group with lobsters and crabs, while fairy shrimp group more closely with insects and remipedes.7Oxford Academic. Major Revisions in Pancrustacean Phylogeny and Evidence of Sensitivity to Taxon Sampling Myriapods (centipedes and millipedes) form the other major mandibulate group, branching off before the pancrustacean split.
This reshuffling has practical consequences for how biologists think about evolution. Any trait shared between a crab and a fly might have been inherited from a common ancestor rather than independently invented. That realization has reshaped research on vision, limb development, nervous systems, and immunity across these groups.
The Exoskeleton and the Problem of Growing
The exoskeleton is arguably the single most important arthropod innovation. It is not just a shell; it is a sophisticated composite material made of chitin fibers embedded in a protein matrix, often reinforced with minerals in crustaceans. The fibers are arranged in layers where each layer shifts its orientation slightly from the one below, creating a helical stacking pattern called the Bouligand structure. Each full 180-degree rotation of fiber orientation appears as a single lamella in cross-section.8PubMed Central. Structural diversity of crustacean exoskeletons and its implications for biomimetics This twisted-plywood arrangement gives the cuticle exceptional resistance to cracking, because a crack that starts propagating in one direction runs into fibers oriented differently in the next layer, which deflects it. Engineers have borrowed this architecture for designing impact-resistant composites.
The trade-off is that a rigid exoskeleton cannot stretch. To grow, an arthropod has to shed its old cuticle and produce a new, larger one underneath, a process called molting or ecdysis. This is one of the most vulnerable moments in an arthropod’s life: the animal is soft, immobile, and exposed until the new exoskeleton hardens. The process is orchestrated by hormones, with ecdysis triggering hormone synthesized by specialized cells on the surface of the tracheal system playing a central role in coordinating the sequence of muscle contractions and behaviors involved in shedding the old skin.9PubMed Central. The Intricate Role of Ecdysis Triggering Hormone Signaling in Insect Development and Reproductive Regulation Crabs and lobsters continue molting throughout their lives. Most insects, by contrast, stop molting once they reach the adult stage.
How Arthropods Breathe
One of the most striking things about arthropods is that they have invented ways to breathe on land not once but multiple times independently. Aquatic arthropods typically use gills, feathery or plate-like structures that extract dissolved oxygen from water. When different lineages moved onto land, they had to solve the gas-exchange problem from scratch each time. The result is that lungs or internal air tubes called tracheae evolved independently in several arthropod groups.10Integrative and Comparative Biology. Handling and Use of Oxygen by Pancrustaceans: Conserved Patterns and the Evolution of Respiratory Structures
Insects breathe through tracheae, a network of branching tubes that carry air directly to the tissues. Spiders breathe through book lungs, layered sheets of tissue inside the abdomen where air and blood exchange gases across thin membranes. Horseshoe crabs, which are aquatic chelicerates, use book gills, the likely evolutionary forerunner of the spider’s book lung. Developmental genetics backs this up: the same genes that pattern book gills in horseshoe crabs also pattern book lungs and even the spinnerets that spiders use to produce silk.11Current Biology. Diverse Adaptations of an Ancestral Gill: A Common Evolutionary Origin for Wings, Breathing Organs, and Spinnerets Some tiny arthropods, like sea spiders (pycnogonids), do not bother with any of these structures and breathe directly through their cuticle.
Arthropods generally use an open circulatory system where blood, called hemolymph, is pumped into body cavities rather than confined to vessels. In most insects, hemolymph does not carry oxygen at all because the tracheal system delivers it directly. However, some insects do have hemocyanin, a copper-based oxygen-transport protein found widely in other arthropods. The stonefly Perla marginata and the firebrat Thermobia domestica both have functional hemocyanin, suggesting that the protein was present in the common ancestor of insects and was lost in most lineages as tracheal systems became more efficient.12PubMed. The occurrence of hemocyanin in Hexapoda
Why Arthropods Are Small (and Why Some Were Once Giant)
The tracheal system works brilliantly at small body sizes, but it runs into trouble as an animal gets bigger. Larger insects have to devote a proportionally larger fraction of their body to tracheal tubes, and eventually, the system cannot deliver oxygen fast enough to deeper tissues. This is one reason insects today are small compared to vertebrates. The connection between oxygen and body size is supported by several lines of evidence: most insects grow smaller when raised in low-oxygen conditions, some grow larger in high-oxygen conditions, and larger species invest more of their body in tracheal infrastructure.13Proceedings of the Royal Society B: Biological Sciences. Atmospheric oxygen level and the evolution of insect body size
During the late Carboniferous period, roughly 300 million years ago, atmospheric oxygen was much higher than it is today. That era produced dragonfly relatives with wingspans approaching 70 centimeters and millipede-like creatures over two meters long. When oxygen levels dropped, the giant arthropods disappeared. For arthropods that breathe through their cuticle rather than tracheae, the constraint is different but still real: the cuticle must be thin enough to let gases pass through yet thick enough to hold the animal together. Measurements of Antarctic sea spiders show that these competing demands create a hard upper limit on body size that the animals cannot exceed without evolving fundamentally new ways to exchange gases.14PubMed Central. Upper limits to body size imposed by respiratory–structural trade-offs in Antarctic pycnogonids
Compound Eyes and Other Senses
Most arthropods see the world through compound eyes, each composed of many individual units called ommatidia. The two dominant designs are apposition eyes, where each ommatidium captures light independently (typical of daytime insects), and superposition eyes, where multiple ommatidia pool their light to form a brighter image (common in nocturnal insects and many crustaceans). The evolutionary history of these eyes is not a single straight line. Insects and crustaceans share a common ancestor that likely already had compound eyes with focusing crystalline cones and the ability to see color and detect polarized light. Myriapods and chelicerates, by contrast, evolved their compound eyes separately from simpler ancestors with basic corneal lenses and probably no color vision.15PubMed. A functional analysis of compound eye evolution
Beyond vision, arthropods have evolved a dizzying array of sensory tools. Many detect airborne vibrations through fine hairs on their exoskeletons. Some spiders sense prey through vibrations traveling along their webs. Crustaceans and insects use chemical receptors on their antennae and even their feet. Mosquitoes can detect carbon dioxide from a human breath at considerable distances. This sensory diversity is possible in part because the exoskeleton is not an inert shell but a platform studded with specialized sensory structures.
Metamorphosis
Many arthropods undergo dramatic changes in body form during development, but insect metamorphosis takes this to an extreme. There are two broad patterns. In hemimetabolous insects like grasshoppers and dragonflies, the juvenile looks like a small, wingless version of the adult and changes gradually with each molt. In holometabolous insects like beetles, butterflies, and flies, the juvenile (larva) looks nothing like the adult and undergoes a complete reorganization of body tissues during a pupal stage.16PubMed Central. Transcription factor E93 specifies adult metamorphosis in hemimetabolous and holometabolous insects
Despite how different these two developmental strategies look from the outside, they share a common hormonal toolkit. Juvenile hormone acts as a brake on metamorphosis in both types of insects, and the same core signaling genes keep young insects in a juvenile state until the right moment. Complete metamorphosis, the kind with a pupa, appears to have evolved by adding a new stage to this existing program rather than reinventing it entirely.17PLoS ONE. Common and Distinct Roles of Juvenile Hormone Signaling Genes in Metamorphosis of Holometabolous and Hemimetabolous Insects The holometabolous strategy has been enormously successful: beetles, flies, moths, and wasps collectively make up the majority of known insect species.
What Arthropods Do for Ecosystems
Strip arthropods from any ecosystem and it collapses quickly. In soil, arthropods like mites, springtails, beetles, and ants drive decomposition, move organic matter between soil layers, regulate populations of bacteria and fungi, and physically reshape soil structure through burrowing.18PubMed. Arthropods in soil reclamation and bioremediation: Functional roles, mechanisms and future perspective Without these activities, dead plant material would accumulate and nutrients would cycle far more slowly.
Aboveground, arthropod pollinators maintain the reproductive success of most flowering plants. The relationship between plants and arthropods is an evolutionary arms race that cuts both ways. Plants have evolved chemical defenses against herbivorous insects, and those same chemicals can end up in nectar and pollen, affecting the pollinators that visit the flowers.19Current Biology. Lingering Effects of Herbivory and Plant Defenses on Pollinators Experimental work with fast-cycling plants has shown that when plants are exposed to both pollinators and herbivores simultaneously, they evolve differently than when exposed to pollinators alone, tending toward self-pollination as a kind of backup strategy.20PubMed. Rapid plant evolution driven by the interaction of pollination and herbivory
In the ocean, tiny crustaceans like copepods and krill perform one of the largest daily animal migrations on Earth: diel vertical migration. They feed near the surface at night and descend hundreds of meters during the day. This movement actively transports carbon from surface waters to the deep ocean through respiration, excretion, and fecal pellets released at depth, a process that helps sequester carbon for decades to centuries.21PubMed. Active Carbon Transport by Diel Vertical Migrating Zooplankton: Calculated and Modeled, but Never Measured The scale of this vertical shuttle is enormous and represents a significant, though still incompletely measured, component of the ocean’s biological carbon pump.22PubMed Central. The biological carbon pump, diel vertical migration, and carbon dioxide removal
Venoms and Chemical Warfare
Arthropod venoms are far more diverse in their chemistry than most people realize. They range from the relatively simple formic acid that ants spray to the elaborate cocktails of proteins, peptide toxins, and small organic molecules found in spider, scorpion, and centipede venoms. These venoms serve multiple purposes beyond killing prey: deterring predators, subduing rivals during competition, and facilitating feeding by keeping blood flowing at a bite site.23PubMed. Arthropod venoms: Biochemistry, ecology and evolution The molecular building blocks of these venoms have been recruited from many different protein families over evolutionary time, and each lineage has independently assembled its own venom toolkit.24Revista ft. Venom Composition and Functionality in Arthropods: A Comparative Review
Parasitoids and Mind Control
Some of the most unsettling biology on Earth comes from parasitoid wasps, insects whose larvae develop inside or on a living host, eventually killing it. These wasps do not just lay an egg and leave. They actively manipulate their host’s biology using venom, viruses integrated into the wasp genome, and specialized cells released from the developing egg, all of which suppress the host’s immune response and alter its behavior to benefit the wasp larva.25PubMed. Molecular Mechanisms Underlying Parasitoid-Derived Host Manipulation Strategies
The emerald jewel wasp offers a dramatic example. It delivers two precise stings to a cockroach’s brain, injecting venom that does not paralyze the cockroach’s muscles but instead eliminates its drive to move. The cockroach can still walk when pulled, but will not initiate movement on its own for seven to ten days, during which the wasp’s larva hatches and feeds on it alive.26Revista ft. Neurotoxicologia da ampulex compressa: mecanismos moleculares e cinética da hipocinesia em baratas – uma revisão integrativa Another well-studied case involves a parasitoid wasp larva that forces its spider host to build an abnormal web, one that better protects the wasp’s cocoon. The trigger appears to be the larva injecting a hormone into the spider’s hemolymph that activates the same web-building behavior the spider would normally perform just before molting.27PubMed Central. Proximate mechanism of behavioral manipulation of an orb-weaver spider host by a parasitoid wasp These are not rare curiosities. Parasitoid wasps are among the most species-rich groups of insects, and host manipulation is widespread among them.
Gut Microbes and Hidden Partners
Arthropods do not operate alone. Their guts harbor microbial communities that influence digestion, growth, defense against disease, and even the ability to break down toxic compounds.28PubMed Central. Insect Gut Microbiota-Research Strategies and Perspectives Termites famously depend on gut microbes to digest cellulose from wood. Bark beetles carry fungi that help them colonize trees. Aphids host bacterial symbionts that synthesize amino acids the insects cannot make themselves. In many cases, these partnerships are so ancient and tightly integrated that neither partner can survive without the other. The host provides a stable environment and nutrients; the microbes provide metabolic capabilities the arthropod’s own genome lacks.
Arthropod Declines and Why They Matter
Reports of insect decline have drawn increasing attention. Evidence points to losses in biodiversity and shifts in community composition across many habitat types, driven by land-use change, loss of suitable habitats, climate change, invasive species, and pollutants including pesticides and fertilizers.29PubMed. Insect declines: challenges and solutions The picture is not uniform: some species are declining sharply while others are expanding, and monitoring data remain patchy across much of the world. But the direction of the overall trend in well-studied regions is concerning, given how deeply ecosystems depend on arthropod functions like pollination, decomposition, and serving as food for birds, fish, and other animals. Losing arthropod diversity does not just mean fewer bugs; it means degraded soil, less productive fisheries, and crops that set fewer seeds.