Insects dominate. More specifically, the phylum Arthropoda, which includes insects, spiders, crustaceans, and their relatives, contains far more described species than any other animal group on the planet. Current counts put arthropods at roughly 1.3 million named species, dwarfing every other branch of animal life combined. But within that enormous phylum, a surprisingly heated debate continues over which specific order of insects truly holds the crown, and the answer depends on how you count the species nobody has found yet.
Arthropods by the Numbers
Arthropoda includes about 1,302,809 described species, a figure that accounts for the vast majority of all known animal diversity on Earth.1Zootaxa. Phylum Arthropoda No other animal phylum comes close. Mollusks (snails, clams, octopuses) have around 85,000 described species. Chordates, which include every mammal, bird, reptile, amphibian, and fish, have roughly 70,000. Arthropoda is, by any measure, more diverse than any other living or extinct animal taxon.2Encyclopedia of Insects. Arthropoda and Related Groups
The sheer scale is hard to grasp. For every species of mammal, bird, or fish that has been catalogued, there are dozens of arthropod species. The gap widens further when you consider that total projections for Earth’s species range from millions to trillions, with a widely cited 2011 estimate landing around 9 million.3Europe PMC. How many species are there on Earth? Progress and problems The vast majority of those undiscovered species are almost certainly arthropods, particularly insects.
The Beetle Question
There is an old saying, sometimes attributed to the biologist J.B.S. Haldane, about the Creator having “an inordinate fondness for beetles.” Beetles, the order Coleoptera, have long held the title of the most species-rich animal order. With more described species than any other order of animals, they are the conventional answer to the question of which single group has produced the most variety.4PubMed Central. The evolution and genomic basis of beetle diversity Roughly 400,000 beetle species have names, spread across habitats from deserts to freshwater streams to the canopies of tropical rainforests. They range in size from tiny featherwing beetles barely visible to the naked eye to palm-sized goliath beetles.
But this headline number reflects what taxonomists have described so far, not necessarily what exists. And that distinction matters enormously, because the groups easiest to spot and collect get named first. Beetles are large enough to pin, colorful enough to attract collectors, and have been the focus of entomological attention for centuries. Other insect orders have not received the same scrutiny.
Why Hymenoptera May Actually Win
A growing body of evidence suggests that the Hymenoptera, the order containing ants, bees, wasps, and sawflies, may actually be the most species-rich animal order by a wide margin. A 2018 analysis modeled the likely number of undescribed parasitoid wasps, a group so small and so poorly studied that most species have never been collected. The results were striking: Hymenoptera may contain 2.5 to 3.2 times more species than Coleoptera.5PubMed Central. Quantifying the unquantifiable: why Hymenoptera, not Coleoptera, is the most speciose animal order
The reasoning is straightforward. Many parasitoid wasps are host-specific, meaning each species attacks only one or a few host insect species. If there are millions of insect species serving as potential hosts, there should be millions of corresponding parasitoid species. Most of these wasps are tiny, often under a millimeter, and they live inside or on other insects during their larval stage. They are extraordinarily difficult to collect, identify, and distinguish from one another using traditional methods. The result is a vast dark matter of unnamed species hiding in plain sight.
This does not mean beetles are rare or unimpressive. Beetles remain one of the most ecologically important insect orders. But the traditional claim that they are the most species-rich animal group likely reflects centuries of collecting bias more than biological reality.
What Made Insects So Diverse
Several factors help explain why insects, and arthropods generally, have generated so many species. One of the most important is their relationship with land. Terrestrial environments cover only about 30% of Earth’s surface, yet they harbor roughly 80% of all species. A study using a time-calibrated phylogeny of animal phyla found that predominantly land-dwelling lineages diversify at faster rates than marine ones, which helps explain both why terrestrial habitats are so rich and why land-based arthropods lead the species count.6PubMed. Faster diversification on land than sea helps explain global biodiversity patterns among habitats and animal phyla
Small body size also plays a role. Smaller organisms tend to have smaller geographic ranges, lower dispersal ability, and weaker tolerance for environmental variation. Those features fragment populations more readily, and fragmented populations are more likely to diverge into separate species over time. Research on Malagasy frogs has demonstrated that smaller species tend to have more fragmented ranges, which facilitates reproductive isolation and thus speciation.7PubMed Central. Speciation in little: the role of range and body size in the diversification of Malagasy mantellid frogs Insects are among the smallest multicellular animals, and their tiny size amplifies this effect across millions of populations worldwide.
Complete metamorphosis, the life cycle that includes a larval stage, a pupal stage, and then the adult form, is another key piece of the puzzle. The majority of described insect species undergo this kind of transformation, which allows larvae and adults to exploit entirely different food sources and habitats.8PubMed Central. Complete metamorphosis of insects A caterpillar eating leaves and a butterfly drinking nectar are the same species at different life stages, yet they occupy separate ecological niches. This effectively doubles the ecological space a single species can fill, reducing competition within species and opening up opportunities for new species to evolve.
Then there is the matter of extinction. Insect diversity in the fossil record shows that insects achieved their extraordinary richness not primarily through unusually high rates of speciation, but through remarkably low rates of extinction, comparable to the slowest-evolving marine invertebrate groups.9PubMed. Insect diversity in the fossil record Species accumulated over hundreds of millions of years, and relatively few were lost along the way. The result is a stockpile of diversity that has been compounding since the Devonian period.
Latitude and Plant Diversity
Insect diversity is not evenly distributed around the planet. It drops off sharply as you move away from the equator, a pattern so consistent that it holds across nearly every insect group studied. Research on latitudinal gradients in insect richness has found that plant diversity is a stronger predictor of insect species numbers than climate factors alone.10Ecologies. Plant Diversity Is More Important than Climate Factors in Driving Insect Richness Pattern along a Latitudinal Gradient Where there are more plant species, there are more insect species feeding on them, pollinating them, or living inside them.
This makes tropical forests the epicenter of insect diversity. A single hectare of rainforest canopy can contain more insect species than an entire temperate country. It also means that tropical deforestation poses a disproportionate threat to global species richness, since the species-dense habitats being cleared are exactly the ones that remain least explored by taxonomists.
The Unseen Contenders
Insects and other visible arthropods dominate the described species count, but several groups of animals may be far more diverse than their current tallies suggest. Nematodes, or roundworms, are the most dramatic example. About 22,000 to 30,000 nematode species have been formally described, a modest number by insect standards.11Encyclopedia of Life Sciences. Nematoda (Roundworms) But estimates of true nematode diversity range wildly, from 100,000 to as many as 10 million species. Nematodes are found in virtually every habitat on Earth, from deep ocean sediments to Antarctic soils to the guts of other animals. They are tiny, morphologically similar to one another, and often require specialized techniques to identify. The gap between described and estimated diversity is larger for nematodes than for almost any other animal group.12IntechOpen. Round Worms from Past to Present: Evolution, Biodiversity, and Ecological Roles
Mites and ticks, collectively the Acari, present a similar story within the arthropods themselves. Around 55,000 Acari species have been described, making them nearly half of all known arachnids.13PubMed Central. Cryptic speciation in the Acari: a function of species lifestyles or our ability to separate species? But actual diversity is thought to be far greater. Mites live in soil, in fresh water, on plants, on other animals, in mattresses, and in house dust. Their microscopic size and cryptic habits mean that new species turn up constantly once someone actually looks for them.14PubMed Central. Diversity and Distribution of Mites (ACARI) Revealed by Contamination Survey in Public Genomic Databases
DNA Barcoding and Cryptic Species
Traditional taxonomy relies on visible physical differences to distinguish species. But many animal species look identical to the human eye yet are genetically distinct, reproducing only among themselves and evolving along separate trajectories. These are called cryptic species, and DNA barcoding has revealed that they are far more common than anyone expected, particularly in tropical arthropods.
A landmark study of what appeared to be a single species of skipper butterfly in Costa Rica, Astraptes fulgerator, used DNA barcoding combined with 25 years of natural history observations to show that the “species” was actually a complex of at least 10 separate species in that region alone.15PubMed Central. Ten species in one: DNA barcoding reveals cryptic species in the neotropical skipper butterfly Astraptes fulgerator Similar work on weevils from the Qinghai-Tibet Plateau found that several morphologically defined weevil species each contained multiple genetically distinct lineages, indicating hidden diversity that traditional methods missed entirely.16PubMed Central. Delimiting species, revealing cryptic diversity, and population divergence in Qinghai-Tibet Plateau weevils through DNA barcoding
Large-scale barcoding efforts have found that this pattern is systematic, not anecdotal. A comprehensive Canadian insect barcoding project found that molecular species counts for Diptera (flies) and Hymenoptera were far higher than either the number of known species or past predictions of total diversity in those orders.17PubMed Central. Counting animal species with DNA barcodes: Canadian insects In other words, even in a well-studied temperate country, the flies and wasps alone contained far more species than entomologists had guessed. The implications for tropical regions, where sampling is thinner and diversity is higher, are sobering. The true number of arthropod species on Earth may be several times the current described total.
Vertebrates in Perspective
For most people, “animal diversity” conjures images of mammals, birds, and fish. But vertebrates represent a tiny fraction of total animal species. The most diverse vertebrate group is the teleost fishes, with nearly 30,000 species, accounting for about half of all living vertebrates and roughly 98% of ray-finned fishes.18PubMed. The Divergent Genomes of Teleosts That is genuinely impressive for a vertebrate group. But it is roughly comparable to the number of described nematode species, and it is dwarfed by even a single family of beetles.
Mammals, the group humans belong to, have only around 6,500 described species. Birds have about 11,000. Reptiles have roughly 12,000. These numbers feel large because vertebrates are the animals we see, name, study, and care about. But in the grand scheme of animal life, they are statistical outliers on the low end of diversity.
The Research Bias Problem
The gap between vertebrate and invertebrate diversity is enormous, yet the scientific literature does not reflect it. An analysis of biodiversity research found that roughly half of published animal biodiversity papers studied vertebrates and half studied invertebrates, even though over 95% of described animal species are invertebrates.19PubMed Central. Scientific research on animal biodiversity is systematically biased towards vertebrates and temperate regions This bias has remained roughly consistent over the past two decades. Vertebrates are dramatically over-represented in the literature relative to their actual share of species richness.
This skew matters for conservation. If most research funding and attention goes to the 5% of animal species that are vertebrates, the remaining 95% receive less study, less monitoring, and less protection. Many invertebrate species may go extinct before they are ever described. The research bias also distorts the public’s understanding of biodiversity itself. When people think of endangered animals, they picture pandas, tigers, and whales, not parasitoid wasps, soil mites, and cave beetles, even though the latter groups contain orders of magnitude more species and face comparable threats from habitat loss.
Species Richness Versus Ecological Dominance
Having the most species is not the same thing as being the most dominant in an ecosystem. Ants illustrate this distinction beautifully. There are more than 15,700 named ant species and subspecies, with possibly as many undescribed ones still waiting to be found. That makes them just 1.2% of all terrestrial arthropod species. But ants punch far above their weight: they make up at least 6% of total terrestrial arthropod biomass.20Proceedings of the National Academy of Sciences (PNAS). The abundance, biomass, and distribution of ants on Earth In tropical forests and grasslands, ants can be the dominant animal in terms of sheer mass, outweighing all wild mammals in some ecosystems.
This contrast between species count and ecological impact shows up across the animal kingdom. Krill, a handful of species in the Southern Ocean, support the entire Antarctic food web. Termites, with only a few thousand species, reshape landscapes and cycle enormous amounts of carbon and nitrogen. Meanwhile, some of the most species-rich groups, like tiny parasitoid wasps, may have relatively low total biomass because each species exists in small, specialized populations tied to a single host. Species richness tells you something about evolutionary creativity, but it does not directly translate to ecological importance. The groups that shape ecosystems and the groups that generate the most evolutionary lineages are often quite different.
Marine Arthropods and the Ocean Gap
Given how dominant arthropods are on land, it is worth asking why the oceans, which cover 70% of the planet, are not similarly packed with arthropod species. Marine arthropods exist in abundance: crabs, lobsters, shrimp, barnacles, copepods, and amphipods all thrive in the sea. Amphipod crustaceans alone have nearly 5,000 described species distributed across a dozen distinct biogeographic regions.21Marine Ecology Progress Series. Global biogeography of marine amphipod crustaceans: latitude, regionalization, and beta diversity But the total marine arthropod species count is a fraction of the terrestrial one.
The faster diversification of land-dwelling lineages, mentioned earlier, is part of the explanation. But the oceans also present fewer physical barriers to gene flow. A mountain range or a river valley can isolate a population of flightless beetles for millions of years, driving speciation. In the open ocean, larvae and adults often disperse across vast distances, keeping gene pools connected and slowing the formation of new species. The result is that marine environments tend to have higher diversity at the phylum level (more fundamentally different body plans) but lower diversity at the species level within each group. The ocean invented more kinds of animals; the land made more versions of fewer kinds.
Coral reefs are the partial exception. Reef systems create the kind of complex, fragmented habitat that drives speciation on land, and they are home to some of the ocean’s most species-rich communities. But even the richest reef does not approach the species density of a tropical forest canopy when invertebrates are counted.